Gnosanopsia – what?

In a previous entry about Riddoch syndrome, I mentioned that I am fascinated by a phenomenon which Zeki calls gnosanopsia – awareness without discrimination. When presented with visual stimuli, patients with gnosanopsia claim that they can see something, but they are unable to describe the visual features associated with that something, such as its color or the direction of motion,

Many questions immediately come to mind. Can we take this claim at face value? Maybe the patients are mistaken. Maybe they merely have a vague sense of something happened, but choose to describe this non-visual experience in visual terms. For example, it is entirely possible that what the patients experience is sensing the automatic motor responses of their eyes triggered by visual stimulation. Can we scientifically differentiate this scenario from gnosanopsia, supposedly a real visual experience but without sufficient content to support discrimination?

This is a case where the phenomenology of vision becomes complicated. It will take cognitive scientists and philosophers decades to disentangle the conceptual complexity. In this entry I merely want to point out that this complexity is not something only found in neurological disorders. In fact our everyday visual experience is complicated.

As an example, let’s consider the phenomenon of crowding. The picture below is from a recent review paper  (Whitney & Levi, 2011):

First, look at the lower left corner of the image. What do you see? A little boy in a green shirt. Now fixate on the bullseye symbol in the middle of the image, and then try to convince yourself that you can still recognize that little boy. It’s not that easy. You know something green is there, but it’s so fuzzy that recognition becomes impossible. It really is a very odd feeling. What is this odd feeling, this confusing sense of green things jumbled together? Is this a real visual experience or is it a vague non-visual experience? It’s very hard to say. I’d like to say it’s vague but not so vague that I can’t say it’s green. I’d like to call the experience visual but there seems to be something more visceral to it.

But maybe this is not so strange, after all. It is very well-established that visual acuity degrades with retinal eccentricity. Maybe we cannot recognize the little boy simple because we don’t have enough visual acuity to resolve the fine details about the little boy. But crowding is not just an effect of reduced acuity. When the little boy is not crowded (for example, the boy on the lower-right corner), recognition is a lot easier. As discussed in Whitney & Levi (2011), the mechanism of crowding is very complicated and we know very little about it.

I started to think about crowding in the context of gnosanopsia and blindsight because I wanted to differentiate real vision (also called phenomenological vision, visual qualia, real visual experience and so on) from merely non-visual, vague feelings. But then I realized that the two are not separate categories of things. Real vision is very often associated with vague feelings. Depth perception is another example. What is the the experience of depth? Is depth, like color, a category of qualia? It’s not very easy to say. Try the following exercise: look at whatever object that is directly in front of you, take a long and careful look, and then close one eye. Can you tell the difference? Was your visual experience changed by closing one eye? On one hand I want to say yes because depth perception is definitely lost after the closure of one eye, but the difference is so subtle that most people can’t tell the difference, at least immediately. Is the percept of depth a visual experience, or is it a non-visual, vague[1] feeling?

[1] Note that vague really isn’t the right word because depth judgement is very precise! It’s not vague at all. We’ll discuss this in a future post.

Reference: Whitney, D. & Levi, D.M. (2011) Visual crowding: a fundamental limit on conscious percept ion and object recognition. Trends in Cognitive Science, 15, 160-168.


~ by hhyu on July 10, 2011.

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