IS THE IMAGERY DEBATE OVER? IF SO, WHAT WAS IT ABOUT?
Zenon Pylyshyn
Source: http://ruccs.rutgers.edu/faculty/pylyshyn-mehler.htm
Many things to many people
In discussing this question, I will begin by laying out what I think is the state of the “debate”. Later I will come back to the question of whether neuropsychological evidence has made (or is likely to make) any difference. What is the problem? To put it as bluntly as possible, the problem over the question of the nature of mental imagery is just that some people find the experience of seeing a picture-in-the-head completely compelling while others find it irrelevant as the basis for a theory of what is actually going on in the mind when we entertain mental images. Beyond that there is no useful general debate, only arguments about the coherence and validity of certain quite specific proposals, and about what morals can be drawn from particular experiments. This is, in the end, not about metaphysics. It is a scientific disagreement, but one in which there are very many different claims falling under the same umbrella term “the imagery debate” and they may have little or nothing in common, other than sharing the same gut reaction to the picture metaphor and to the question of what to make of the phenomenology of imagery.
Among the many things involved in this debate are a variety of substantive disagreements.
· A disagreement about whether the form of representation underlying mental imagery is in some way special, and if so, in what way it is special – whether it is special because it uses distinct mechanisms specific to imagery or whether it is special because it deals with information about how things look (i.e., because of what imaginal representations are about or their subject matter);
· A disagreement about whether images are “depictive” or whatever the opposite is (perhaps “descriptive”);
· A disagreement about whether or not mental imagery “involves the visual system”, which itself raises the question of what exactly is the visual system and in what way it may be involved;
· A disagreement about whether certain phenomena observed during episodes of mental imagery are due to the fact that the brain evolved in a particular way resulting in a “natural harmony” between the way things unfold in one’s imagination and the way they unfold in the world or in one’s perception of the world (what Shepard, 1975, has called “second order isomorphism”).
· A disagreement about whether some of the phenomena observed during episodes of mental imagery arise because (1) people are reasoning from what they know about the situation being imagined, and are simulating what they believe would have happened if a real event were being observed, or because (2) special image-specific mechanisms are deployed when one reasons using mental images. This is the disagreement that I wrote about in (Pylyshyn, 1981) and, I believe, remains one of the main questions about mental imagery.
One of the things that makes this debate both ironic and ill-posed is that it is hard to disagree with most of the picture theory[1] views in this discussion, since there is something right about the claims. It is true that when we solve problems in which the geometry of a display plays a roll we usually do so by imagining the figure, and that when we do imagine the figure we are able to do things we could not do if we were to approach the problem symbolically – say by thinking in words. This much is not in dispute. What is in dispute is what is going on in our head when we engage in the sort of activity we call “imaging”, and in particular what an adequate theory of this process will need to postulate as the bearer of the imaginal information. This is not a case of believing that images do not exist or are “epiphenomenal”. It is a question of whether theories of mental imagery that posit 2D displays or “depictive representations” are empirically correct, or perhaps even coherent. In every case I have looked at, hypothesizing pictures or depictions does not provide any explanatory advantage over what I will call the “null hypothesis” that image content is represented as symbolic expressions (see below), even though it may feel more comfortable because it comports with one’s subjective impression of what is going on. I, for one, get very nervous when I find a theory in psychology making claims that are consonant with how it looks to me from the inside – I know of too many examples where how it feels on the inside is exactly the wrong kind of theory to have. Things rarely are how they seem in any mature science, and this is especially true in a nascent science like psychology or psychobiology!
Intrinsic and extrinsic constraints
The basic dilemma is that while the following two claims may both seem to be true, they are incompatible – at least in their naïve form:
(1) Since the mental images we have are of our own making we can make them have whatever properties we wish, and we can make them behave in any way we choose. Having an image is like having a thought – it seems as though we can think any thought there is to think. Consequently which image property or thought we have depends on what we want to do, and this generally depends on what we believe.
(2) Mental images appear to have certain inherent properties that allow them to parallel many aspects of the perceived world. For example images do not seem to unfold the way thoughts do, following principles of inference (including heuristic rules), but in a way that directly reflects how we perceive the world. An image looks a certain way to us, therefore we “see” things in them independent of our explicit knowledge about what we are imaging. If we imagine a geometrical figure, such as a parallelogram, and imagine drawing its diagonals we can see that one of the diagonals is longer than the other and yet that the two cross at their mutual midpoints, and we can see that this is so apparently without having to infer it from our knowledge of geometry. Similarly, when we imagine a dynamic situation or event unfold in our mind (such as a baseball being hit), the imagined event behaves in ways that appear to be at least partially outside our voluntary control, and maybe even outside our intellectual capacity to calculate.
There are anecdotes to illustrate each of these two perspectives and the opposing factions in the “imagery debate” generally emphasize one or the other of the above claims. The picture-theorists argues that if you have a visual image of an object, then you have no choice but to imagine it from a particular viewpoint and having a particular shape and orientation, etc. Similarly, if you imagine an object as small in size it follows from the inherent nature of mental imagery that it will be harder to “see” and therefore to report small visual details than if you imagined it large; or that if you focus your attention on a place in your imagine and then try to report a property that is far away, it will take longer than if you attempted to report a property that was nearer to where you are focused. The critics of picture theories argue equally cogently that you can just as easily imagine an object without imagining it as having any particular properties, that there is no reason (other than the implicit requirement of the instruction to “imagine something small”) why you can’t imagine something as small but highly detailed and therefore not take longer to report its visible details, or to imagine switching your attention from one place on an image to another in a time independent of how far away the two places are, as long as you are not attempting to simulate what would happen if you were looking at the real object being represented or are not attempting to simulate a situation in which you believe it would take more time to get from one place to another if the places were further apart (in fact I reported the results of several experiments showing that this is exactly what happens Pylyshyn, 1981). The imagery-as-general-reasoning adherents can point to many examples where the way a dynamic image unfolds is clearly under our voluntary control. For example, imagine sugar being poured into a glass full of water (does it overflow?), or imagine yellow and blue transparent colored filters being moved together so they overlap (what color do you see where they overlap?). The answer you give to these questions clearly depends on what you know about the physics of solutions and what you know (or remember) about the psychophysics of color mixing.
It is easy enough to come up with examples that go either way; some empirical phenomena appear to be a consequence of inherent properties of an image representation and others appear to arise because of what you know or believe (perhaps falsely) about how the visually perceived world works. The substantive empirical question is: Which properties are inherent properties of the imagery system (or the medium or mechanisms of mental imagery) and which are properties that the person doing the imaging creates in order to simulate what he or she believes would be true of a real situation corresponding to the one being imagined.
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