Eyes offer window into brain’s timekeepers

The eyes may reveal whether the brain’s internal stopwatch runs fast or slow. Pupil size predicted whether a monkey would over- or underestimate a second, scientists report in the Nov. 2 Journal of Neuroscience.

Scientists knew that pupils get bigger when a person is paying attention. They also knew that paying attention can influence how people perceive the passage of time. Using monkeys, the new study links pupil size and timing directly. “What they’ve done here is connect those dots,” says neuroscientist Thalia Wheatley of Dartmouth College. More generally, the study shows how the eyes are windows into how the brain operates. “There’s so much information coming out of the eyes,” Wheatley says. 

Neuroscientist Masaki Tanaka of Hokkaido University School of Medicine in Japan and colleagues trained three Japanese macaques to look at a spot on a computer screen after precisely one second had elapsed. The study measured the monkeys’ subjective timing abilities: The monkeys had to rely on themselves to count the milliseconds. Just before each trial, the researchers measured pupil diameters.

When the monkeys underestimated a second by looking too soon, their pupil sizes were slightly larger than in trials in which the monkeys overestimated a second, the researchers found. That means that when pupils were large, the monkeys felt time zoom by. But when pupils were small, time felt slower.

The differences in pupil size were subtle, but Tanaka and colleagues think these changes are meaningful. Pupil size, the results suggest, offers a readout of the brain as it keeps track of passing milliseconds.

This pupil readout may reflect a specific type of signaling in the brain. As a chemical messenger called noradrenaline puts the brain into a heightened state of alertness, pupils get bigger, previous research has shown. That link is why this study makes sense, Wheatley says. Attention is known to make time fly, a distortion that would lead a monkey to think a second has elapsed sooner than it has. The opposite is also true. When the brain is a little more sluggish or not paying attention, time ticks by slower and seconds stretch out.

By finding that the eyes hold clues to how the brain perceives time, Tanaka says that the study may motivate further research into how brain cells actually make this split-second calculation (SN: 7/25/15, p. 20).