SEEING
A number of Vanderbilt researchers are interested in the same question:
How do we see?
That interest blends the study of the organization of the brain, anatomy
and physiology,
development and visual performance.
Randolph Blake, left, professor of psychology, chair of the department
and Kennedy Center investigator, and Jeffrey Schall, right, associate
professor psychology and Kennedy Center investigator, are two of the researchers
in that group. They search for the answer to the "Big Question"
in different ways. Blake uses human subjects and computers and Schall uses
monkeys and computers to discover how the brain computes visual information.
The work of Blake and Schall builds on existing knowledge about the structure
of the visual nervous system. It's long been known that the optic nerve
fibers carrying information from the eyes to the brain crisscross at the
optic chiasm. As a result about 50 percent of the fibers from the left eye
send their neural messages to the occipital lobe of the right cerebral hemisphere
and the remaining 50 percent to the left hemisphere. The same is true for
the nerve fibers originating from the right eye.
Because of this, the visual cortex at the very back of the brain, receives
input from both eyes, and it is here that messages from the two eyes begin
to be assembled into meaningful descriptions of objects and events. From
here, however, the story is incomplete, and Blake and Schall are attempting
to write at least some of the remaining chapters.
In his attempt to understand how the brain creates visual reality, Blake
adopts a two-pronged attack. First, he and his students create computer
software to simulate what they believe is occurring within the visual nervous
system when people perceive given aspects of the visual world. These simulations
or "neural models" generate predictions about what people should
and should not be able to see. Then, Blake tests people on specially designed
visual tasks to discover if they perform like the computer models predict.
Over repeated tests, the neural model is refined to bring its predictions
into closer correspondence with actual visual performance.
At present, Blake and his colleagues are concentrating on two aspects of
vision, motion perception and stereoscopic depth perception.
"In our laboratory, people view animation sequences depicting three-dimensional
objects, and they're asked to make judgments about the appearance of these
objects," Blake explained.
Results from some of his research shed light on how the brain combines the
left and right eyes' images into a single, stable 3-D view of the world.
"We aren't aware of seeing the world through two eyes, because this
process of binocular combination transpires at a preconscious level,"
Blake noted. "In other words, we don't know we're doing it when we
do it."
Blake said his research is an "example of learning what the brain can
do without going inside the brain."
Schall's research is more hands-on. It directly explores the brain function
mediating visually-guided behavior. Schall and his research associates record
the pattern of eye movements made by macaque monkeys looking at a computer-generated
display as they locate a target among distractors. While the monkeys are
performing their tasks, the activity of single neurons in the frontal cortex
is monitored.
"We have been investigating how the brain selects the target for each
eye movement and regulates when to shift gaze," Schall says. "We
have discovered that the brain processes the image to locate the target,
but then procrastinates before moving the eye."
After an interval amounting to as much as one-half second, the monkey's
brain finally acknowledges the differentiated target and generates the eye
movement to the target.
"The brain may have procrastination built into it to prevent unwise
movements in an unpredictable world," Schall says.
-Kelly C. Lockhart
-Photos by Billy Kingsley
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