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In the ‘mind’s eye’: two visual systems in one brain

By Mel Goodale and David Milner

Vision, more than any other sense, dominates our mental life. Our visual experience is so rich and detailed that we can scarcely distinguish that subjective world from the real thing. Even when we are just thinking about the world with our eyes closed, we can’t help imagining what it looks like. Our language is full of visual metaphors. We can ‘see the point’, if we are not ‘blind to the facts’. We occasionally show ‘foresight’ (though perhaps more often ‘hindsight’) by ‘seeing the consequences’ of our actions in our ‘mind’s eye.’

But where does that rich visual experience come from? Most of us have the strong impression that we are simply looking out at the world and registering what we see—as if we were nothing more than a rather sophisticated video camera that delivers a faithful reproduction of the world on some kind of screen inside our heads. This idea that we have an internal picture of the world is compelling, yet it turns out to be not only misleading but fundamentally wrong.

There is much more to vision than just pointing our eyes at the world and having the image projected onto an internal screen. Our brain has to make sense of the world, not simply reproduce it. In fact, the brain has to work just as hard to make sense of what’s on a television screen in our living room as it does to make sense of the real world itself. So putting the television screen in the brain doesn’t explain anything. (Who is looking at the screen in our heads anyway?) But an even more fundamental problem is that our visual experience is not all there is to vision. It turns out that some of the most important things that vision does for us never reach our consciousness at all.

Who is looking at the screen in our heads anyway?

One way to get a handle on how vision works is to study the visual life of people with damage to different ‘visual’ parts of the brain. Research of this kind is revealing just how misleading our intuitions about how vision works can be. In some cases, it is easy to get a feel for what such individuals might experience. For example, some patients lose the capacity to see any colours at all; in other words, they see the world in shades of grey. A depressing but easily imagined experience. After all, most of us are familiar with black and white drawings, photos, and films. But imagine the opposite scenario, being able to see colour and even the fine texture of objects, but losing the ability to discern the geometry of those objects. Such patients, although rare, do exist. There are other equally rare patients who appear to have lost all conscious experience of visual motion, despite being able to see that objects have changed their positions – an experience that might be akin to seeing disco dancers under a strobe light. All of these examples help us understand how the brain constructs our conscious visual experience of the world.

But the role of the brain in creating our conscious visual experience is not all we can learn from studying patients with brain damage. When asked to talk about their visual problems, of course, patients will describe their conscious experience of the world. That’s all any of us can do. But there are other ways of finding out what people can ‘see’. If we look at their behaviour rather than simply listening to what they tell us, we may discover that they have other visual problems not apparent to their own awareness—or conversely that they may be able to see far more than they think they can.

Trying to understand the visual problems that brain damage may cause can cast light on a more fundamental question: Why do we need vision in the first place? A compelling argument can be made that we need vision for two quite different but complementary reasons. On the one hand, we need vision to make sense of the objects and events in the world beyond our bodies. On the other hand, we also need vision to guide our actions in that world from moment to moment. These are two quite different job descriptions, and nature seems to have given us two different visual systems to carry them out. One system, the one that allows us to recognize objects and build up a database about the world, is the one we are more familiar with, the one that gives us our conscious visual experience. The other, much less studied and understood, provides the visual control we need in order to move about and interact with objects. This system does not have to be conscious, but it does have to be quick and accurate. We have investigated a unique neurological patient (D.F.) over a long period who has no conscious perception of object size and shape, but who, through an intact visual control system, is able to orient and shape her hand to pick up objects without any trouble. It was D.F.’s remarkable ability to interact visually with objects whose features she cannot perceive that first led us to the idea of separate visual systems for perception and action. 

The idea of two visual systems in a single brain initially seems counterintuitive or even absurd. It seems to conflict with all of our everyday assumptions about how the mind works. In fact, the idea of separate visual systems for perceiving the world and acting on it was not entertained as a plausible scenario even by visual scientists until quite recently. Our visual experience of the world is so compelling that it is hard to believe that a quite separate visual system in the brain, inaccessible to visual consciousness, could be guiding our movements. It seems intuitively obvious that the visual image that allows us to recognize a coffee cup must also guide our hand when we pick it up. But this belief is an illusion. Studies of D.F. and other neurological patients, and complementary research on the normal brain, particularly using brain imaging, is making it abundantly clear that the visual system that gives us our conscious experience of the world is not the same visual system that guides our movements in the world.

Melvyn Goodale (University of Western Ontario, Canada) and David Milner (Durham University, UK) are the authors of Sight Unseen: An Exploration of Conscious and Unconscious Vision published by Oxford University Press.

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Image credits: Family watching television, c. 1958. Photo by Evert F. Baumgardner. National Archives and Records Administration. Public domain via Wikimedia Commons.

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