Primate Saccade Rhythmicity

Abstract

Active sensing behaviors in rodents display theta (4-8 Hz) rhythmicity. Whether similar rhythmicity exists in primate saccadic eye movements has remained a matter of debate. We studied saccade dynamics in 22 human participants and two macaque monkeys, examining the influence of different visual stimuli and tasks. Inter-saccadic intervals (ISIs) reliably revealed a characteristic duration and under certain conditions clear theta rhythmicity. Rhythmicity was strongest for saccades with short ISIs. Surprisingly, the degree of rhythmicity was not due to spatial regularity of the visual scene, but it was shaped by task demands. Macro- and micro-saccade ISIs shared similar characteristic durations. Naturally occurring micro-saccades provided evidence that ISIs can become more regular without becoming faster. During free-viewing, subsequent ISIs were not independent, but the visual system switched between states of low and high rhythmicity. Humans and macaques showed similar saccade dynamics, suggesting a potential common evolutionary trait in primate active visual sensing.Significance StatementActive sensing, wherein organisms deliberately orient their sensory organs to probe their environment, is a cornerstone of perceptual experience. While certain animals like rodents showcase rhythmic patterns in such behaviors, the presence of similar rhythms in primate vision remains contested. In our two-species study, involving human participants and macaque monkeys, we unveil a distinctive rhythmic pattern of saccades, the main visual sensing behavior of primates. Interestingly, this rhythm resonates with known theta rhythms. Notably, this rhythmicity was not necessarily determined by the visual scene but was modulated by the task at hand. Our findings shed light on a potentially shared evolutionary trait in primate vision, underscoring the intricate interplay between perception, action, and cognition.

Memory’s pulse: episodic memory formation is theta-rhythmic

“…We rarely notice how even a relaxed conversation moves at breakneck speed; evanescent syllables 55 and microexpressions last for only around a theta cycle. And when the world is static, our visual perception of it still whirls by, with each saccade sampling different locations at a theta pace 56 and resetting hippocampal theta oscillations 57 . Thus, there is converging evidence suggesting many facets of experience are discretized at a theta frequency.…”