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More answers from Professor Melvyn Goodale

May 23rd, 2008

What are the most remarkable things you have learnt about human vision during your career?

One important thing that we have learned is that we have more than one visual system.  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 camera that delivers a faithful reproduction of the world on some kind of television screen inside our heads. This idea that we have an internal picture of the world is compelling, but our research has shown that this is not only misleading, it is 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, after all, is looking at the screen in our heads?) 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 consciousness at all.

Although it is true that we need vision to give us detailed knowledge of the world beyond ourselves, knowledge that allows us to recognize things from minute to minute and day to day, we also need vision to guide our actions in that world at the very moment they occur. These are two quite different job descriptions, and nature seems to have given us two different visual systems to carry them out.  The idea of two visual systems in a single brain might initially seem counterintuitive or even absurd.  In fact, when we first put forward this proposal, many of our colleagues were quite sceptical.  After all, our visual experience of the world is so compelling that it is hard to believe that some other quite independent visual system-one that operates completely outside of consciousness-is guiding our movements.  For most of us, it seems intuitively obvious that the visual image that allows us to recognize a coffee cup is the same one that guides our hand when we pick it up. But this belief is an illusion. Over the last decade or so, our research has made it clear that the visual system that gives us our visual experience of the world is not the same system that guides our movements in the world.

What are some examples of the way that we use the 'vision-for-action' system? 

Although we use vision-for-perception to recognize our coffee cup, once it has been 'flagged' the task of figuring out how to pick up the cup is 'handed off' to the 'vision-for-action' system which does the work of computing just how far away the cup is from our hand and how our fingers should be postured to pick up the cup successfully.

Are the two systems equally balanced, or can a person have a really strong 'vision-for-perception' system and a weak 'vision-for-action' system, and vice versa? 

We have never systematically investigated individual differences in the way people use the two systems, it would not surprise me if such differences exist.  We can certainly learn to recognize subtle differences between visual patterns - and some people really excel at doing this in particular domains.  Individuals with particularly well-developed 'vision-for-perception' systems could perhaps more easily become expert bird watchers, art dealers, pathologists, or fashion designers.  By the same token, it would not surprise me if natural athletes had some advantages in the 'vision-for-action' department!

What kind of problems do you experience if your 'vision-for-action' system doesn't function properly? Does this relate at all to the tendency of some people to bump into door-frames while others glide effortlessly through their environments? Does it affect driving? 

People who have damage to the 'vision-for-action' pathways can show profound difficulties in the visual control of even simple movements such as reaching out and grasping a cup of coffee - even though they have no difficulty recognizing what they are looking at.  Although individuals unlucky enough to have this kind of brain damage cannot use vision to direct their movements, they can use the sense of touch and even auditory information.  In other words, their problem is not purely visual or purely motor - it is a visuomotor problem.

Clearly, disturbances in the 'vision-for-action' system could disrupt one's ability to navigate through the world - including driving - even though such an individual might easily pass a conventional eye exam.

Can you improve your 'vision-for-action' system or is it something that you're born with?

We don't know very much about how training affects the vision-for-action system - although recent studies have shown that young people who play a lot of video games exhibit improved performance in visual search tasks and other visually demanding problems - suggesting that visual systems in the brain can change in fundamental ways with practice.

Are there differences in the capabilities of this system in men and women, and adults and children? Do we lose it as we get older and our eyesight deteriorates?  

We are only beginning to explore how the two visual systems change as we grow older.  We do know that since our 'vision-for-action' system makes more use of our peripheral vision than does our 'vision-for-perception' system, some diseases of old age such as macular degeneration tend to interfere more with activities that depend on visual perception - such as reading or watching television - than they do with the visual control of many physical activities, such as walking or even playing some sports.

Professor Melvyn Goodale

Professor Melvyn Goodale