Primate Saccade Rhythmicity

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 show long-range correlation structure and 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.