imaginationimaginationPerceptual Systems: Five+, One, or Many?

by Nigel J.T. Thomas
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California State University, Los Angeles.
Published in Behavioral and Brain Sciences, 24, 241-242 (2001).
© Cambridge University Press

This is a commentary on the target article "On Specification and the Senses," by Thomas A. Stoffregen and Benoît G. Bardy: Behavioral and Brain Sciences, 24, 195-261 (2001).


The target article's value lies not in its defence of specification, or the "global array" concept, but in its challenge to the paradigm of 5+ senses, and its examples of multiple receptor types cooperatively participating in specific information pick-up tasks. Rather than analysing our perceptual endowment into 5+ senses, it is more revealing to type perceptual systems according to task.

Stoffregen and Bardy can and should not hope to persuade us that the traditional five senses (plus, presumably, a few more, like vestibular sense, proprioception, etc.) just do not exist. Clearly for many purposes it is valid and useful to think of the senses in this way. The circularities they point to, arising from defining the senses in terms of receptor types or energies transduced, do not seem to be vicious.

However, their argument opens up a very important pragmatic and heuristic question that has received almost no previous attention: Is regarding our sensory endowment as consisting of 5+ independent perceptual systems the most useful and perspicuous way to think about the mechanisms of ecological perception? Here they make a good case for a negative answer, challenging deeply entrenched and, up to now, virtually unexamined assumptions. They show that certain important, real-world perceptual tasks require the coordinated deployment of more than one type of receptor. If we think of the senses as 5+ channels, our attention is diverted from such cases (which may well be the rule rather than the exception), and, even if we do notice them, the separate senses framework leads us toward positing unnecessarily complex and conceptually suspect inferential or computational theoretical accounts of them.

But if 5+ senses is not the most useful picture of things, does it follow that the heuristically best alternative is to think of the perceptual environment as a single global array, presumably to be perceived by a single global perceptual system? Stoffregen and Bardy apparently think that if the ambient energies available to our perceptual system do not unambiguously specify what is really out there then we must be doomed to perceive the world only "indirectly", our experience mediated through representations and inferential processes. They are thus led to the notion of the global array in the hope of finding an information source sufficiently rich to ensure specification. But unless we understand "direct perception" to mean "invariably veridical perception" (in which case perception certainly is not direct) it simply does not follow that directness requires unambiguous specification. In fact, we do not ultimately rely on mere perception to tell us what is really out there, but on science, which certainly involves inferential processes. Specification is a red-herring, and the theory of the global array is a (probably inadequate) solution to a non-problem.

Of course, the global array undoubtedly exists, and our sensory endowment as a whole undoubtedly exists too. Thus, (granting the general framework of Gibsonian direct perception theory) it will inevitably be true to describe any perceptual episode as the pick-up of an invariant of the global array by the global perceptual system. But this is not to say very much. In fact, it is surely the case that many instances of perceptual information pick-up do make use of only one receptor type, and even the examples given by Stoffregen and Bardy each involve only a small subset of the receptor types we have. To insist on treating ambient arrays and perceptual systems only as "global" wholes would be to obscure this point, and threatens to be just as misleading as the paradigm of 5+ independent channels.

A more revealing analysis might be to type perceptual systems in terms of the specific sorts of environmental information that they gather. The target article's examples suggest that, instead of saying "this is an instance of vision, this of audition," etc., we might do better to say things like "this is an instance of perceiving that your conveyance has come to a stop", "this is an instance of perceiving that a surface affords sitting," and so on. In this vein, we can think of our sensory endowment as comprised of a number - probably quite a large number - of perceptual instruments, each specialized for the pick up of particular sorts of environmental information, and actively deployed as and when that information is needed for the guidance of behavior. A perceptual instrument (alternatively a "smart perceptual mechanism" [Runeson, 1977], or "smart sensor" [Burt, 1988]) is a complex of anatomical and cognitive structures that is capable of actively testing for the presence or amplitude of some specific type of environmental property. It consists not only of receptors, but also of efferent systems that "tune" them, the musculature that orients them and moves them so as to sample the ambient energy arrays appropriately, and the neural structures and algorithms that control these "tunings" and movements, and orchestrate appropriate responses to the receptor outputs (Thomas, 1999). I take it that by switching neural algorithms, and thus the way in which receptors are deployed, our fairly limited array of receptor types can be recruited to do a large number of different perceptual jobs, or putting it another way, to form parts of a large number of perceptual instruments (cf. Ballard [1991] on "sensor fission"). We do not so much have 5+ general purpose senses as a large array of anatomically overlapping, specialized perceptual instruments, a capacious "box of tricks" (Ramachandran, 1990).

From the entrenched standpoint of the orthodox paradigm of 5+ senses, this theoretical perspective must seem strange and counterintuitive, but Stoffregen and Bardy throw that paradigm into deep question. Furthermore, they direct our attention toward the significant, but previously under-explored possibility that many perceptual instruments may cooperatively employ more than one receptor type. The real and considerable value of the target article lies, I think, in these challenges to entrenched orthodoxy, rather than in the unnecessary and heuristically rather unhelpful notion of the "global array".


Ballard, D. H. (1991). Animate vision. Artificial Intelligence 48:57-86.

Burt, P. J. (1988). "Smart sensing" in machine vision. In: Machine vision H. Freeman (Ed.) (pp. 1-30). San Diego, CA: Academic Press.

Ramachandran, V. S. (1990). Visual perception in people and machines. In A. Blake & T. Troscianko (Eds.), AI and the Eye (pp. 21-77). Chichester, U.K.: Wiley.

Runeson, S. (1977). On the possibility of "smart" perceptual mechanisms. Scandinavian Journal of Psychology 18:172-179.

Thomas, N. J. T. (1999). Are theories of imagery theories of imagination? An active perception approach to conscious mental content. Cognitive Science 23:207-245.

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