Devoid of any external time cues, monkeys can still tell time. Animals learned to move their eyeballs once every second, a completely internal timing feat made possible by the rhythmic behavior of small groups of nerve cells, researchers propose online October 30 in PLOS Biology.
Time is often measured with clues from the environment, says study coauthor Geoffrey Ghose of the University of Minnesota in Minneapolis. A quick glance at a clock indicates that your meeting will start soon, and a look outside at a low sun tells you that it’s time to leave work. But some time-telling abilities rely on purely internal processes — just a feeling that minutes, hours or days have ticked by, Ghose says.
Ghose and Blaine Schneider, also of the University of Minnesota, studied this internal sensation of time by creating a situation in which two monkeys had to generate their own pattern without any outside help. The animals were trained to switch their gaze rhythmically between a red dot and a blue dot on a computer screen once every second, a job that looks like “they’re watching an extremely boring tennis match,” Ghose says.
After a while, the monkeys got good, on average just tens of milliseconds off their tempo. Meanwhile, the researchers used electrodes to eavesdrop on the behavior of neurons in a part of the brain called the lateral intraparietal area. Earlier monkey studies found that neurons there build up activity with time, firing messages more and more frequently as the milliseconds tick by.
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But Ghose and Schneider saw the opposite. As time passed, these neurons actually grew quieter. While the cells were getting quieter overall, there were some important variations: the firing of some neuron groups was dropping off faster than others. And this variability seemed to predict which way the monkey was going to move its eyes, so that the two sides of the brain seemed to be in a tug-of-war. When a signal from one side of the brain got strong, the monkey moved its eyes.
This timing signal would probably be very specific to this kind of eye-moving task, says Ghose. Other jobs that require precision timing might rely on other parts of the brain.
“This is a really interesting and unexpected kind of finding,” says neuroscientist Michael Shadlen, a Howard Hughes Medical Institute investigator at Columbia University Medical Center in New York. He cautions, though, that it’s not clear that the neurons in this study are in the same exact spot as those studied previously.
These neurons must have something to do with timing, Shadlen says, but they might not be the ultimate clock for this eye-movement task. “I believe these signals are related to timing, but whether they are causing timing is an open question,” he says.
Ghose plans to test that idea by interfering with these neurons’ behavior to see if that speeds up or slows down the monkeys’ rhythm.