Time-keeping Brain Neurons Discovered
ScienceDaily (Oct. 23, 2009) — Keeping track of time is one of the brain's
most important tasks. As the brain processes the flood of sights and sounds
it encounters, it must also remember when each event occurred. But how does
that happen? How does your brain recall that you brushed your teeth before
you took a shower, and not the other way around?
For decades, neuroscientists have theorized that the brain "time stamps"
events as they happen, allowing us to keep track of where we are in time and
when past events occurred. However, they couldn't find any evidence that such
time stamps really existed -- until now.
An MIT team led by Institute Professor Ann Graybiel has found groups of
neurons in the primate brain that code time with extreme precision. "All you
do is time stamp everything, and then recalling events is easy: you go back
and look through your time stamps until you see which ones are correlated
with the event," she says.
That kind of precise timing control is critical for everyday tasks such as
driving a car or playing the piano, as well as keeping track of past events.
The discovery, reported in this week's issue of the Proceedings of the
National Academy of Sciences, could lead to new treatments for diseases such
as Parkinson's disease, where the ability to control the timing of movements
is impaired.
Construction of time
The research team trained two macaque monkeys to perform a simple
eye-movement task. After receiving the "go" signal, the monkeys were free to
perform the task at their own speed. The researchers found neurons that
consistently fired at specific times -- 100 milliseconds, 110 milliseconds,
150 milliseconds and so on -- after the "go" signal.
"Soon enough we realized we had cells keeping time, which everyone has wanted
to find, but nobody has found them before," says Graybiel, who is also an
investigator in MIT's McGovern Institute for Brain Research. The neurons are
located in the prefrontal cortex and the striatum, both of which play
important roles in learning, movement and thought control.
The new work is an elegant demonstration of how the brain represents time,
says Peter Strick, professor of neurobiology at the University of Pittsburgh,
who was not involved in the research. "We have sensory receptors for light,
sound, touch, hot and cold, and smell, but we don't have sensory receptors
for time. This is a sense constructed by the brain," he says.
Key to the team's success was a new technique that allows researchers to
record electrical signals from hundreds of neurons in the brain
simultaneously, and a mathematical way to analyze the brain signals,
spearheaded by team members Naotaka Fujii of the RIKEN Brain Institute in
Japan and Dezhe Jin of Penn State. Though this study focused on the
prefrontal cortex and striatum, Graybiel says she expects other regions of
the brain may also have neurons that keep time.
Graybiel suggests that the new research could help patients with Parkinson's
disease, who often behave as if their brains' timekeeping functions are
impaired: they have trouble performing tasks that require accurate rhythm,
such as dancing, and time appears to pass more slowly for them. Rhythmic
stimuli such as tapping can help them to speak more clearly.
Targeting the timekeeping neurons with neural prosthetic devices or drugs --
possibly including the natural brain chemicals dopamine and serotonin -- may
help treat those Parkinson's symptoms, she says.
Future studies in this area could shed light on how the brain produces these
time stamps and how this function can control behavior and learning. The work
also raises questions regarding how the brain interprets the passage of time
differently under different circumstances.
"Sometimes time moves quickly, and in some situations time seems to slow
down. All of this ultimately has a neural representation," says Strick.
Funding: National Eye Institute, National Parkinson Foundation, Alfred P.
Sloan Foundation, and the Huck Institutes of the Life Sciences at Penn State
University.
Journal reference:
1. Jin DZ, Fujii N, Graybiel AM. Neural representation of time in
corticobasal ganglia circuits. Proceedings of the National Academy of
Sciences, Week of Oct. 19 2009
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