Spatial Dynamics of Microsaccades: Investigating the Influence of Fixational Proximity to Visual Stimuli on Saccade-Induced EEG Modulation

Microsaccades have emerged as a valuable tool for examining EEG activity during tasks requiring eye fixation. While it is established that microsaccades generate a distinct evoked potential and EEG topography (Dimigen et al., 2009), it is unclear to what extent this modulation is due to motor processing, sensory input, or a combination of both. This is particularly relevant as there is significant spatial variability of visual stimuli across experiments, representing different sensory input. This study aims to determine if the post-saccade proximity of the fovea to high-contrast visual stimuli influences saccade-locked EEG activity. Examining data from an experiment involving a Posner reaction time task before and after neurofeedback and tACS stimulation, we focused on microsaccades during the 2.5-second cue period of the Posner task. Microsaccades were categorized into leftward and rightward directions, further stratified into 'close' and 'far' bins based on their landing proximity to lateralized visual stimuli. Employing a between-conditions cluster-based permutation test, we statistically assessed the evoked response of 'close' and 'far' saccades for both directions. The results revealed a saccade-locked evoked potential with timing and topography matching that which has been found in previous studies (Dimigen et al., 2009; Meyberg et al., 2015). This evoked potential was characterized by a biphasic muscle spike potential at saccade onset, followed by a positive peak at 106ms and a negative peak at 162ms in occipital electrodes. However, statistical analysis demonstrated no significant differences between 'close' and 'far' conditions for left or right saccades. The replication of saccade-locked ERPs and the absence of significant differences between ‘close’ and ‘far’ saccades suggest that fixational proximity to visual stimuli minimally influences the saccade-induced modulation of the EEG signal. Moreover, the saccade-locked evoked potential appears to reflect purely motor processes, rather than changes induced by the change of visual input on the retina.