Visual Imagery and Consciousness
Nigel J.T. Thomas
Page 4
http://www.imagery-imagination.com/viac.htm
Theories of Imagery, and their Implications for Consciousness
There is much controversy in the contemporary scientific and philosophical literature about the underlying nature and mechanisms of imagery. The rival theories fall into three broad classes, each of which have distinctive implications for our understanding of imaginal consciousness:
- Picture theory (its contemporary advocates often prefer to call it "quasi-pictorial," or "analog" theory)
- Description theory (also, rather misleadingly, known as "propositional" theory)
- Enactive theory (there is no established name for this type of theory; different versions are known as "motor theory," "sensorimotor theory," "perceptual cycle theory," "role taking theory," and "perceptual activity theory.")
Because of the vigorous, high-profile dispute (known as the "analog-propositional" debate, or sometimes just "the imagery debate") that flared up between advocates of the first two theory types in the 1970s, and that continues to rumble on even today, picture and description theories have become by far the better known of these theoretical alternatives. Unfortunately, this debate has been conducted almost entirely without regard to the conscious nature (or, indeed, the intentionality) of imagery. Each of the three theory types purports to be able to account for the full range of established empirical facts about imagery, and, in fact, each is flexibly enough conceived that it is likely to prove difficult to choose between them on narrowly empirical grounds. That being so, it seems reasonable that our evaluation of them should look to broader theoretical concerns, such as their respective implications for the theory of consciousness.
Picture theory
Picture theory, as the name suggests, holds that visual mental images may be identified with representations in the mind and/or brain that are in some significant way like pictures, or, at least, that represent things in much the same way that pictures represent things. (Whatever that way may be: Although it is widely believed that pictures represent through resemblance, it does not seem likely that resemblance can be the basis of mental representation. Resemblance is a fundamentally subjective relationship that is recognized when a conscious mind focuses upon certain similarities between things while, at the same time, discounting or ignoring other, equally objectively real dissimilarities. In order to do this, the mind must already be able to represent the things, and their relevant aspects, to itself.)
Picture theory is undoubtedly the oldest and most widely accepted theory of imagery. After all, since prehistoric times people have used pictures as a means of causing one another to have visual experiences of things that are not really there, and the analogy with mental imagery (also experience of things that are not really there) is all too easily drawn. The theory goes back at least to Plato, and some rudimentary version of it seems to be deeply entrenched in "folk" (i.e., lay or "commonsense") beliefs about how the mind works. Indeed, many of the expressions used to talk about imagery in ordinary colloquial language (such as, "mental picture," "the mind's eye," and even "image" itself) seem to be derived from this "folk" version of picture theory. The fact that the theory is embedded in our language in this way can make it difficult to express or understand criticisms of it, or to clearly grasp alternative accounts of the actual phenomena. Nevertheless, many twentieth and twenty-first century philosophers have raised telling objections against it, leading some of them to be accused (quite unjustifiably) of denying that people have quasi-visual experiences.
However, the picture theory today is not just a folk theory. From the 1970s onward, cognitive scientists, most particularly Stephen Kosslyn, have developed the basic idea of the mental image as an inner picture into a sophisticated and detailed scientific model designed to account for a wide range of experimental findings. More recent versions of this theory hold that the visual mental image is embodied as a spatially extended two-dimensional pattern of neural excitation in the retinotopically mapped visual cortices of the brain. Such patterns, isomorphic to the pattern of optical stimulation on the retinae of the eyes, are known to occur during vision. Kosslyn holds that our mental images are similar patterns, but generated from internal sources rather than sensory input.
This is not the place to review all the alleged empirical shortcomings of quasi-pictorial theory. However, both introspective and empirical evidence suggest a number of ways in which the experience of imagery and the experience of looking at a picture vary. Perhaps the strongest evidence comes from experiments involving pictures that invite more than one interpretation, such as the Necker cube and the duck-rabbit (figure 1). Subjects find it very much harder to see both interpretations in their mental images of these than they do when the picture is physically displayed in front them, and this applies even when the picture is their own drawing, based upon their mental image.
Figure 1: The Necker cube and the duck-rabbit.
Kosslyn maintains that the pattern of neural excitation that (he thinks) constitutes the mental image is only a picture in an extended, metaphorical sense: a "quasi picture." After all, unlike regular pictures (drawings, photographs, projected optical images, etc.), we do not need to see it with our eyes in order to derive information from it, or consciously experience it. The neural excitation pattern is like a picture only inasmuch as it represents spatial relationships within the two-dimensional projection of the represented visual scene by topologically equivalent spatial relationships in the spatially extended pattern of excitation. Kosslyn maintains that because his theory does not identify mental images with pictures in the full, literal sense, but only with these "quasi pictures," it avoids all of the many powerful objections that have been raised against naïve "folk" versions of picture theory. Whether this is so, however, remains controversial.
Although the modern version of pictorial theory can account for a large amount of empirical data about how people use mental images as representations, its supporters have given very little attention to the question of how it might account for the fact that imagery is (at least sometimes) consciously experienced. There seem to be two basic ways in which this issue might be approached. Either the quasi picture in the cortex might be conscious in and of itself, or, alternatively, consciousness arises when some other, more inward cognitive structure, a "mind's eye" as it were, somehow "quasi-sees" the quasi picture.
The first option, however, runs immediately into the notorious "hard problem" of consciousness in its most intractable form: we really have no idea how a pattern of excitation in part of the brain (excitation that is ultimately nothing but an elaborately choreographed dance of molecules and ions around and through membranes) could possibly be, or give rise to, a conscious experience of any sort, let alone an experience of a spatially extended structure isomorphic to the excited brain region.
In any case, the second option appears to be favored by Kosslyn. He speaks of (and diagrams) a "mind's eye function" gathering information from the quasi-pictorial image in the brain, and some of his explanations of empirical findings seem to depend upon this idea. Perhaps then, just as ordinary visual consciousness arises when people take in visual information with their eyes, imaginal consciousness arises when the mind's eye takes in visual information from the inner picture.
But the "mind's eye" metaphor strongly suggests that the theory commits the homunculus fallacy. To whom might this eye belong if not to a conscious little man inside my head, who experiences the inner world of quasi pictures in a way analogous to that in which I, a whole person, experience the external world through my bodily eyes? Compounding this worry is the fact that, at a formal, structural level (abstracting away from implementational details), Kosslyn's quasi-pictorial theory of imagery looks very like a more detailed version of Descartes' theory of imagery. For Descartes too, mental images are material quasi pictures formed within the brain and consciously experienced not simply because they are there, but because they are somehow "seen" by the soul. Kosslyn makes it clear that he does not intend to follow Descartes in attributing our consciousness of these brain pictures to an immaterial soul (a ghostly homunculus) beyond the reach of science; but although quasi-pictorial theory may not necessarily imply Cartesian dualism, it does seem to be at least committed to Cartesian materialism, the notion that subjective experience depends upon some structure in the brain that (like a soul or a homunculus) acts as a conscious inner spectator of internal representations.
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