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
Chambers and Reisberg (1985) briefly showed subjects examples of single "reversible figures", such as the Necker cube or the duck-rabbit (figure 4), with instructions to "form a mental picture" of the figure, so they could later draw it. Next, the subjects were asked to visualize the figure and attempt to "see" the alternative interpretation in their image. (It had been determined that the 5 second exposure time was too short for subjects to see more than one of the two possible interpretations whilst actually viewing the figure.) Subjects were unable to find a second interpretation from their image in any of the 55 trials made. This was despite being allowed as much time as they liked; despite having previously been familiarized with reversible figures of a similar type; and despite being given hints as to how to get a figure to reverse by spatial attention shifting. Nevertheless, when they subsequently made their drawings, subjects were able to recognize the alternative interpretation in their own drawings in all but four trials. Similar failings at image reinterpretation tasks have been reported by many other experimenters, including: Reed (1974); Palmer (1977); Hinton (1979); Reisberg, Smith, Baxter, and Sonenshine (1989); Reisberg and Chambers (1991); and Slezak (1991, 1995).
Picture theory would lead us to expect that, provided sufficient detail has been encoded (and the success with the drawings seems to confirm this), we ought to be able to reinterpret a mental image quite as easily as we can reinterpret the equivalent real picture. It has been shown that reinterpreting the "reversible figures" depends on shifting the focus of visual attention across the picture (Tsal & Kolbert, 1985), and Kosslyn (1994) explicitly holds that we can spatially shift our attention across the visual buffer; nevertheless, it is easy to reinterpret the duck-rabbit picture and very hard to reinterpret its image. True, it has now been shown that images of certain rather different figures can be reinterpreted somewhat more easily, and that under special conditions even images of the duck-rabbit and its ilk can sometimes be reversed (Finke, Pinker, & Farah, 1989; Peterson, Kihlstrom, Rose, & Glisky, 1992; Brandimonte & Gerbino, 1993; Cornoldi, Logie, Brandimonte, Kaufmann, & Reisberg, 1996). Nevertheless, this work does nothing to undermine the integrity of the original results, and others like them. An important disanalogy between imagery and pictures remains (Slezak, 1993, 1995). People clearly experience significant difficulties in reinterpreting images under conditions where they have very little trouble with the equivalent picture.
Kosslyn (1980, 1994) attempts to account for these results in terms of the differential fading of parts of the image. According to his theory, a complex image is built up in the visual buffer by the sequential placing of its various sub-parts, and a part will begin to fade as soon as it is written to the buffer. Thus, by the time a complex image is fully assembled the earlier parts will be considerably fainter than the newer. Although the fading parts can be successively "refreshed", the whole image can never attain a uniform vividness. He then argues that if reinterpreting an image involves reconstruing the relationships between parts, or the way that it is segmented into parts, (as is plausibly the case with the experiments in question) then the differential vividness of the relevant parts explains the difficulties of such reinterpretations.
This seems coherent, but is hardly without problems. For one thing, it is not clear that it accords with the phenomenology of imagery. Folk psychology does acknowledge that images fade, but I, at least, can never recall experiencing an image distinctly more faded in some structural parts than in others. I may focus in on one part, so that the rest fades from my attention, but this does not seem to be what Kosslyn is describing. Another, less subjectively based, objection is that many experimental tasks that subjects can do well seem to involve simultaneously attending to, and even manipulating, multiple parts of images that are at least as complex as those used in the reinterpretation experiments. For example, some of the foundational experiments of Kosslyn's own research program call for the simultaneous visualization of two animals (Kosslyn, 1975), and many of the mental rotation experiments of Shepard and his students (Shepard & Cooper, 1982) involve manipulation of images of complex irregular polygons or three-dimensional assemblages. The duck-rabbit and the Necker cube seem simple by comparison.
Chambers and Reisberg themselves (1992; Reisberg, 1994) provide an alternative pictorialist explanation of their results. They suggest that pictorial images occur paired with a second form of representation, a "reference frame", which biases either how the image is read by the "mind's eye", or else helps to determine which of the telling details specified in the "deep representation" actually gets a detailed instantiation in the "surface display". However, this epicycle only seems to push the problem back a step. Instead of wondering why it is hard to reinterpret images we are left to wonder why it is hard to switch reference frames appropriately during imagery, but not during perception.
It is also surely a considerable embarrassment for picture theory that almost all the "classic" effects that originally served to convince the psychological community of the reality and cognitive significance of visual imagery (§2) have since been demonstrated in the congenitally blind, who can hardly be capable of forming mental reconstructions of former visual sense impressions. Mnemonic effects of imagery in the blind were found by Craig (1973); Jonides, Kahn, and Rozin (1975); Zimler and Keenan (1983); and Kerr (1983). Kerr also reports mental scanning, and size/inspection time effects. Mental rotation in the blind is reported by Marmor and Zaback (1976), and Carpenter and Eisenberg (1978). Hampson and Duffy (1984) found interference effects. Effects may be weaker, or reaction times slower, than in the sighted, but formally the results are very similar.
No doubt the blind subjects rely mainly on "tactile/kinaesthetic" (i.e. haptic) imagery, as is evidenced by the way that they fail at more purely visual imagery tasks, such as perspectival judgements (Arditi, Holtzman, & Kosslyn, 1988). However, quasi-pictorialist forms of explanation do not readily carry over to the haptic mode. Although a "passive-touch buffer" (mapping the skin much as the visual buffer is supposed to map the retina) seems conceivable, it would not be remotely adequate to the tasks in question. Haptic knowledge of objects and spatial relations is mediated by active, exploratory movements, and involves a complex coordination between tactile sense, proprioception, and motor control. There would seem to be little place for any equivalent to the sort of bottom-up, relatively peripheral object representations that are supposed to inhabit the visual buffer. This does not, of course, prove that quasi-pictorialist explanations will not work for visual imagery. However, it would be very remarkable for two quite disparate forms of imagery mechanism to engender such formally similar experimental results.
2.4.2 Problems for Description Theory
According to Intons-Peterson and Roskos-Ewoldsen (1989) the results on imagery in the blind helped to persuade many that "images are amodally spatial, and perhaps not visual in nature" (presumably an allusion to description theory). However, as Kosslyn and Pomerantz (1977) have argued, description theory does not deal very satisfactorily with the "classic" imagery effects (§2). Although it is possible to devise explanations for them in terms which are consistent with basic descriptionist tenets, these explanations generally require otherwise quite unmotivated, ad hoc assumptions. None of the effects would have been predicted from the theory's core principles. Of course, we would have to tolerate such ad hoc explanations if there were no coherent alternatives to description theory, and Pylyshyn (1978, 1984) tries to establish that there are none by bringing conceptual objections against the very notion of fundamentally non-language-like mental representation. However, these arguments do not appear to be sound (Tye, 1991). With the demolition of these metatheoretical arguments of Pylyshyn's, much of the motivation for taking description theory seriously would seem to have dissipated.
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