Temporal dynamics of saccades explained by a self-paced process

Abstract

Sensory organs are thought to sample the environment rhythmically thereby providing periodic perceptual input. Whisking and sniffing are governed by oscillators which impose rhythms on the motor-control of sensory acquisition and consequently on sensory input. Saccadic eye movements are the main visual sampling mechanism in primates, and were suggested to constitute part of such a rhythmic exploration system. In this study we characterized saccadic rhythmicity, and examined whether it is consistent with autonomous oscillatory generator or with self-paced generation. Eye movements were tracked while observers were either free-viewing a movie or fixating a static stimulus. We inspected the temporal dynamics of exploratory and fixational saccades and quantified their first-order and high-order dependencies. Data were analyzed using methods derived from spike-train analysis, and tested against mathematical models and simulations. The findings show that saccade timings are explained by first-order dependencies, specifically by their refractory period. Saccade-timings are inconsistent with an autonomous pace-maker but are consistent with a “self-paced” generator, where each saccade is a link in a chain of neural processes that depend on the outcome of the saccade itself. We propose a mathematical model parsimoniously capturing various facets of saccade-timings, and suggest a possible neural mechanism producing the observed dynamics.

The role of motor and environmental visual rhythms in structuring auditory cortical excitability

“…Next we examined the effect of saccades on auditory cortical activity, reasoning that since our results show that visual inputs modulate A1 excitability, saccade related inputs, which has been shown to modulate excitability in primary visual cortex (Barczak et al, 2019;Rajkai et al, 2008), should as well. If entrainment could be brought about by the quasi-rhythmic sequence of saccades (Amit et al, 2017;Henderson, 2003), we expected that the neuronal activity would be entrained to the same excitability phase as in the case of the LED stream, as both saccades and LED flashes are associated with a post-event volley of visual input. We tracked eye movements at 500Hz resolution using an infrared system (see Methods).…”

The limits of fixation—Keeping the ametropic eye on target

“…More surprisingly, fixational saccade rates showed a dependency on refractive error as well and did not settle on the level known for sustained fixation from the literature. In general, saccades occur quite regularly, either as large-scale saccades or as microsaccades, at rates of 1-2 per second, with a distribution skewed to long latencies (Amit, Abeles, Bar-Gad, & Yuval-Greenberg, 2017;Cherici et al, 2012;Ko, Poletti, & Rucci, 2010). Under sustained fixation as well as high acuity tasks, saccades occur at lower rates than under free viewing (Amit et al, 2017;Ko et al, 2010).…”

“…In general, saccades occur quite regularly, either as large-scale saccades or as microsaccades, at rates of 1-2 per second, with a distribution skewed to long latencies (Amit, Abeles, Bar-Gad, & Yuval-Greenberg, 2017;Cherici et al, 2012;Ko, Poletti, & Rucci, 2010). Under sustained fixation as well as high acuity tasks, saccades occur at lower rates than under free viewing (Amit et al, 2017;Ko et al, 2010). Saccade rate furthermore depends on training status and task (Bonneh et al, 2010;Cherici et al, 2012;Rolfs, Kliegl, & Engbert, 2008), but at a given task and target, fixational saccade rate is expected to be constant.…”

Reducing saccadic artifacts and confounds in brain imaging studies through experimental design

“…A few characteristics of saccades make them an exceptionally problematic confound. First, their relatively frequent occurrences (1–3 per second; Amit, Abeles, Bar‐Gad, & Yuval‐Greenberg, ; Martinez‐Conde, Macknik, & Hubel, ) guarantee their presence in the vast majority of trials of most experimental protocols. Second, they are not random in time relative to the stimulus: a perceptual transient triggers a stereotypical temporal modulation of saccade rate (Rolfs, Engbert, & Kliegl, ), which makes it impossible to eliminate them by averaging repetitions.…”