THE ROUGH GUIDE TO PSYCHOLOGYof visual processing has been supported by the study of brain-damaged
patients. For example, there’s a condition known as optic ataxia, associ-
ated with damage to the “where” pathway. These patients can recognize
what things are, but can’t reach for them appropriately. By contrast,
patients with damage to the “what” pathway show the opposite deficit.
For example, they won’t be able to tell you which way a post-box slot is
oriented, but give them a letter and they’ll post it at just the right angle.
We tend to think that we either can or cannot see something. Another
form of brain damage shows how this is an oversimplification. Blind-
sight has been studied extensively by the psychologist Larry Weiskrantz
and is associated with damage to the primary visual cortex. Patients with
this problem report that there is a whole part of their visual field that
is effectively blind – they feel as if they can’t see anything there. Curi-
ously, however, when forced to make a decision about whether there
is, say, a square or circle, in that part of their vision, they will perform
better than if they were simply guessing. Quite how they do this isn’t
fully understood. However, it’s likely that the subcortical pathway to
the superior colliculus plays a role. There may also be islands of intact
functioning in the part of their visual cortex that is damaged. What the
syndrome shows is that it’s possible to be consciously blind while still
being able to “see”.
VISUAL ILLUSIONS
Illusions, especially of the visual variety, have proven extremely useful
to psychologists because they expose the short cuts and assumptions
used by our brains to create as accurate a representation of the world
as possible. One of the best known is the Kanizsa triangle, named
after the Italian Gestalt psycholo-
gist Gaetano Kanizsa. The observer
perceives a triangle with edges
that don’t really exist. The illu-
sion occurs because of the way the
brain uses statistical probabilities
to deduce what’s out there in the
world. In this case it calculates
that it’s more likely that there is a
white triangle occluding the three
circles than that there happen to
be three circles with the exact same