Are Theories of Imagery Theories of Imagination? An Active Perception Approach to Conscious Mental Content
Nigel J.T. Thomas
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Source: http://cogprints.org/5018/1/im-im-cp.htm
Considering the possibility of perceptual tests directed at the retinotopic areas should also help to make it clear how the PA view of imagery and perception fits with theories of perceptual consciousness of the sort proposed by Marcel (1983) and Gray (1995). These authors assemble a considerable range of empirical evidence supporting the view that we are not conscious of "automatically" produced (i.e. bottom-up, neural) perceptual representations, but, rather, that "conscious percepts" arise from "a constructive act of fitting a perceptual hypothesis to its sensory source" (Marcel, 1983). Marcel explicitly identifies his concept of perceptual hypotheses with the concept of schema that we have been using, and, except for the fact that he only considers anentirely intra-cranial process (i.e. testing hypotheses only against neural representations, rather than, say, the optic array, or the object itself), "fitting a perceptual hypothesis to its sensory source" would seem to mean much the same as carrying out a set of perceptual tests under the direction of a schema. Gray (1995) explicitly notes the similarity of his account of conscious perception with the PA theory of Neisser (1976).
The left-hand box in figure 3 represents the schemata, the stored procedures that determine the sequence in which the various perceptual instruments are brought into play. During normal perception the current schema activates certain instruments which then proceed to make their tests. The results of the tests are reported back to the schema and contribute to determining which instruments will next be activated6, and so on in a continuous cyclical process (the "perceptual cycle" of Neisser, 1976). I suggest that this active process, equivalent to Marcel's "hypothesis fitting", may be identified as the conscious mental act of perceiving. It is an activity directed at an object of perception (or, possibly sometimes, misdirected [Anscombe, 1965]), and thus (unlike bottom-up, neural "representations") has intentionality, the quality that Brentano (1874/1973) and many subsequent philosophers have seen as the distinctive mark of the mental. Searle (1992) has argued that intentionality and consciousness (or potential consciousness) are interdependent and co-occurring7. (For further considerations suggesting the relevance of active perception to consciousness see Cotterill, 1995, 1997.)
The suggestion is that during imagery, the schema is active in much the same way that it is during perception. It still sends out at least some of its "orders" to the perceptual instruments, and selects procedural branches to follow. However, the reciprocal control of the schema's activity by the perceptual instruments is lost, or at least much attenuated. Hypotheses are still being put forward, as it were, giving rise to (potentially) conscious experiences, but they are not subject to testing against reality. This might either be because the instruments themselves (or some of them) are inhibited from carrying out their tests (Marks, 1973; Jacobson, 1932), and so fail to return any meaningful result, or it might be that results that are returned are disregarded, or not given full weight, by the schema. Either or both these processes may well occur in particular cases. However this may be, the processes that do occur will still display the intentionality of perceptual consciousness, being still directed towards an object, albeit a merely intentional ("intentionally inexistent") object rather than a material one (Anscombe, 1965; Ishiguro, 1967). It is important to stress that, on this theory, no thing orstate in the mind or brain corresponds to the percept or image. Although the schema may be considered the repository for what we learn about the world through perception, and although it controls our conscious perceptual activity, it should not be identified with that activity. As has been independently argued (Rabb, 1975; Heil, 1982; Tye, 1984), strictly speaking there is no percept or mental image, only the activity of perceiving or imagining, which takes different forms according to what is being perceived or imagined.
The above account of PA theory admittedly needs further development. Little has been said about the internal structure of perceptual schemata, how they interact with more general aspects of behavioral control, or how they are modified in response to new information that conflicts with the perceptual expectations they embody. In this regard, however, we can appeal to standard accounts of schemata (e.g. Rumelhart, Smolensky, McClelland, & Hinton, 1986), which will serve no worse (if no better) than they serve the numerous other psychological theories which rely on the concept. We also lack much detail about what perceptual instruments there are, and how they might work (and details that are given are speculative). Such facts will only be ascertained through detailed empirical investigations from a PA perspective (although there are surely many suggestive leads in the existing, especially the Gibsonian, literature). But these details do not, in any case, belong to the core commitments of the PA research program, any more than specific claims about neural realization or coding schemes are constitutive of picture or description theory. The core commitmentsare: the view of categorical perception (perceiving as) as the active exploration and interrogation of the environment; the view of imagery as the (partial) commission of such an exploration in the absence of the appropriate object (going through the motions of an interrogation without regard, or full regard, to answers received); and the rejection of CM (i.e. of the identification of conscious contents with neurally realized data structures). A schema may very well be best understood as a computational data structure, but we are not conscious of our schemata. It is not the schema but the activity it supports that carries intentionality and embodies our perceptual and imaginal awareness.
2.4 The Empirical Standing of the Theories
The following sub-sections will review certain issues for the three theories that arise from experimental findings. They do not aim at comprehensiveness; neither do they aim definitively to refute or establish any of the theories. Research programs are not susceptible to refutation or verification. The aim is merely to raise some questions about the explanatory power of two the better known approaches, and to suggest that PA theory may have at least a comparable explanatory potential. The fact is that the methods of experimental psychology have not, as yet, been sufficient to bring about a resolution of the imagery debate (Pylyshyn, 1994), and some authors have gone so far as to suggest that they cannot be (Anderson, 1978; Palmer, 1978). Thus, in §3 and §4, I will turn to broader considerations of theoretical scope and context.
2.4.1 Problems for Picture Theory
Kosslyn's picture theory was designed to explain the "classic" imagery effects (rotation, scanning, mnemonics, etc.) outlined in §2, and, although doubts have been raised (e.g. Pylyshyn, 1979; Yuille, 1983), it does a generally creditable job (Kosslyn, 1994; Kosslyn, Holyoak, & Huffman, 1976; Kosslyn & Pomerantz, 1977). However, results on the reinterpretation of images, and on imagery in congenitally blind subjects may be less easily accommodated.
The Necker cube and the duck-rabbit.
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