November 7, 1995
Contact: Ellie Shick, Information Officer (615/322-2706)
Vanderbilt research:
Procrastination is built into the brain
NASHVILLE, Tenn.--Vanderbilt University researchers are making
inroads in how the brain aims the eye, bringing science a step closer to
solving the mystery of how the brain makes decisions. Surprisingly, their
most recent research reveals that the brain locates a visual target for
a subsequent eye movement without necessarily initiating the movement (Journal
of Neuroscience, October 1995).
"It seems procrastination is built into our brains," said
Jeffrey D. Schall, associate professor of psychology. The researchers hypothesize
that because the real world contains many possible targets that are always
changing, this extra delay may provide the advantage of being able to alter
the choice of what to look at before the commitment to move is made. "In
other words, procrastination allows time to change your mind," Schall
said. Implications for this research include providing visually guided robotics
for the disabled and diagnosis of brain disorders. An impaired ability to
voluntarily guide eye movements are symptoms of numerous mental and neurological
disorders, such as schizophrenia, AIDS and Parkinson's and Alzheimer's diseases.
"If you want to fix a machine, in this case the human brain, you
need to understand how it works," said Schall, adding that understanding
how the brain decides where and when to move the eyes is essential in understanding
how people see.
The Basics
"To see, we must move our eyes," said post-doctoral fellow
Kirk Thompson. "In fact, without realizing it, we move our eyes between
three to five times per second. Like a photographer aiming his camera at
objects of interest, our brain moves our eyes to focus on meaningful objects
in the world. This is done without any appreciable effort on our part.
"But what seems effortless is, in reality, a rather difficult
problem our brains solve before each gaze change," Thompson said. "That
is, what to look at next?"
"Until now, we didn't know how long it takes for the brain to
decide where to shift gaze because we didn't know how to measure it,"
said Thompson. "We now know precisely when single neurons in the brain
locate the target for an eye movement. We were surprised to find that making
that decision does not immediately result in a gaze shift."
Their discovery indicates that the brain's decision-making process
preceding an action has more than one component, the researchers said. By
understanding the seemingly simple decision of
where to shift gaze, they will better understand how the brain makes more
complex decisions.
Methodology
The researchers trained macaque monkeys to examine a video display
and look at one target object that was conspicuously different from several
distractor objects. They monitored the activity of single brain cells to
determine when the target is selected and when the eye subsequently moves
to focus on it.
Located in the frontal lobe of the cerebral cortex, the frontal eye
field converts the product of visual processing into a command to move the
eyes. The frontal eye field is populated by neurons that are active from
the time visual information reaches the brain until the eye moves to the
target. Each of these neurons has a "window on the world" or receptive
field, a region of the visual scene from which a stimulus will evoke a response.
To determine precisely when the decision to move the eye occurs, the
researchers used an analysis based on signal detection theory, a technique
developed to identify meaningful signals in noisy transmissions. They compared
the activity of a brain cell when the target landed within the cell's receptive
field to the activity when a distractor landed within its receptive field.
What's New and Different
"We found that the frontal eye field neurons respond initially
equally to both the target and distractor stimuli when they are presented,"
Thompson said. However, over time, the response to the target persists,
but the response to the distractor shuts off, even though the distractor
is still on the video display. The neural activity begins to differentiate
the stimuli after approximately 100 milliseconds.
"What this indicates to us is a principle of coding that can be
compared to a seesaw," Schall said. "As one end rises, the other
end falls. It seems that the brain uses this very basic `winner take all'
mechanism to perceive and mediate action."
They next compared the decision time to the time it took for the eyes
to move. "We were surprised to discover that, although we could pinpoint
when the decision was made to move the eyes (about 100 milliseconds), the
time it took to actually move them varied widely (100 to 500 milliseconds),"
said Thompson, adding that this indicates procrastination is a basic property
built into the brain.
The study follows previous work at Vanderbilt that described the evolution
of a target location signal within the brain (Schall and Hanes, 1993, Nature,
366:467-469). Thompson, Schall and graduate students Doug Hanes and Narcisse
Bichot will present their findings at the 1995 Annual Meeting of the Society
for Neuroscience in San Diego Nov. 11-16, expected to draw 20,000 participants.
Ongoing research includes experiments on how and when the brain triggers
an eye movement, on the visual processing involved in discriminating different
visual targets from distractors and on how the brain reacts when the target
for an eye movement suddenly changes.
-VU-
Vanderbilt
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Document last updated Jan. 27, 1997