New Support for the Perceptual Activity
Theory of Mental Imagery
An Ongoing Bibliographic Essay
by Nigel J.T. Thomas
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Since the publication of my "Are Theories of Imagery Theories of Imagination? An Active Perception Approach to Conscious Mental Content," (Thomas, 1999 - henceforth abbreviated as ATOITOI on this page), a good deal of published material has appeared or has come to my attention that either provides additional support for the Perceptual Activity Theory (PA theory) of mental imagery presented in ATOITOI, or that throws further doubt on the rival (picture and description) theories that are criticized there. Other relevant evidence was not mentioned in ATOITOI because I lacked the space for a proper explanation of its relevance. I hope eventually to write and publish a new account of PA theory, that will make use of much of this material. In the meantime this page provides citations (and, where possible, links) to the "new" support, and discussion sections that briefly explain the relevance of the cited material. Quite apart from presenting new lines of supporting evidence and argument, I hope this page will help to clarify many aspects of PA theory for those seeking a deeper understanding of it. Also, as any empirical or theoretical challenges to PA theory are published (and come to my attention), they will also be discussed here, together with my rebuttal (if I have one). Published misintepretations of the theory, that provide significantly misleading accounts of my actual views, will also be listed and discussed here as they come to my attention.
In some of my more recent writings (notably Thomas, 2008) I have adopted Noë's (2004) term enactive theory in place of the expression Perceptual Activity theory (or PA theory) that I used in ATOITOI and elsewhere. This is merely a terminological change, and does not reflect any substantive change in my views. "Enactive" serves, I think, to capture the essence of the theory a little more clearly, and also helps to bring out the close affinity between my views about imagery and Noë's (now well known) account of perception and perceptual consciousness. (This affinity is explicitly noted by O'Regan & Noë (2001 p.945, 947, 972).) For the sake of consistency, however, I will continue to use the expression "Perceptual Activity [or PA] theory" on this page.
Please note that this page is an ongoing project. Not only is the discusion incomplete at the moment, but I also expect to be finding new material to add to it periodically.
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Links I have provided to articles online are all "open access," and so should be accessible to all. Other articles may be available to you online via a library or personal subscription.
Please note that citations in the discussions below may refer either to items listed in this table, or to works listed in the Supplementary Bibliography further down the page.
As items in this table are not arranged alphabetically, you may want to use your browser's within-page search (Find) capability to locate citations from the discussion sections.
(mostly chronological within sections - items that appear in more than one section are marked with a *)
|1. Direct evidence for the involvement of content-appropriate exploratory perceptual activity in imagery||
|2. Support from neurology (clinical studies of people with brain lesions); especially representational (imagery) neglect syndrome||
objections to pictorial theories of imagery (including the reasons why
evidence of activity in retinotopically mapped cortex during imagery does
not strongly support pictorialism)
(Discussion not yet available)
on why description ('propositional') theory is unworkable
(Discussion not yet available)
and arguments for regarding conscious perception as embodied exploratory activity
(Discussion not yet available)
visual experience depends upon top-down attentional mechanisms
(Discussion not yet available)
instruments and active perception in animals and humans
(see also 3b, above)
(Discussion not yet available)
conceptual relationship between imagery and imagination
(Discussion not yet available)
|9. An argument from psychological phenomenology.||
|10. Computational modeling of Perceptual Activity Theory.
(Discussion not yet available)
from Imagery and Efferent fibers in the Optic Nerve
(Discussion not yet available)
|12. Misrepresentations of Perceptual Activity Theory||
|13. Alleged or apparent problems for Perceptual Activity Theory||
|A bibliography of other publications that are cited in the discussion below, but have either already been cited in ATOITOI, or else have only indirect relevance to the truth or otherwise of PA theory, may be found at the end of the discussion sections below.|
The heart of the Perceptual Activity Theory of Imagery (PA theory) is the contention that the experience of imagery arises from our more or less covert enactment of the specific exploratory perceptual behavior that would be appropriate for exploring the imagined object if it were actually present. Direct experimental support for this now comes from recent work on eye movements during imagery. In the account of PA theory given in ATOITOI I stressed covert enactment processes occurring within the perceptual instruments or even entirely within the brain, assuming that enactment of more overt exploratory actions, such as eye movements, would be largely inhibited. Although I still think such covert processes play the major role in imagery, I am now gratified to discover that the inhibition of certain more overt enactment processes is not so strong or so pervasive as I had feared. A real correlation between eye movements during perceptual exploration and movements during imagery has now been demonstrated in several experiments.
The most direct demonstration of this builds upon earlier research by Stark and others that shows that looking at a visual presentation results in cognitively directed saccadic eye movements that actively explore the display in a structured, way (Ellis & Stark, 1979; Stark & Ellis, 1981; Noton, 1970; Bozkov, Bohdanecky, & Radil-Weiss, 1982; Gaarder, 1975; Kowler, 1996; Yarbus, 1967). In fact, it was demonstrated that under the right conditions, and with suitable stimuli, people soon establish a regular scanpath, a sequence of saccades and intervening fixations through which they cycle repeatedly, apparently exploring the stimulus over and over in a stereotyped sequence (Noton & Stark, 1971a, 1971b, 1971c). (It should be noted that scanpaths vary both from person to person and from stimulus to stimulus: i.e. each experimental subject established their own distinctive set of scanpath patterns, a different one for each stimulus drawing.)
Brandt & Stark (1997) have now shown that when people form mental images of previously learned visual stimuli, they also spontaneously move their eyes in a regular scanpath demonstrably similar to the one established whilst viewing the specific stimulus now being imagined. The subjects were first presented with a simple visual pattern to be memorized, and shortly afterwards they were asked to imagine that pattern. Their eye movements were recorded throughout the procedure, and it was possible to show that the unique scanpaths established during presentation of each stimulus pattern spontaneously reappeared when the subjects tried to imagine that same pattern. Laeng & Teodorescu (2002) have since replicated and confirmed these findings, demonstrating that "Eye scanpaths during visual imagery reenact those of perception of the same visual scene." Furthermore, they found that when subjects were free to move their eyes whilst visually inspecting a pattern, but were required to fix their gaze when recalling it (via imagery), they did not remember the pattern as well as the did when allowed to move their eyes freely during both inspection and recall phases of the experiment. This suggests that the enacted eye movements play a real functional role in the production of the imagery, and are not mere "epiphenomena" of internal processes. This clearly constitutes direct experimental support for a PA theory of imagery (Laeng & Teodorescu, 2002).
Further confirmation, but this time with imagery of moving (rotating) objects, comes from the finding that the pattern of saccades made by subjects in visually following the motion of a rotating disk is spontaneously enacted when they are asked to asked to visualize such a rotation (de’Sperati, 2003a,b). There is also evidence that, under the right conditions, the tracking (as opposed to saccadic) eye movements that people make when following a moving stimulus are also enacted when they are merely imagine such a motion (Antrobus, Antrobus, & Singer, 1964; Deckert, 1964; Zikmund, 1972; but see Brown, 1968).
Both Brandt & Stark (1997) and Laeng & Teodorescu (2002) used rather simple visual stimuli in their experiments, which thus, presumably, gave rise to rather simple mental images. However, very similar results have now been found using more complex and "naturalistic" stimuli. Johansson, Holsanova, & Holmqvist (2005, 2006) used either a picture of a visual scene or a verbal description of a relatively complex visual scene as stimuli, and recorded their subjects' eye movements both as they encoded the stimulus, and as they recalled the scene from memory (by verbally describing it - they were not explicitly asked to recall it as a mental image). As in the earlier experiments, the scanpath patterns in the recall phase were measurably similar to those recorded during encoding (presumably the subjects who heard a verbal description were forming an image as they heard it).
Other recent studies, by Demarais & Cohen (1998), by Spivey and his collaborators (Spivey et al., 2000; Spivey & Geng 2001), and by Gbadamosi & Zangemeister (2001), also provide evidence that eye movement patterns during imagery reflect the spatial structures even of complex, naturalistic mental images.
The imagery in these experiments was generated in response to verbal descriptions of naturalistic scenes, and must surely have been relatively complex and detailed. Demarais & Cohen (1998) studied eye movements made whilst subjects solved transitive inference ("n-term series") reasoning problems involving such tasks as the following (concerning an arrangement of items on imaginary kitchen shelves): "A jar of pickles is below a box of tea bags; the jar of pickles is above a can of coffee; where's the can of coffee?" (Demarais & Cohen, 1998 p. 231). The subjects were not explicitly told to generate imagery, but it is well known that most subjects report spontaneously generating and using visual images in thinking about problems of this type (Huttenlocher, 1968; Shaver, Pierson, & Lang 1974; Sternberg, 1980; Nyíri, 2001). Demarais & Cohen found that, with problems such as the one quoted, subjects (who were not aware that their eye movements were being recorded) made significantly more up and down eye movements than they did during otherwise similar control tasks that did not involve linearly arrayed items. When solving similar problems in which the items were described as arrayed in a horizontal rather than a vertical line, the subjects made more side to side eye movements than when doing control tasks. Clearly (modulo assumptions about what aspects of perceptual testing get suppressed during imagery) this result is just what PA theory would predict would happen during the experiencing of such images.
Demarais & Cohen (1998) also suggest an interesting and quite plausible explanation of why the results of some (not all) earlier investigations imply that subjects' eyes tend to move less when they are doing mental tasks that supposedly depend upon imagery than when they are doing other sorts of task (e.g. Weiner & Ehrlichman, 1976; Hiscock & Bergstrom, 1981; Ehrlichman & Barrett, 1983; Bergstrom & Hiscock, 1988). Many experimental imagery tasks, it is suggested, do not involve the inspection of the extremities of a linearly extended image, but, rather, internal structural details of an image that may be relatively compact. Most eye movements they evoke are thus likely to be low amplitude "fixation" movements (microsaccades) rather than the large, more easily measurable saccades evoked by spatially linear transitive inference tasks.
Spivey & Geng (2001 experiment 1; see also Spivey et al., 2000) found very similar results in a related experiment. This differed from the experiment by Demarais & Cohen mainly in that the subjects were given explicit instructions to form imagery (on the basis of verbal descriptions) rather than the imagery being induced indirectly, via the demands of a transitive inference task. Subjects were instructed to form images in accordance with passages such as the following:
Imagine you are standing across the street from a 40 story apartment building. At the bottom there is a doorman in blue. On the 10th floor, a woman is hanging her laundry out the window. On the 29th floor, two kids are sitting on the fire escape smoking cigarettes. On the very top floor, two people are screaming. (Spivey & Geng, 2001 p. 237)
The subjects' eye movements were recorded whilst they were hearing the italicized portion of this passage, and it was found that they made a significantly greater proportion of upwardly directed saccades than were seen during a control imagery task (forming an image in response to a similar descriptive passage, but without the strong suggestion of the more salient aspects of the imagined scene being high up, or, indeed, in any particular direction). Similar passages that suggested the interesting aspects of the imagined scene would be in a downwards direction (e.g., watching from the top of a canyon as people rappel down its opposite wall), or to the left or right, produced significantly more saccades directed downwards, leftwards, and rightwards respectively, as compared to the control (and the earlier results of Jacobson (1932) suggest that even more such eye movements would probably be observed if they were not often being actively inhibited during imagery). Once again, this is very much what one would expect on the basis of PA theory, and nothing inherent in picture theory or description theory suggests the likelihood of such results (or those of Demarais & Cohen).
Further corroboration comes from Richardson & Spivey (2000) (and experiment
2 of Spivey & Geng (2001); and see also Richarson & Spivey, 2004 part 2), where it is shown that saccadic eye movements
during the recall of visually presented information tend to reflect the spatial
structure of the original stimulus (which is, presumably, being visualized).
Also, Hong et al. (1997) found that the pattern of REMs (rapid eye movements)
of dreaming subjects are correlated with the reported contents of the
subjects' dream imagery. Admittedly, it is now well recognized that dreams, including dreams involving vivid visual imagery, may occur during periods of sleep when there are no such eye movements, but this may be simply because in non-REM sleep the eye movements associated with the dream imagery have been inhibited (as is most motor activity during sleep), and have thus become covert (Nielsen, 2000). After all, according to PA theory, overt exploratory activity during imagery is the exception rather than the rule.
• [It should be noted that the question of whether the eye movements of REM sleep are directly related to dream imagery was also much studied and debated during the 1960s and early 1970s (Dement & Kleitman, 1957; Roffwarg et al., 1962; Moskowitz & Berger, 1969; Jacobs, Feldman & Bender, 1972; Firth & Oswald, 1975). No very clear answers emerged from this research, however. Koulack (1972) concluded, from his review of this literature, that although some of the eye movements of REM sleep may well be related to dream imagery, others are not. Insofar as I can follow their argument, it appears to me that more recent research has brought Doricchi et al. (2007) to a very similar conclusion.]
The experiments discussed above indicate that (as PA theory
predicts) eye movements, or the eye movement control system, are closely involved
in the generation of the experience of visual mental imagery, and the specific
pattern of such movements (or the specific sequence of states of the control
system) is specific to the particular experiential and intentional content of
the imagery. This suggests that if the eyes of someone who is experiencing imagery
are made to move in ways that are inappropriate or irrelevant to what is being
imagined (i.e., that would not be those made in visually exploring the thing
being imagined) then the imagery will be disrupted. This prediction is experimentally
confirmed both by Ruggieri (1999), who found that if subjects who are imagining
a moving object are instructed to stop moving their eyes or their head, the motion
in the image is (more often than not) stopped or noticeably slowed, and by several
researchers who have found that the vividness (and,
to some extent, the emotional impact) of people's mental images are significantly
reduced when the subjects had to carry out a visual task requiring
eye movements while imagining (Andrade, Kavanaugh, & Baddeley, 1997; Barrowcliff
et al., 2004; Gunter & Bodner, 2008). Perhaps unsurprisingly, the vividness
was also somewhat attenuated when the subjects were required to do an otherwise
similar visual task that did not require eye movement, and when they had to do
a task that called for spatial control of hand rather than eye movements. However,
the effect of the eye movement task was markedly stronger. These findings also
make sense of the fact that when people want to get rid of an unpleasant or disturbing
mental image they may deliberately move their eyes about in a violent and apparently
random manner (Antrobus, Antrobus, & Singer,
1964; Singer & Antrobus, 1965). (It may also help to explain why Marks (1973)
found that when they recall pictures from memory, vivid imagers tend to display
less eye movement than do those whose imagery is less vivid. Perhaps a greater
ability to keep irrelevant and disruptive eye movements under control is a part
of the "skill" of
being a vivid imager.)
•[Andrade, Kavanaugh, & Baddeley (1997) and Gunter & Bodner (2008), discuss their results in the light of Baddeley's theory of the "visuo-spatial sketchpad" function of working memory (Baddeley, 1976, 1994; Baddeley & Hitch, 1974). This, however, is essentially a theory about the function of imagery in the broader cognitive system, and, as such, there is no reason to think that it is incompatible with PA theory, which is a theory of the nature of imagery (see Thomas, 2008 §4.2 & Supplement: Dual Coding and Common Coding Theories of Memory, note 1).]
Of course, as noted in ATOITOI (§2) none of the extant imagery theories (or rather, "research programs") is such that it can be decisively refuted (or verified) by any particular set of experimental results. Certainly Kosslyn's quasi-pictorialism is sufficiently loosely formulated that just about any empirical evidence can be accommodated by making suitable auxiliary assumptions or ad hoc maneuvers (and as Pylyshyn (1981) has noted, defenses of pictorialism have frequently taken advantage of this fact). Thus it is no surprise to find that Mast & Kosslyn (2002) have wasted no time in spinning the Laeng & Teodorescu (2002) results in such a way as to make them seem consistent with pictorialism. Given the pervasive influence of pictorialism, and the general lack of awareness that there might be any alternative explanations of imagery (apart from descriptionism) (Thomas, 2003), it should come as no surprise that Demarais & Cohen (1998) also sketch a pictorialist account of their results along similar lines, and even Laeng & Teodorescu (2002) themselves seem to be very open to this sort of interpretation of their work (although they also explicitly recognize how well their findings jibe with PA Theory and a thoroughgoing rejection of inner pictures). Mast & Kosslyn (2002) suggests that the eye movements observed during imagery reflect the movement of pictorial images (as based on individual "visual snapshots," corresponding to eye fixations) across, or in and out of, the visual buffer where they are supposedly displayed. What seems to be being suggested is a process akin to the stitching together of a number of relatively small inner pictures into a more panoramic imaginal experience of a scene. (It is implied that this may be necessary because of the fact that, in a single eye fixation, only about 2° of visual angle is focused on the fovea, and thus seen with high acuity. Some theorists, such as O'Regan (1992) and, implicitly, Ballard (1991) take this as evidence that vision itself does not involve any picture-like inner display of the sort envisaged by Kosslyn. However, from a determinedly pictorialist perspective it means that an individual image, as displayed at any one moment on the visual buffer, would only be detailed in its central 2°.) But although, when suitable auxiliary assumptions are introduced, the new eye-movement results can undoubtedly be rendered compatible with the core commitments of pictorialism, it is far from clear that these assumptions are consistent with ways that Kosslyn has interpreted his pictorialism in the past in dealing with other empirical findings (such as his favored interpretation of his own "mental scanning" research (Kosslyn, Ball, & Reiser, 1978; Kosslyn, 1980, 1994), and his vaunted claims to have measured the "visual angle of the mind's eye" (Kosslyn, 1978), which he at that time held to be very much greater than 2°). In any case, it is important to stress that, unlike picture theory, PA theory does not merely accommodate but, rather, strongly suggests the likelihood of phenomena of the sort found by the recent eye movement studies. This prediction arises not from any auxiliary assumptions, but rather from the core commitments of the PA research program.
Furthermore, there is recent evidence for the involvement of overt perceptual exploratory actions in olfactory as well as in visual imagery. Bensafi et al (2003) measured the sniffing behavior of subjects instructed to imagine various smells, and found that it parallels what occurs when real smells are being perceived. If subjects were prevented from sniffing (by a nasal clip) the vividness of their olfactory imagery was significantly decreased. Furthermore, when they imagined pleasant odors they spontaneously took large, deep sniffs, and when they imagined unpleasant odors they took short, shallow ones (just as happens when people really smell pleasant or unpleasant smells). This experiment is significant not only, because it extends the sorts of findings discussed above to a non-visual mode of imagery, but also because the sort of quasi-pictorialist interpretation of the eye-movement findings that is suggested by Mast & Kosslyn (2002) does not seem applicable in this case. What could possibly be the olfactory equivalent of the little pictorial sub-images that Mast & Kosslyn imagine as being stitched together by the imagery eye movements?
There is also evidence for yet another sort of eye movement being involved in visual imagery (in addition to the saccadic and tracking movements already discussed). Ruggieri & Alfieri (1992) measured changes in the length of the optical axis (i.e., the thickness) of the crystalline lens of people's eyes when they were asked to imagine either nearby objects ("reading a word on the page of a book"), or faraway ones ("a ship on the horizon"). Sure enough, when asked to imagine the nearby objects, the length of the optical axis increased, just as it does when we are bringing a real nearby stimulus into focus. Once again, these results make good sense if we conceive of the adjustments in the lens during imagery as part of the acting out of the actual perceptual tests we would make in actually seeing something, but it is not at all clear how they could be accounted for in terms of a pictorialist (or, come to that, a descriptionist) story about imagery, even one amended in the way that Mast & Kosslyn (2002) suggest.
According to Bridgeman (1992) "most human behaviors are eye movements. . . . Saccades occur several times per second throughout waking life: even during what we think of as steady fixation, microsaccades continue at the same rate." Although we are not conscious of making them, and have little if any conscious control over them, even these microsaccades appear to be under cognitive control, adjusted appropriately to the stimulus (Gaarder, 1975). Thus it is not so implausible as it might at first seem that such movements are closely involved in such a ubiquitous aspect of our mental lives as imagery. (Much the same could probably be said for sniffs.) Although we normally have little if any conscious awareness of these exploratory perceptual behaviors, we are conscious of both the perceptions and the mental imagery that they make possible.
Nevertheless, it is worth re-emphasizing that, as I conceive PA theory, there is a lot more to visual exploration than just eye movements (and, I am sure, a lot more to olfaction than sniffing), and I doubt whether actual eye movement enactments are either sufficient, or even, in most cases, necessary for experiencing visual imagery. During visual perception, much of the visual exploration process, as I conceive it, goes on internally, within the brain, and involves the exploration (or querying) of not only the (external) optic array, but also its internal informationally correlated causal products, such as the momentary bottom-up excitation pattern of the retinotopically mapped brain areas, such as V1 (what Pylyshyn (2002b) calls the Tootell Display). During imagery, although there is neither a relevant optic array nor a relevant Tootell Display, we enact the relevant exploratory procedures as if there were.
Furthermore, what you see and what you imagine will depend not only on where
you direct your attention, but, equally, on what you look for there.
Differences in what you are looking for (what perceptible features or characteristics)
ought to be reflected in fine grained differences at the neural level, in
the brain, but could hardly be expected to produce reliably and consistently
measurable differences in eye movements. Although eye movements are certainly
a significant aspect of visual exploration, the focus on eye movements by
many researchers inclined toward active theories of perception seems to be
largely a result of the fact that they are much easier to observe and measure
(at the requisite fine level of detail) than the relevant brain events. It
is gratifying that, despite these limitations, eye movement studies have now
produced clear evidence in favor of the PA theory of imagery.
• [Note: The development of my version of of PA theory was, in fact, significantly influenced by the scanpath research of Noton & Stark (1971a, 1971b; Noton, 1970), and I discussed this work in my doctoral thesis (Thomas, 1987). Unfortunately I did not have the space, in ATOITOI, to cover it adequately. I would like to thank Lawrence Stark, for alerting me to his work with Brandt that now explicitly extends the scanpath research program to encompass imagery. Thanks also to Bruno Laeng for alerting me to Spivey's work.]
Back to table
Several reviews of the clinical literature on deficits to mental imagery and to visual perception caused by brain damage, have come to the conclusion that this evidence is not consistent with the quasi-pictorial theory of Kosslyn (Trojano & Grossi, 1994; Goldenburg, 1998; Bartolomeo, 2002; Dulin et al., 2008). People whose are blinded or visually impaired due to brain damage, even when that damage is in the retinotopically mapped regions of the visual cortex, often do not show any signs of impaired visual imagery (although Kosslyn's theory clearly implies that it should be), and people whose imagery does seem to be impaired by brain damage may not be visually impaired in an equivalent way. The first two of these reviews were published before ATOITOI, so they do not consider PA theory. However, Dulin et al. (2008 p. 1398) argue that although the neurological evidence (together with evidence that they also review from neuroimaging studies and from TMS – transcranial magnetic stimulation, a technique for inducing temporary brain "lesions" with intense, focused magnetic fields), is not consistent with quasi-pictorial theory, it can be reconciled with PA theory. Bartolomeo (2002) is less tentative, and quite explicitly argues in favor of PA theory (as presented in ATOITOI) on the basis of the clinical neurological evidence.
Bartolomeo & Chokron (2002a) come to the same conclusion in a brief discussion that focuses specifically on the strange neurological syndrome of unilateral neglect (see Thomas, 2008 §4.5 sup.) wherein (in typical cases) people with damage to the right parietal cortex of their brain show a strong tendency to ignore or fail to notice things to their left (although they are not blind to things to the left, and can see them if their attention is explicitly drawn to them). People with this syndrome may fail to eat food on the left hand side of their plate (even if they are hungry), or fail to notice that a picture of a house shows it as being on fire, if the flames and smoke are coming only from what is, from their perspective, its left hand side (Young & de Haan, 1990; Halligan & Marshall, 1993; Bartolomeo & Chokron, 2001a; Bartolomeo, 2007).
Bisiach & Luzzatti (1978) were the first to show that some patients with this syndrome may also experience similar problems when they are asked to recall visual features of a familiar scene from memory by forming a mental image of it as seen from a particular vantage point: They asked two neglect patients to imagine being in the Piazza Del Duomo, a well known square in Milan (the patients' native city) and to describe the buildings and other features around the square. (One of the patients was also questioned about the items in “the studio where he had spent most of his life,” with very similar results.) When asked to imagine that they were standing on the steps of the cathedral that is at one end of the Piazza, nearly all of the features recalled, by both subjects, were ones that would have been to their right from that viewpoint, and few things on the left were remembered. When asked to imagine standing at the opposite end of the square, facing the cathedral, most of the features they recalled were ones on the other, previously neglected, side, which was now to their right. Presumably, the patients were forming a mental image of the Piazza, as viewed from the specified location, and attempting to read off the features around it from their imagery. Clearly knowledge of features on both sides (presumably mostly gained before they became ill) was in their memory, but they were unable to access all of it normally from their imagery.
The reality of this latter phenomenon, sometimes called representational neglect (or sometimes imaginal neglect), is now well established (see, e.g., Ogden, 1985; Meador et al., 1987; Bisiach & Berti, 1990; Bartolomeo & Chokron, 2001a; Rode et al., 2007; Thomas, 2008 §4.5 sup.), and I did discuss it briefly in the antepenultimate draft of ATOITOI. It seemed to me that it was very hard to see how this phenomenon might be explained in a principled way from the perspective of description theory. (Pylyshyn (2002 b §R53) does have a try, but in the process seems to move a long way toward embracing PA theory!) The phenomenon might conceivably be explained in terms of the patients failing to pay attention to (or perhaps failing to properly construct) the left hand side of an inner quasi-picture (see Meador et al., 1987); but why should we expect attention to an inner picture to be affected in the same way as attention to the actual visual environment? Peripheral actions such as eye and head movements play a large and crucial role in the directing of attention during visual perception, but they clearly cannot be playing anything like the same role in directing the putative "attentional spotlight" around inside the brain.
On the other hand, it seemed to me that, because unilateral neglect seems to be essentially a defect of active attention (a failure to look for things to the left, to explore to the left), and because PA theory regards imagery as a product of just such active, attentional, exploratory processes, PA theory would accommodate the representational neglect phenomenon in a very straightforward and natural way. Whatever it is that causes neglect patients to fail to explore to the left when they are actually looking at something may be expected also to cause them to fail to go through the motions of exploring to the left when they are going through the motions of looking at it (i.e., imagining it).
I was particularly encouraged in this view by the interpretations of the phenomenon offered by Sunderland (1990), and especially Bisiach, Luzzatti & Perani (1979), who explicitly discuss the phenomenon in terms of schemata (although, admittedly, they use the term in a sense that is not quite the same as that given it by Neisser (1976) and myself). I would probably have been even more encouraged if I had then been aware of the interpretation of the phenomenon offered by Bartolomeo, D'Erme, & Gainotti (1994). However, I decided to cut the discussion of this out of ATOITOI, mainly because I became aware that occasional reports had appeared in the clinical literature of visual neglect apparently occuring without representational neglect, and of representational neglect without apparent visual neglect (Coslett, 1997), and I came to feel that my armchair grasp of the complex clinical neurological issues was insufficient for me to be able to give an authoritative account of them. It is very gratifying for me now to find that experts on the unilateral neglect syndrome, with first hand clinical experience of its manifestations and complexities, have come to much the same conclusions as I initially did. Bartolomeo & Chokron (2002a; Bartolomeo, 2002) argue that PA theory (in the form outlined in ATOITOI) provides much the most plausible and parsimonious framework for explaining the representational neglect phenomenon. (It is notable also that, despite his ingrained commitment to the description theory of imagery, Pylyshyn (2002b) largely accepts their arguments.)
In another recent study of representational neglect, Rode et al. (2004) also note the consistency of their results with PA theory, and express doubt as to whether they can be explained in terms of an inner "mental screen". The study involved a patient suffering from visual neglect, but with an intimate knowledge of the geography of France, who was asked to form a mental image of a map of the country and, in two minutes, to name as many as possible of the towns that he could "see" in his image. Nearly all the towns he named were toward the right of the map (i.e. in eastern France). When asked to name French towns without being instructed to imagine the map, he was able to name a similar number of towns in the same amount of time, but in this condition they were more or less evenly distributed around the country.
Admittedly the direct support this particular study lends to PA theory is relatively weak (alternative accounts of the results are probably possible), but I mention it because of something the authors themselves somehow fail to point out: the results are clearly difficult to reconcile with common coding theories of memory (which are are usually associated with description theories of imagery such as those of Pylyshyn (1973, 1981, 2002a, 2002b, 2003b) or Hinton (1979)). On the other hand, the fact that when the subject was asked to visualize a map, different towns were recalled from those remembered without visualization, is very much what would be expected from the perspective of the rival Dual Coding theory of memory (Paivio, 1971, 1986, 1991, 2007). (As argued in ATOITOI, Dual Coding theory is equally consistent with both PA and pictorial theories of imagery.)
A somewhat different experiment involving the visualization of a map of France by neglect patients is reported by Bartolomeo, Bachoud-Lévi, Azouvi, & Chokron (2005). In this work, instead of being asked to report as many cities as they could from the visualized map in a set time, the subjects were instead asked to visualize an outline map and were then aurally presented with the names of French cities which they were asked to indicate (by pressing an appropriate button) as being either to the left or right of a north-south line passing through Paris (such a line in fact bisects the country more or less evenly). Unsurprisingly, unlike both normal control subjects and patients suffering from other neurological syndromes, the patients suffering from representational neglect performed significantly more poorly with town to the left of Paris as compared with those to the right. What is interesting, however, is that this difference showed up not in the accuracyof their responses – in general they were able to indicate correctly that towns to the left of Paris are indeed to the left – but in the speed of response. That is, it tended to take them significantly longer to make a determination about a town to the left of Paris than it did for a town to the right. The authors argue that this shows that the relevant information about the left side of the map is available in these subjects' imagery. Their problem is not that the left half of some inner picture is missing or somehow degraded. The information is there in their memories, but representational neglect sufferers have more difficulty in accessing it when it concerns things to the left. This again suggests that representational neglect is fundamentally a deficit in directing asttention (as perceptual neglect also appears to be), and thus that, as the authors point out, it is best explained by PA theory.
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It is notable that not only perceptual, but also representational neglect can be, at least partially and temporarily ameliorated by procedures that, at least on the face of things, manipulate purely external aspects of a patient's attention. Meador et al. (1987) found that simply having a patient physically turn his head and eyes toward the left while recalling an image of a familiar scene caused him to remember significantly (about 26%) more details from the left of the scene than he had been able to manage otherwise. Likewise, it has been found that squirting ice cold water in the patient's left ear (euphemistically called "vestibular stimulation"), which is a procedure well known to temporarily ameliorate the symptoms of perceptual neglect (presumably because it sharply draws attention to the left), also reduces the symptoms of perceptual neglect (Geminiani & Bottini, 1992; Rode & Perenin, 1994).
Further striking evidence of this kind comes from research on the use of prism adaptation as a treatment for neglect. This involves the use of goggles with prisms in them that bend the path of light coming into the eyes so that, initially, objects appear to be further to the right than they actually are, so that, for example, in reaching for an object that is actually in front, a subject would tend to reach well to the right of its real position. However, after quite a short time wearing such goggles, both normal subjects and neglect patients adapt to the goggles so that they are no longer confused by them and reach normally (and, presumably, objects directly in front will seem to be in front once again). When the goggles are removed from a normal subject, this adaptation persists for a while, so that the opposite of the original effect occurs, and objects now appear to be to the left of their actual position, and until the adaptation wears off and they re-adapt to normal vision without goggles, the subject will tend to reach to the left of the real position.
It has been found that when left unilateral neglect patients undergo this prism adaptation procedure the symptoms of neglect are considerably reduced (that is to say, they become much more likely to notice things actually to their left than they were before) and, remarkably, although the adaptation effect wears off quite quickly in a normal subject, the improvement in neglect symptoms can persist for hours or even days following just a few minutes of wearing the prism goggles (Rossetti et al. 1998; McIntosh et al., 2002; Frassinetti et al., 2002). The mechanisms behind this improvement are (so far as I can tell) still not fully understood, but there seems to be a consensus that they are largely or entirely due to an effect upon eye movements (Danckert & Ferber, 2006). Unlike normal people, neglect patients move their eyes to look towards the left far less often than they do so towards the right, but in the wake of adaptation to the goggles, their eye movement patterns are much more normal in this respect. This change in eye movement behavior seems to underlie the improvement in symptoms, and, in fact, it appears that only certain the aspects of the syndrome, presumably those caused by these external aspects of directional attention (by where one is inclined to point ones eye's) are actually ameliorated; other aspects of neglect do not get better. In particular, Ferber et al. (2003) found that a neglect patient who had undergone prism adaptation still failed to notice aspects of the left hand side of certain pictures, aspects that were obvious to normal subjects, even though the patient now moved his eyes to look toward the left side of the picture (which he had not done before undergoing prism adaptation). This suggests that prism adaptation acts only, or mainly, on external aspects of direction attention, such as control of eye movements (and probably other muscular behaviors, such as reaching) and does not significantly affect other more "internal" aspects of visual processing or memory that have also been damaged in neglect syndrome (see Danckert & Ferber, 2006 for discussion).
Despite this, however, the prism adaptation technique has been shown to ameliorate representational neglect just as it does perceptual neglect (Rode et al., 1998; Rode et al., 2001). That is to say, after prism adaptation, neglect patients can report significantly more features than they could before from the left of a scene recalled from memory (formed before their disease began) as a mental image.
This evidence, I think, effectively rules out the possibility that representational neglect might arise through the formation of a quasi-picture with its left side missing or damaged. If this were the case, how could mere mainipulations of attention, such as those described above, possibly ameliorate the neglect symptoms? In any case, the brain lesions of neglect patients are not in the retinotopically mapped areas that Kosslyn supposes to comprise the visual buffer, and although lesions of the retinotopic maps on one side of the brain may give rise to imagery deficits (Farah et al., 1992; Butter et al., 1997; but see Chatterjee & Southwood, 1995; Goldenberg et al., 1995; Goldenburg, 1998; Bartolomeo et al., 1998).), the symptoms (like the more obvious perceptual symptoms) seem to be quite distinct from those of neglect.
Meador et al., (1987) accept this point, but suggest instead that representational neglect may result from to a failure to pay attention to the left side of a (presumably undamaged) quasi-picture formed in the visual buffer. However, if this were the case, it would indicate that the problem lies with purely internal attentional processes, of the sort that Kosslyn envisages as functioning to move an "attention window" across the visual buffer (Kosslyn, 1994, 2005). It is not at all clear why manipulation of external attentional mechanisms, such as the manipulations of eye movements and head orientation described above, should affect these internal processes. Unlike quasi-pictorial theory, however, PA theory holds that both internal and external attentional mechanisms are centrally involved in (indeed, constitutive of) imagery. Thus, PA theory predicts what has in fact been found: procedures that help patients to orient their gaze more to the left should also increase their ability to vicariously re-enact the exploration of the left side of space, and thus to imagine it (because that re-enactment is the imaging). The fact that internal attentional mechanisms, which are probably also damaged in neglect, are not affected by these procedures, no doubt contributes to the fact that some symptoms of neglect linger on even after prism adaptation.
A study by Chokron et al., (2004) also seems, on the face of things, to point towards much same conclusion. In this case, external perceptual attention was not manipulated by the positive types of techniques discussed above (which, as it were, help to push attention toward the left) but by the negative technique of having the neglect patients close their eyes, so as to eliminate the attentional pull of right-side stimuli. Once again, the symptoms of representational neglect seemed to be diminished by this simple procedure, and on an earlier version of this page I argued that this experiment provided yet another piece of evidence favoring PA theory. However, a similar but more recent study (Rode et al., 2007) completely failed to find any evidence that blindfolding their subjects reduced the symptoms of representational neglect. Consideration of this latter experiment has led me to have second thoughts about how the Chokron et al., (2004) experiment ought to be interpreted, and I have thus moved further discussion of both these experiments to the section of this page, below, where I deal with alleged evidence or arguments against PA theory. To anticipate that discussion a little, I am now inclined to think that, when properly interpreted, neither of these results provide significant positive support for PA theory (although neither are inconsistent with it). Blindfolding (or closing the eyes) just may not be a relevant method of attentional manipulation.
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Kubovy (2003) points out certain aspects of the phenomenology of imagery, in particular: the intentionality of imagery (and perception); the fact that in imagery (unlike perception) we experience the intentional object as absent; and the fact that when we visually imagine a scene (such as imagining the front of one's house, in order to count how many windows it has) we necessarily imagine it as seen from a particular vantage point (pace Gibson, 1974). For Kubovy, this last fact implies that the "act of imagining contains the experiencer." He then argues as follows:
A theory of mental imagery must take account of these observations. But current theories do not. Regarding the fact that you knew where you stood when you counted windows, perhaps we need a theory of embodied imagination (perhaps in the direction proposed by Ballard et al. (1997)), according to which imagining a visible object is a partial reenactment of many of the bodily activities involved in perceiving something, not just an activation of the visual system. (Kubovy, 2003 p. 581)
It is quite clear from an earlier passage in Kubovy's article that the "current theories" of imagery that are being judged as inadequate here are the quasi-pictorial theory of Kosslyn and the description theory of Pylyshyn. Like so many, Kubovy appears to be unaware that there might be any alternative theories of imagery, such as PA theory, in the literature. (He seems unaware not only of my version of PA theory, but also of the several other earlier versions: see ATOITOI §2.3 for references. The article by Ballard et al. (1997) that he cites [and that I cite above] presents an active, embodied theory of perception, but does not address the issue of imagery at all.) Nevertheless, with its call for a "theory of embodied imagination," and its description of imagining as "a partial reenactment of many of the bodily activities involved in perceiving something," this is clearly an argument for some form of PA theory. (Note that ATOITOI endorsed, and drew significantly on, Ballard's earlier (1991) work on active perception.)
As it stands, Kubovy's very terse argument does not seem particularly compelling to me, but perhaps it might become more persuasive if it were fleshed out a little more. In any case, it is an argument for PA theory quite different from any that I have made (and, so far as I know, from any anyone else has made) and I am happy to find a fresh line of support.
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The main concern of Kosslyn & Thompson (2003) is to suggest an explanation for why it is that many neuroimaging studies of visual imagery have found evidence for elevated activity in primary visual cortex during imagery, many other such studies indicate that there is no such elevated activity. The explanation that they suggest seems quite plausible to me, and is broadly consistent with the commitments of Perceptual Activity (PA) theory as outlined in ATOITOI (Thomas, 1999). However, although it does not really vitiate their main argument, in the course of their exposition they seriously misrepresent the nature of PA theory.
Their argument involves the classification of imagery theories into two broad classes, which they call "perceptual anticipation theories" and "propositional theories" respectively. The latter class, of course, corresponds to the well known "propositional" or "description" theory associated mainly with Pylyshyn (see Thomas, 2003, 2008). However, the class of "perceptual anticipation theories" is supposed to encompass both quasi-pictorial theories of the sort long championed by Kosslyn (e.g., 1980, 1994; also Miyashita, 1995), and PA theories (they cite both ATOITOI (Thomas, 1999) and Neisser's (1976) version of PA theory, which was what originally inspired my interest in this way of thinking about imagery). I think they make a good case that "perceptual anticipation theories" (of either subtype) are able to give a much more plausible account of the range of neuroimaging data than are "propositional" theories. My objection is to their account of what "perceptual anticipation theories" hold:
According to perceptual anticipation theory, mental images arise when one anticipates perceiving an object or scene so strongly that a depictive representation of the stimulus is created in early visual cortex (cf. Kosslyn, 1994; McCrone, 2001; Miyashita, 1995; Neisser, 1976; Thomas, 1999). According to this class of theories, visual long-term memories of shapes are stored in an abstract code in the inferior temporal lobes. . . . According to perceptual anticipation theory, the local geometry of shapes is only implicit in the longterm memory (LTM) representation, and is made explicit by generating patterns of activation in topographically organized early visual cortex. (Kosslyn & Thompson, 2003 pp.724-5)
It is not wrong to characterize PA theory as being about anticipation; indeed, the notion of perceptual anticipation plays a particularly central role in Neisser's (1976) version of the theory. [I am not sure how relevant anticipation really is to quasi-pictorial theory, however. I do not recall anything like anticipation playing any very large role in Kosslyn's earlier theoretical work on imagery.] However, the point about depictive representation, is completely at odds with a central commitment of PA theory (certainly as I understand it, and I think as Neisser understood it too (see Neisser, 1979)). PA theory explicitly and emphatically rejects the notion that mental images are depictive representations or spatial patterns of neural activation (whether in early, topographically organized visual cortex or anywhere else). Although it is quite possible that such activation patterns, isomorphic to the spatial structure of whatever is being imagined, may sometimes arise in topographically organized cortex during imagery, according to PA theory they are not to be identified with the mental image, nor are they the cause of the experience of imagery. Rather, they are collateral effects of the covert enactment of exploratory perceptual behavior. It is this enactment, and not any pattern of brain activation that may arise from it, that constitutes imagery, and gives rise to quasi-perceptual experience. (I have no strong views about where in the brain the information that guides this enactment, what Neisser and I call the schema, is stored long-term. Perhaps, as Kosslyn & Thompson imply, it is all in the inferior temporal lobes, although I am inclined to suspect that it is distributed much more widely.)
Although the Kosslyn & Thompson paper is well worth reading for its plausible explanation of the discrepancies between the different neuroimaging studies, and for the good case it makes for thinking that both quasi-pictorial and PA theories can account for these findings better than "propositional" theory can, please do not be misled by its quite false account of PA theory itself.
Dartnall provides a brief account of PA theory that, although not necessarily incorrect if carefully interpreted, may be misleading. He says that, according to ATOITOI, "rather than storing inner analogs of the external world, we generate them by running our perceptual abilities offline" (Dartnall, 2005 p. 139, emphasis in original). What concerns me, here, is the metaphor of generation (which he repeats twice more). Perhaps it is fair to say that according to PA theory, the process of running our perceptual abilities offline generates imagery experience (i.e., quasi-perceptual experience). However, it would be very natural to read Dartnall's words as implying the generation of some sort of representational product (an inner analog, an activation pattern in a topographical map, or some other sort of representational token), and that this product is the mental image (which then, perhaps, causes the quasi-perceptual experience). For PA theory, however, there is no such product. Imagery is the running of our perceptual abilities offline, not something generated by it.
Dartnall's mistake (if it is one) probably does not affect the cogency of the overall argument of his paper. I just want to forestall the possibility of his readers being misled as to the real commitments of PA theory.
UPDATE June 5, 2009: In Dartnall's more recent article (2007) it is apparent that he does not misunderstand PA theory in the way that it appeared he might have done in 2005. However, although he represents PA theory quite accurately in the newer article, he also ventures a brief criticism of it. I rebut his criticism here.
According to Eshach & Schwartz (2006 p.740), "Thomas (1999) asserts that ‘Visual imagery involves having entities, in the head or in the mind, which are like, or functionally equivalent to inner pictures.’" That is, indeed, a quotation from ATOITOI, but I do not assert it, I deny it! It is part of my explanation of picture theory, which is, of course, a view that I am actually at some pains to refute. By taking this sentence out of context, Eshach & Schwartz misrepresent me as believing the opposite of what I in fact believe.
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Kosslyn, Thompson, Sukel & Alpert (2005) describe an experiment in which subjects were asked to form, and memorize, mental images of (fairly simple) visual shapes, either on the basis of a verbal description, or else by being shown segments of the shape which they then had to mentally "glue" together to form an image of the whole. After memorizing the imagined shapes, the subjects were placed in a PET scanner and asked to recall the patterns and imagine them projected on a screen in front of them. An X then appeared on the screen and the subjects had to press a foot pedal to indicate whether or not the X fell onto the shape.
In the event, the PET scan results showed only relatively minor differences in brain activation, as reflected in differences in the pattern of regional cerebral blood flow (rCBF, which is what a PET scan actually measures) between the two conditions (original image formed from verbal description vs. formed by segment "gluing"):
Although we did find evidence that distinct processing underlies the two ways of generating images, the differences between the conditions were subtle. More striking was the evidence for many common processes. Specifically, in both conditions we detected a nearly identical swath of bilateral activation spreading from the middle and inferior frontal lobes back through the middle and superior portions of the temporal cortex to the posterior parietal lobe .(p. 50)
Kosslyn et al. seem at a loss to explain many of the specifics of these differences, but they nevertheless spend some time arguing that the "subtle" differences they observed support their original experimental hypothesis (derived from a particular elaboration of quasi-pictorial theory) that there should be two importantly different types of image generation invoked by the two conditions. (It is notable that they do not claim to have observed extensive occipital activation in either condition, although it is the occipital lobe, where the retinotopic maps are located, that Kosslyn (1994, 2005) holds to be the crucial area where all visual mental images are actually formed. It is also fairly clear that they hoped and expected to see a much greater difference in rCBF between the two conditions than that which they actually observed.)
They then go on to argue, however, that the lack of "gross differences" between the conditions is inconsistent with Perceptual Activity theory, as presented in ATOITOI:
Indeed, given the gross differences between the processes that would be used in "perceptual exploration" of the two sorts of stimuli, perceptual activity theory would have led us to expect large differences between the two conditions, but we found only subtle differences in rCBF between them. The results rather suggest that the participants were able to generate the same image representation in the two conditions and could use it in the same ways once it was generated. (P. 51)
Not only do they seem to trying to have it both ways with this argument (arguing that the "subtle" difference in rCBF between conditions confirms the quasi-pictorialist prediction of a difference, but that the lack of a "gross" difference undermines PA theory), it also seems to be based on an almost perverse misunderstanding of PA theory. PA theory holds that we experience visual imagery of some visual object or pattern when we partially enact (or "go through the motions of") the processes of perceptual exploration that we would carry out if we were actually viewing such an object or pattern. As both conditions in this experiment led the subjects to visualize the same actual shapes, and as the PET scan was given not when the images were originally formed, but when the previously formed images were later recalled, PA theory, quite as much as Kosslyn's quasi-pictorial theory (or indeed, any other theory of which I am aware), predicts that the same shape image will be recalled by the subjects in the PET scanner, regardless of any differences attending its original, earlier formation. Although the stimuli (verbal instructions, and pictures of parts) were indeed "grossly" different in the two conditions, the images formed, and later recalled in the scanner, were, by design, the same, so it should be no surprise that they activated the brain in essentially the same way.
Kosslyn et al. appear to think that PA theory holds that experiencing a mental image involves re-enacting the initial image formation process ("mimicking encoding," as they put it), whatever that initial formation process may have been. They are mistaken. In the paradigmatic case where an image arises as a direct memory of an actual former perceptual experience, what happens might indeed be described as re-enactment of original encoding (at least if we set aside any qualms a PA theorist might feel about describing the percetual process as an "encoding"). However, quite regardless of any elaborate brain scanning experiments, this way of expressing the central claim of PA theory would be bound to fail as soon as we tried to apply it to other common types of imagery, such as "imagination images" (images of things never actually seen, like the images formed in this experiment). In order to cover such cases, PA theory is expressed in subjunctive, counterfactual terms: We experience imagery of, say, a unicorn when we enact (some of) the perceptual, exploratory process that we would perform if (contrary to the actual fact) we were looking at a unicorn, there in front of us. Of course, to be able to do that we need to have some knowledge of what unicorns are supposed to look like, but it does not matter how that knowledge was aquired.
In fact, in this respect, PA theory and quasi-pictorial theory (and probably description theory too) are in the same boat. Both need to use this subjunctive strategy in order to account for imagination imagery. Kosslyn does not actually have a great deal to say about the difference between memory images and imagination images, but presumably he would say that when we form a memory image of something we recreate a state of the visual buffer that is similar to the state it was in when we were actually seeing and "encoding" that thing, but that when we have an imagination image we recreate a state of the visual buffer similar to the state it would have been in if we had actually seen the thing. If this raises problems for either theory, it raises them equally for both (and probably for any remotely plausible imagery theory that might be devised). (Of course, I am putting words in Kosslyn's mouth here, but I cannot imagine how else he might address this issue, or, indeed, why he might want to address it otherwise.)
PA theory as such, then, does not predict any differences in rCBF between the two conditions of this experiment. The fact that there are, nevertheless, “subtle” differences is, however, readily explained. After all, there is bound to be more going on the subjects' heads than just the relevant image formation (however one may conceive that to work), and, in particular, it seems plausible that some memory might incidentally be evoked of the rather contrived procedures that they fairly recently went through in originally forming the images. That could well account for the systematic, if subtle, rCBF differences between conditions. Indeed, because of this very real possibility, I am not convinced that the results provide more than very weak support for the authors' hypothesis about two types of quasi-pictorial image generation. Be that as it may, they are certainly in no way discomforting for PA theory.
Rode et al. (2007) have recently reported an experiment on people suffering from representational neglect that, so they claim, contradicts the standard interpretation of neglect as arising primarily from a defect in attention, and this raises a problem for the view that PA theory provides the best account of the representational neglect syndrome. [See above, or alternatively here, for a brief explanation of neglect syndrome and its relevance to PA theory]. Subjects (some neglect patients and some normal control subjects) were asked to visualize a map of France, and then to report as many as possible of the French towns that they could "see" on the map. (We are to presume that all subjects had a good knowledge of French geography, at least before the onset of their disease.) All the subjects tried this task under two conditions, once with their eyes open, and once whilst blindfolded. The rationale for this was that, according to the influential view of Posner et al., (1984: see also Bartolomeo & Chokron, 2002b) the symptoms of unilateral neglect arise mainly from a deficit in the ability to disengage attention from stimuli to the right (rather than a deficit either in moving the attentional focus to the left, or in fixing it upon stimuli there). It was hypothesized, therefore, that, with a blindfold concealing any potentially attention grabbing and holding stimuli to the right, the symptoms of representational neglect should be diminished.
As had been found in earlier experiments (e.g., Rode et al., 1998, 2001, 2004; Bartolomeo et al., 2005), in the eyes open condition, the neglect patients did much more poorly with towns from the left of the imaginary map (i.e., western France) than from the right (eastern). The normal, control subjects recalled towns from west and east more or less equally well. When the subjects were blindfolded, the control subjects did even better, recalling more towns from both sides. However, contrary to expectations, blindfolding did not seem to affect the performance of the neglect patients at all, and they recalled, on average, no more towns to the left (or, indeed, the right) than they had with their eyes open.
Rode et al. (2007) argue that this negative result shows that representational neglect is not the result of a failure to direct sufficient attention to, and thus fully explore, the left-representing side of a mental image, but rather that "Representational neglect may represent a failure to generate the left side of mental images" (p. 432). What they mainly appear to believe is that they have refuted is the theory (embraced by Meador et al. (1987)) that neglect patients generate essentially normal quasi-pictorial images, but then fail to pay attention to the left sides of them. However, Rode et al. also seem to associate this view (quite wrongly) with PA theory (they cite both ATOITOI and Bartolomeo & Chokron (2002a)), and they draw the broader conclusion that their results imply that "a working memory/image generation defect account of representational neglect" is to be preferred over "a directed attention defect account" (p. 436).
Of course, I am quite happy to see the quasi-pictorialist view of Meador et al. controverted. However, given that PA theory explains imagery as, at root, a result of the active direction of attention, any account of representational neglect based upon PA theory is bound to be some sort of "directed attention defect account." Thus, despite the fact that Rode et al. seem to have misunderstood PA theory, their broader conclusion nevertheless raises a challenge to it.
However, this broader conclusion is in direct conflict with much larger mass
of evidence, from a variety of experiments, that points in precisely
the opposite direction, indicating that a defect of directed attention is very
much involved in representational (as well as perceptual) neglect, and suggesting
that representational neglect cannot be due to the formation of one-sidedly
damaged image representations [some of this evidence (much of it due to the work
of Rode and his collaborators!) is reviewed above]. Most strikingly,
al. (2004; see also Bartolomeo & Chokron, 2001b) apparently found just
the opposite to the result reported by Rode et al. (2007): they claim
that symptoms of representational neglect are often ameliorated when
the subjects were blindfolded (but see below). So far as I can see, there is
no reason for a PA theorist to deny that (as Danckert & Ferber (2006) argue)
some more general deficit of working memory acts alongside the more specific
directional deficits of attention to produce the full range of neglect symptoms.
However, it is hardly justifiable to completely throw out the well-supported
hypothesis that attentional deficit plays a major role in causing neglect on
the basis of this one, isolated negative result (even taken together with the
couple of other studies that Rode et al. (2007) cite in support of their
•[This is especially true when we consider that, even on their own anti-attentional intrepretation, aspects of Rode et al.'s (2007) findings remain puzzling. In particular, they do not seem to have an explanation (although no more do I) of why the blindfolding produced no improvement whatsoever in the neglect patients' performance, on either the left or the right side, even though it improved the performance of the normal control subjects on both sides of the map.]
I do not truly know why Rode et al. (2007) did not find evidence that
blindfolding can ameliorate representational neglect, but I will offer some speculations
that, I believe, are consistent with the evidence. Perhaps this result does not
show that defective attentional processes are not the cause of representational
neglect, but rather, that blindfolding is not a very effective way to manipulate
the relevant attentional processes. This might be the case if representational
neglect arises primarily from damage to mechanisms that steer attention to the
left, rather than (as the experimenters assume) damage to those that disengage
attention from stimuli to the right. The former, indeed, is what we might expect
on the basis of PA imagery theory, which relies upon the notion
of active, top-down, schema-driven, steering of attention (endogenous attention),
but gives no role to the mechanisms that enable the bottom-up capturing of perceptual
attention by external stimuli that "catch the eye" (exogenous attention).
(Both of these sorts of directional attentional mechanisms undoubtedly exist,
but exogenous mechanisms are probably not very relevant to imagery.
Imagery, on anyone's account, is endogeously generated.)
•[Please note that this distinction between exogenous and endogenous attention is not the same as the distinction between external and internal aspects of attention that I made earlier. The internal/external distinction marks the difference between different types of attentional processes, namely those that operate entirely within the brain and those that also involve musculature and bodily movements. The endogenous/exogenous distinction has to do with whether the immediate cause of some attentional act comes from within the organism itself, or is some thing or event in the environment. For example, external attentional mechanisms, such as eye saccades, can clearly be caused either exogenously (by something catching one's eye) or endogenously (as part of schema-driven perceptual exploration, or even by a conscious decision to look in a certain direction).]
•[Actually, in the first century B.C., Lucretius (and presumably therefore Epicurus, whose doctrines he is expounding) does seem to have believed that imagery has an exogenous origin. Vision, he tells us, occurs when films of atoms thrown off the surfaces of material objects, enter the eyes. However, some of these films may float around unperceived for a long time, and during that time they may become randomly intermingled with one another. Also they will decay and become so attenuated that they can enter our bodies through pores even when our eyes are shut in sleep. Thus they produce dreams and visions of centaurs or other chimeric monsters, or of men long dead, or whatever (Lucretius, De Rerum Naturae, Book IV lines 722-748). Of course, not only does this sort of theory fail to account for the fact that waking imagery is, to a very considerable extent, subject to the will (Thomas, 2008 §§ 1.2-1.3; McGinn, 2004), but it is incompatible with even a rudimentary modern (or even medieval) understanding of optics.]
However, there do appear to be good reasons to believe that defects of exogenous attentional processes (viz. the capture and holding of attention by right-side stimuli) play a very significant, even predominant, role in perceptual unilateral neglect (Posner et al., 1984; Bartolomeo & Chokron, 2002b; Bartolomeo, 2007). Thus it has seemed reasonable to suppose that they will also be relevant to representational neglect. Perhaps, however, that supposition is mistaken.
I would like to suggest that the brain lesions that cause neglect may affect either exogenous or endogenous attentional mechanisms singly, or both together. Neglect patients, after all, vary widely both in the severity and in the precise detailed character of their symptoms, and it remains controversial amongst neurologists precisely where in the brain's right hemisphere lesions need to occur in order to produce left neglect. In part this is because different neglect patients seem to have lesions in rather different places, and many have quite large lesions, that seem likely to disrupting more than one function (Bartolomeo, 2007).
Consider, now, patients who have suffered damage to both exogenous and endogenous perceptual mechanisms. During perception the problems caused by the defective exogenous mechanisms may well predominate over, and, indeed, largely swamp, any effects due to damage to endogenous attention. After all, it is not going to matter if you are not very good at steering your attention to the left if it has already been captured and held by right-side stimuli. This supposition seems to be consistent with the evidence discussed by Bartolomeo & Chokron (2002b; Bartolomeo, 2007): although they stress the predominant role of exogenous attentional processes in perceptual neglect, they are careful to note that endogenous attention is also generally affected in these patients.
However, when people focus on their mental imagery, they normally need to temporarily disengage much of their attention from their immediate perceptual environment. This is borne out not only by introspection (McGinn, 2004), but also by a mass of empirical evidence showing that imagery and perception (especially in the same sensory modality) compete for attentional resources (e.g., Brooks, 1967, 1968; Atwood, 1971; Segal & Fusella, 1971; Baddeley et al., 1975; Janssen, 1976a, 1976b; Eddy & Glass, 1981; Logie & Baddeley, 1990; De Beni & Moè, 2003). I see no reason to believe that neglect patients are impaired in this general ability to "zone out" when they want (or are required by an experimenter) to pay attention to their imaginings instead of to their surroundings. (If they were so impaired, that ought to give rise to a general, bilateral difficulty in evoking eyes-open imagery, which is not what is seen.) That being so, defects of exogenous attention, even without blindfolding, may be largely irrelevant to the aetiology of representational neglect. When someone is imagining, their attention is disengaged from the outside world and, thereby, less than usually susceptible to exogenous capture. [Of course, when the eyes are open a very salient stimulus, such as a sudden movement, can engage exogenous attention, and disrupt imagery, even for normal subjects, but this was not what was happening in the Rode et al. (2007) experiment. The experimenters assumed that, for neglect patients, just the ordinary, static features of the right side of the environment would be enough to grab and hold the attention, even when the subjects were trying to focus on their imagery.] Representational neglect might then be a result entirely (or mainly) of damage to endogenous attention (as PA theory implies), even if this plays only a subordinate role in the aetiology of the same subjects' perceptual neglect. If this is so, blindfolding, which (unlike prism adaptation and other techniques described above) affects only exogenous attention, will not affect the condition.
To speculate further along these lines, if only their exogenous mechanisms are damaged we might expect that patients would show symptoms of perceptual, but not representational, neglect. This is, in fact, the pattern of symptoms shown by the majority of neglect patients (Bartolomeo et al., 1994; Bartolomeo, 2007; Bourlon et al., in press)). If, on the other hand, only the endogenous mechanisms are damaged, we might expect to see definite representational neglect accompanied by relatively mild perceptual neglect. Even though I am inclined to think that endogenous attention is important in normal perception, so that this damage to endogenous attention should produce some perceptual neglect, having normal exogenous attention, with things to the left catching the eye just as much as normal, might be sufficient to mask the more obvious symptoms, giving rise the occasional claims to have found patients suffering from representational but not perceptual neglect (Guariglia et al., 1993; Bartolomeo et al., 1994; Beschin et al., 1997; Coslett, 1997; Peru & Zapparoli, 1999; Ortigue et al., 2001).
But if exogenous attention is largely irrelevant to representational neglect, how then are we to explain the findings of Chokron et al. (2004)? These authors compared drawings that neglect patients made (from memory) with their eyes open with ones made with their eyes closed. Unsurprisingly, the eyes-closed drawings were inferior in most respects, showing less detail and less accuracy. However, for several patients, their eyes-closed drawings showed few if any signs of left neglect, whereas the equivalents made with the eyes open omitted much or most of the detail on the left-hand side of the object.
The authors interpret this result as further evidence (adding to that discussed above) that representational neglect can be ameliorated by the manipulation of attention, and I was formerly inclined to accept this interpretation, and to argue that (as the authors themselves imply) the finding should thereby be added to the roster of evidence favoring PA theory. However, this interpretation is thrown into question by the fact that Rode et al. (2007), found that blindfolding had no effect whatsoever on representational neglect (as just discussed). As we have seen, these latter results can be reconciled with PA theory if we postulate that blindfolding (or closing the eyes), because it manipulates only exogenous attention, is irrelevant to the endogenous attentional processes responsible for imagery.
I believe the results of Chokron et al. (2004) can be reconciled with this assumption. Indeed, I am now inclined to believe that their experiment tells us nothing directly about imagery, because their findings can be explained entirely in terms of purely perceptual neglect. Although they apparently believed that their experiment dealt with imagery and representational neglect, this may have been merely because they implicitly assumed that the drawing-from-memory task necessarily involved forming a mental image, which could then be simply copied to the paper. If that were so, the missing left side of the eyes-open drawings would be evidence for representational neglect because it suggests that the patients were copying from an image with its left side missing or unattended. But do even normal subjects really draw from memory by first forming a complete mental image and then just copying this exemplar? I doubt it. In any case, neglect patients typically leave out the left side from their drawings even when the exemplar they are copying from is a picture that is right there in front of them, so we certainly do not need to postulate a defective mental image in order to account for the defective drawing. We do not even need to appeal to defective attention to an image exemplar. It is not at all difficult to account for the neglect shown in the drawings made in the eyes-open condition entirely in terms perceptual neglect and the capture of exogenous attention. After all, as soon as a patient sullies their paper with any detail to the right, there is something there to hold on to their attention and thus hinder them from redirecting it toward the left.
But, apart from the incomplete eyes-open drawings (entirely explicable by perceptual, exogenous effects), Chokron et al. (2004) appear to have had no grounds for believing that their subjects actually did suffer from representational neglect (remember, most neglect patients suffer only from perceptual, and not from representational neglect). The subjects had been assessed with several standard tests of perceptual neglect, but there is no sign in the published article that any of them had been tested for specifically representational neglect, using tasks that clearly rely upon imagery, such as reporting the details of a familiar remembered scene (the Piazza Del Duomo test of Bisiach & Luzzatti (1978)), or the Map of France Test developed and used by Rode. It thus seems entirely possible that those subjects who correctly drew both sides of their object when their eyes were closed (by no means all the subjects) were, like the majority of neglect patients, not suffering from representational neglect at all. Although their exogenous attentional mechanisms were damaged, the endogenous (schema driven) aspect of their attention may well have been spared, leaving their imagery, and thus their eyes-closed drawing ability, essentially normal. For them, closing their eyes did not fix their representational neglect because it did not need fixing. Rather, their unimpaired mental imagery abilities were able to take proper control of the drawing process once the exogenous perceptual effects that had been interfering with their eyes-open drawing were nullified.
On the other hand, the subjects studied by Rode et al. (2007) were selected using the Map of France test, and thus were all suffering from genuine representational neglect. This was not ameliorated by blindfolding because, as argued above, blindfolding acts only on exogenous attentional processes, and not the endogenous ones responsible for imagery.
This leads me to make a tentative empirical prediction: Although some (probably most) neglect patients will do better at putting in the left side of their drawings when they are blindfolded, those who can independently be shown to suffer from representational neglect (by something like the Piazza Del Duomo or the Map of France test) will show little if any improvement. (Note that by no means all the patients tested by Chokron et al. (2004) showed the improvement.)
If the above speculations are on the right track, the upshot is that the findings of Rode et al. (2007) are not, despite superficial appearances, inconsistent with PA theory. However, it also follows that, (again despite some superficial appearances) the findings of Chokron et al. (1994), although they are also certainly consistent with PA theory, do not provide positive support for it.
If the speculations prove to be mistaken, I would return to my contention that, however they are to be explained, the findings of Rode et al. (2007) are an isolated negative result quite insufficient to rebut the mass of positive evidence suggesting that representational neglect is caused (at least in very large part) by attentional deficit.
Dartnall (2007 p. 264) provides quite a good brief account of PA theory, and then, in an endnote (note 5, p. 279), ventures a brief criticism, that was apparently suggested to him by Kosslyn. He, quite rightly, points out that according to PA theory there is no distinction to be drawn between a mental image and the experience of a mental image. The term "mental image" is just a name for a particular sort of experience. As the pioneering PA theorist Hebb (1968 p. 476-478) pointed out, when we imagine an object, we have an experience of an apparent or imagined object, not an experience of an image of the object. This may be contrasted with the perspective of Kosslyn's quasi-pictorial theory, according to which a mental image is a quasi-pictorial representation created in the brain, and is distinct from any experience that (according to circumstances) it might or might not cause.
Dartnall then points out that according to Kosslyn, "people can reliably rate the vividness of the image itself (not the vividness of the object) and estimate the visual angle within which objects can appear (treating the image itself as an object to be evaluated). In both cases, he [Kosslyn] said that it is the image and not the object that is being addressed," (Dartnall, 2007 n.5 p. 279). This is his entire argument against PA theory.
Now it is certainly true that people find it quite natural to make judgements about the relative vividness of their imagery, and it would certainly be very odd to claim that, in doing so, they are really making judgements about the vividness of objects (let alone objects that, being imaginary, are not there, and may not even exist anywhere). However, the PA theorist does not make any such peculiar claim. Remember that, according to PA theory, mental images are experiences. There is nothing at all odd about saying that an experience might be more or less vivid (very arguably, only experiences can be more or less vivid: it is not at all obvious to me that it makes any sense to say that a mental representation is vivid, as opposed to being vividly experienced). This "argument" from vividness, then, depends upon a subtle misconstrual of PA theory, and has no force against the theory as it actually is.[It is actually quite controversial whether people can reliably and meaningfully "rate" the vividness of their imagery (Thomas, 2009 pp. 449-450), but that is not material to the current issue. People certainly do make judgements, reliable or otherwise, about the matter.]
The claim that people can estimate the visual angle of their imagery (and that, in doing so, they are making a judgement about an inner representation rather than about an experience or about the things being imagined) is not at all self evident. Indeed, Colin McGinn (2004 pp. 22ff) argues to the direct contrary. For McGinn, one of the crucial differences between visual perception and visual imagery is that what we can perceive is subject to the limitations of the eyes' visual angle, whereas what we can visually imagine is not: "There is no boundary to the image imposed by the constraints of optics and retinal anatomy" (2004 p. 22).
It would thus, presumably, surprise McGinn to hear that Kosslyn and his collaborators have actually done experiments to measure "the visual angle of the mind's eye," and have found it to be not very different from the visual angle of the bodily eyes (Kosslyn, 1978; see also: Finke & Kosslyn, 1980; Finke & Kurtzman, 1981). Has the armchair philosopher, then, been confounded by the experimental scientist? Not at all. The experiments in question in fact depend upon asking people to imagine themselves staring fixedly ahead at some scene, not turning their eyes, heads, or bodies as we normally do, and then asking them about how much they can "see" in their mental image of the scene under those circumstances. They were not asked to look within and examining images, rather, they were asked to imagine what they would be able to see in certain circumstances. For example, in one experiment they were asked to imagine looking at the side of a bus, from some distance away, and then walking towards it until the image "overflowed" so that they could no longer imagine taking in the whole bus in a single glance. The "visual angle of the mind's eye" could then be calculated from the known length of a bus and the distance that the subjects estimated themselves to be from the bus when its image began to overflow their visual field. As such, these experiments constitute virtually a textbook example of how results in psychology can sometimes say much more about the demand characteristics of the experiment, and thus the preconceptions that the experimenter built into the experimental design, than about any phenomenon allegedly being investigated (Orne, 1962; Rossnow, 2002; Thomas, 2008 §4.3 sup. 2). If the subjects had not reported that they could see in their images about as much as they would have been able to see if they had actually been looking, they would have been failing to follow the experimental instructions. [Other better known experimental findings by Kosslyn, particularly his discovery of what he calls "mental scanning" (see Thomas, 2008 §4.3), have also been criticized as being contaminated by the effects of experimental demand characteristics, and Kosslyn has vigorously defended this work from these criticisms, both with arguments and with more carefully designed experiments. In my view this defense is at least partially successful. His work on the "visual angle" of imagery, however, has not receive either the same level of scrutiny or of defense. So far as I can see his arguments in defense of mental scanning cannot be straightforwardly extrapolated to cover the "visual angle" experiments. In this latter case the effects of experimental demand are far more direct and obvious.]
My point here is not that Kosslyn is wrong and McGinn is right, or vice versa – in fact I think they both, in their way, misconceive the matter – but that, contrary to the implicit claim made by Dartnall, Kosslyn's experiments provide no unequivocal evidence (indeed, scarcely any prima facie evidence) that people can report on the characteristics of their inner quasi-pictorial representations as opposed to the characteristics of their imaginal experience. The fact that Kosslyn interprets his subjects' reports in the former way is a fact about Kosslyn's theoretical views, not a fact about the phenomena under study. There is nothing to prevent the PA theorist from interpreting the reports as descriptions of the things that are being imagined (or of how much of the things, such as the bus, are being imagined).
In the body of his article, despite having shown himself to be aware of the existence of both PA theory and description theory, Dartnall uncritically embraces quasi-pictorialism, and Kosslyn's interpretations of the relevant evidence, and completely ignores the many serious objections to it raised by Pylyshyn, myself, and others (e.g., Pylyshyn, 1973, 1978, 1981, 2002a,b, 2003 a,b; Thomas, 1999, 2008 §4.4 sup., 2009; Hinton, 1979; Slezak, 1991, 1995; Bartolomeo, 2002; Bennett & Hacker, 2003; McGinn, 2004 ch 5; Dulin et al., 2008).
Back to table
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