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How the brain tells time
Oct. 28, 2005
Courtesy Duke University
and World Science staff
The brain is a time machine, say neuroscientists Catalin Buhusi and Warren
Meck of Duke University in Durham, N.C.—and understanding how it tracks time is
key to grasping all its functions.
In an article in the October 2005 issue of the
journal Nature Reviews Neuroscience, Buhusi and Meck discuss the current state of
research into one of the brain’s most important, and mysterious, clocks: that governing timing intervals in the seconds-to-minutes range.
Such timing, they say, occupies a middle ground between two other
brain “clocks.” These are the circadian clock that governs sleep over a
24-hour cycle, and the millisecond clock that regulates such functions as
muscular coordination and speech generation and recognition.
Interval timing is central to broader coordination of tasks such as walking, manipulating objects, carrying on a conversation and tracking objects in the environment, they said.
“Interval timing is necessary for us to understand temporal order of events, for example when carrying on a conversation,” said Meck. “I need to process the pacing of speech, to organize my thoughts coherently and to respond back to you in a timely manner.
... in fact it’s hard to find any complex behavioral process that timing isn’t involved in.”
Deciphering the mechanisms of such clocks may be even more basic to understanding the brain,
they said, than figuring out how the brain processes space and movement.
Said Buhusi, “I would argue that time is more fundamental than space, because one can just close one’s eyes and relive memories, going back in time; or prospectively go forward in time to predict something, without actually changing your position in space.”
Understanding the machinery of interval timing is profoundly difficult because it is “amodal,” said Buhusi and Meck. That is, the interval timing clock is independent of any sense—touch, sight, hearing, taste or smell. Thus, it cannot be localized in a discrete brain area, as can the circadian clock, which has inputs from the visual system and outputs that control the cyclic release of
certain hormones.
“So, this process has to be distributed so it can integrate information from all the senses,” said Meck.
“But more importantly, because it’s involved in learning and memory, you could argue that time isn’t directly perceived, but that we make temporal discriminations relative to memories of previous durations. Such features have made the machinery of interval timing more elusive, and some even questioned whether an internal clock of this sort even exists.”
In the 1980s Meck and colleagues at Brown and Columbia Universities proposed a theory for explaining interval timing
that involved a “pacemaker-accumulator” model.
This holds that somewhere in the brain lurks an independent biological pacemaker that regularly emits timing pulses or “ticks.”
But more recent research by Meck and colleagues
at Duke has led to the development of what he calls a “striatal beat frequency” model of interval timing.
This involves a system in
which different groups of brain cells emit repeating pulses of electrical
activity. The striatum, a structure deep in the brain in an area known as the
basal ganglia, picks up these oscillations and coordinates them.
“Each structure in the brain contributes its own resonance,”
Buhusi said. And the striatum or basal ganglia are “like a conductor who listens to the orchestra, which is composed of individual musicians. Then, with the beat of his baton, the conductor synchronizes the orchestra.”
Basically, the model holds, the entire brain is a timing machine, in which individual structures busy with their own
tasks generate resonances that integrate to become ticks of
a neural clock.
Meck, Buhusi and their colleagues are conducting an array of experiments to try to identify this “baton” timing signal and to refine the theory. These include studies using genetically modified mice,
drugs, recording of electrical brain signals in ensembles of brain cells and
brain imaging.
For example, they are studying how the clock’s ticking changes in Parkinson’s patients as they change levels of their medication, which effects the amount of dopamine in their brains. Dopamine has been implicated as a key signaling molecule in the
timing circuitry.
“When Parkinson’s patients are on their medication, they time quite normally,” said Meck. “But as their medication wears off, we can see their clock slow down by recording their brain signals.”
Said Meck, “We’re addressing two challenges. One is to find the molecular processes that underlie this internal clock. And the second challenge is to build more realistic models of how this timing process works, with constant, parallel input from throughout the brain.” In such studies, the researchers face the daunting process of trying to monitor the intricate swirling of neural activity throughout the entire brain, said Meck.
“Looking at only one place in the brain for the interval clock is like the blind man feeling just the toe of the elephant and trying to describe how it works,” he said. “While we’re very excited about our success so
far... We are blind men touching just one part of this elephant.”
“Our new review paper, to the best of our knowledge, is the first to try to integrate the different fields and levels of analysis that contribute to understanding timing and time
perception.”
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