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DURHAM, N.C. -- The human visual system does not generate a
picture of what actually exists in front of the viewer at any
given moment, asserts a new book by neurobiologists at Duke
University Medical Center.

Rather, the researchers theorize that evolution -- as well
as individual experience during development -- have created a
visual system in which perceptions represent what a given
visual stimulus has typically signified in the past, rather
than simply representing what is presently 'out there.'

Despite the seemingly commonsensical belief that everyday
experience with visual perceptions corresponds precisely with
the characteristics of the 'real world,' in Why We See What
We Do: An Empirical Theory of Vision, (Sinauer Associates,
2003), Dale Purves, M.D., and Beau Lotto provide detailed
scientific evidence to the contrary.

Purves is the George B. Geller Professor of Neurobiology at
Duke University Medical Center, and Lotto is a faculty member
in the Institute of Ophthalmology at University College
London.

Basically, say Purves and Lotto, their evidence demonstrates
that what humans and other mammals see is a reflex response to
the accumulation of possible sources that a given stimulus has
turned out to be in past experience. This way of generating
vision explains why visual perceptions are often at odds with
physical measurements of the underlying objects -- the angles
and line lengths of a simple geometrical figure, for
instance.

According to Purves and Lotto, their empirical theory of
vision explains a wealth of striking visual illusions of
brightness, color, form, depth and motion that have puzzled
vision scientists for decades, sometimes centuries. (See
http://www.purveslab.net
for demonstrations of these effects and their explanation in
terms of the new theory.).

The authors emphasize that any successful theory of vision
must address a conundrum recognized more than a century ago
that visual stimuli are inevitably ambiguous.

"The physicist Hermann von Helmholtz was the first to
clearly state the fundamental problem in vision, namely that
there is no way to directly specify objects and conditions in
the world by means of the information conveyed to eye by
light," Purves said in an interview. "Even though it doesn't
seem that way to us, the information carried by the light that
falls on the retina is inevitably ambiguous. A particular
stimulus can have many different physical sources. Since the
goal of any visual animal is to react appropriately to the
sources of visual stimuli, this fact presents a major
problem."

For example, he said, it is impossible to know whether a
given amount of light reaching the retina signifies a highly
reflective surface in weak illumination, or a weakly reflective
one in strong illumination. Since the amount of light and
therefore the effect on the retina is the same in either case,
the viewer's perception cannot be a simple "report" of the
amount of light reaching the eye from the surface in
question.

"What we see has to correspond to what is really out there
-- the shiny surface or the dull one," says Purves. "If that
weren't the case, we would all be in deep and continual trouble
with respect to the responses we make to retinal stimuli."

Ironically, Purves explained, this central problem
identified more than a century ago has taken a back seat in the
twentieth century, primarily because of rapid progress in
anatomical and neural recording techniques that have proven
enormously successful in determining how the nerve cells in the
visual system are wired. Despite the scientific success from
using these techniques, said Purves, the successes have not
kept the implicit promise that understanding the detailed
wiring of relevant parts of the brain will lead to a general
theory of vision.

"The problem is that the quite wonderful knowledge vision
scientists now have about nerve cells and the cells'
connections has not been able to explain what we actually see,"
he said. "Especially puzzling have been the obvious
discrepancies that exist between physical measurements of the
real world with photometers, rulers and the like, and the
corresponding perceptions."

Thus, Purves, Lotto and their colleagues set out to collect
the data that would, explain the range of striking visual
illusions that has long fascinated and perplexed
neurobiologists, psychologists, philosophers and others curious
about vision.

"The work that we began in the mid-nineties was motivated by
our conviction that the problem of visual stimulus ambiguity
shouldn't continue to be relegated to the back burner," said
Purves. "Of course, we're not the only people since Helmholtz
to have wrestled with this problem, but I think we've done a
pretty good job of showing how the visual system goes about
solving it, at least in general terms."

The premise of the book -- and of the many scientific papers
on which the theory is based -- is that the only way the
problem of stimulus ambiguity can be solved is to generate
perceptions on the basis of what a given image on the retina
has, in statistical terms, turned out to be in the past.

"The tutoring in this process comes from the feedback of the
results of visual behavior in response to visual stimuli," said
Purves. "If you make a visual mistake, you suffer the
consequences," said Purves. "The connectivity of the visual
brain has thus been shaped gradually by this trial-and-error
process over the eons of human evolution and the decades of
individual development," said Purves. "As a result, what we see
at any moment is, quite literally, always predicated on the
probability distribution of the possible sources of the retinal
stimulus." The obvious implication of this fact is that the
nuts and bolts of the visual system are going to have to be
understood in these statistical terms, said Purves.

Purves also pointed out that this strategy of vision has
implications for other aspects of brain function, such as the
generation of auditory perceptions. As with the light reaching
the retina, the sources of auditory stimuli at the ear are
ambiguous. The sound pressure waves at the ear, like the light
waves affecting the retina, cannot specify the physical source
of the sound. Thus, understanding the sources of sound stimuli
in these probabilistic terms may be just as useful in
understanding what we hear as it has been in understanding what
we see.

"If we are to make real progress in understanding how the
human brain makes sense of the physical world," said Purves,
"then visual behavior and its underlying physiology will have
to be understood in terms of this empirical theory of neural
function."