Friday, August 17, 2012

contrast or inference?

Norma Graham makes an interesting point, which I've seen quoted many times, in her book on spatial vision. She notes that it is as if the brain is transparent to what is happening in the early levels of visual processing, and that this is curious. It's curious, but it's a typical stance for someone who studies spatial vision; we assume that discrimination or identification or detection of signal strength is mediated almost entirely by the filters that are transducing the signal, not those that understand it, or respond (overtly) to it.

Whether or not this transparency holds when complex images are viewed is, I think, totally unknown. It may be that the percept elicited by a complex scene really is the sum of its parts, and that it simply provokes additional sensations of meaning, identity, extension, etc., which are tied to spatial locations within the scene. So, the visible scene that we are conscious of is indeed an object of spatial vision. This is the point of view I generally adopt, and I think it is common.

Another view is that the percept is entirely inference. Boundaries, surfaces, colors, textures, etc., are qualities in themselves, inferred from particular organization of spatial structure, and these then are organized in such a way that objects and identities and meanings can be inferred in successive stages. These inferences are what is seen consciously; perhaps inferences, and the evidence for them (the matter of spatial vision), are experienced simultaneously, but the substantive inferences, being the important elements of experience, are what dominate consciousness. So, only a small part of the phenomenal scene is actually constituted by e.g. luminance contrasts, and much more of it is constituted by higher level inferences. I think this point of view is also common, maybe especially in the current generation of visual neuroscientists.

The latter view is not exclusive of Graham's observation. If the patterns that are viewed are simple enough, they will not form objects, and will not have meaning. Or, they will be interpreted only as what they are, which doesn't require much inference, or only circular inference (which isn't a bad thing necessarily, when you really do want to conclude that a thing is itself, e.g. a gaussian blob of light, on the basis of its being a blob of light; usually, you want to infer that there is a letter on a page on the basis of a particular arrangement of blobs of light).

So, in the experiment that I'm currently analyzing to death, I am clearly taking the first view, in which case I think my conclusions are solid. If the second view is more accurate, what does the result mean? It could mean that inferences about image strength are based on higher frequencies just because they are the more susceptible to loss in a weak signal. If I'm asking subjects to judge image contrast, they could easily interpret this as judging image strength, and then their judgments would be biased towards the most delicate parts of the image, but they would still take everything into account.

This latter interpretation is still interesting, but it doesn't require "suppression". It is worth mentioning and I should at least include it in the manuscript, although the FVM talk probably will not have space... already there's barely space for the default story.

No comments:

Post a Comment