Microsaccades Are Coupled to Heartbeat

Abstract

During visual fixation, the eye generates microsaccades and slower components of fixational eye movements that are part of the visual processing strategy in humans. Here, we show that ongoing heartbeat is coupled to temporal rate variations in the generation of microsaccades. Using coregistration of eye recording and ECG in humans, we tested the hypothesis that microsaccade onsets are coupled to the relative phase of the R-R intervals in heartbeats. We observed significantly more microsaccades during the early phase after the R peak in the ECG. This form of coupling between heartbeat and eye movements was substantiated by the additional finding of a coupling between heart phase and motion activity in slow fixational eye movements; i.e., retinal image slip caused by physiological drift. Our findings therefore demonstrate a coupling of the oculomotor system and ongoing heartbeat, which provides further evidence for bodily influences on visuomotor functioning.

Significance statement: In the present study, we show that microsaccades are coupled to heartbeat. Moreover, we revealed a strong modulation of slow eye movements around the R peak in the ECG. These results suggest that heartbeat as a basic physiological signal is related to statistical modulations of fixational eye movements, in particular, the generation of microsaccades. Therefore, our findings add a new perspective on the principles underlying the generation of fixational eye movements. Importantly, our study highlights the need to record eye movements when studying the influence of heartbeat in neuroscience to avoid misinterpretation of eye-movement-related artifacts as heart-evoked modulations of neural processing.

Keywords: eye movements; heartbeat; microsaccades.

Figure 1.

Coupling of FEM and heartbeat (R peaks). Microsaccades (orange) and R peaks in the ECG (blue) are detected simultaneously. Microsaccade onset is determined relative to the preceding and following R peak.

Figure 2.

a, Deviation from null hypothesis of uniformly distributed microsaccades in the interval between two R peaks. Shown is a comparison of circular density of R-peak-locked microsaccades (orange; 95% confidence interval in gray) and circular density of a uniform distribution (blue). b, Circular densities of relative microsaccade onsets for each individual observer (orange), along with the 95% confidence interval (gray). Computation is based on resampling from the original pool of microsaccades of a given participant. In addition, the number of microsaccades for each observer is presented on top of the circular densities.

Figure 3.

a, Box count analysis showing the number of boxes (width = 0.01°) necessary to cover the eye trajectory in a time window of 50 ms. Mean (±95% confidence interval) centered box count is shown in black and the smoothed time course for each observer in gray (spline, df = 15). b, Time course of pupil size around the R peak. Mean (±95% confidence interval) z-transformed pupil size is shown in black and smoothed time course for each observer in gray (spline, df = 15).