Abstract
INTRODUCTION. Microsaccades, or small recurring eye movements, typically occur ~1-2 times per second. Although generally considered involuntary, the characteristics of these eye movements are task-dependent and affect performance. For example, microsaccades are flexibly allocated to precisely relocate gaze during high acuity tasks in the fovea and suppressed prior to the onset of a stimulus outside of the fovea during a motion discrimination task. Here we investigated whether and how microsaccade rates are adaptively modulated by trial difficulty when observers discriminate motion directions in the perifovea. METHODS. We used a 2AFC task to measure the discriminability of a Gabor drifting for 500ms in 1 of 8 reference directions (4 cardinal, 4 oblique) at 8 isoeccentric locations (7°). Observers reported a Gabor’s drift direction, which was slightly clockwise or counterclockwise with respect to a reference direction. The difficulty of each trial was varied in two ways: (a) cardinal vs. oblique directions; and (b) tilt offset between reference and Gabor direction. The tilt angles were randomized using a method of constant stimuli: the Gabor’s drift direction was offset from the reference direction by ± 0.5, 1, 2, 4 or 8°. RESULTS. First, we found that microsaccade rates were suppressed prior to stimulus onset. Second, microsaccade rates were modulated by trial difficulty in two ways: they decreased (1) as the angular offset between the target and standard decreased; and (2) when stimuli drifted in oblique rather than cardinal directions. CONCLUSION. Microsaccades were suppressed prior to stimulus presentation, and during the stimulus period for difficult discrimination tasks in the perifovea. This flexibility is consistent with the proposals that greater fixational stability can (1) mitigate potential blur during microsaccades, and (2) prolong the duration of evidence accumulation. As a result, microsaccades may serve as a marker of cognitive effort for visual tasks in the perifovea.