Abstract
Highly localized perceptual enhancements at the microsaccade goal have been reported to occur before the onset of these tiny gaze shifts. Yet, these modulations have been examined only in the unnatural context of instructed microsaccades. It remains unknown whether they unfold with the same magnitude for spontaneous microsaccades and whether they are broadly or selectively tuned for spatial frequency (SF). Here we investigate how naturally occurring microsaccades impact discrimination of foveal stimuli of different SFs. Eye movements were recorded with a high-resolution digital Dual-Purkinje Image eye-tracker while subjects (N=7) performed a 2AFC discrimination task. Two vertically oriented gabor patches (0.5deg diameter) at different SFs (4,8,12 and 16 cpd) were presented 0.5 deg to the left and right of the display center. After a variable delay, one of the two gabor patches briefly (50 ms) tilted ±45 deg and observers reported the direction of the orientation change. Contrast thresholds for each SF were assessed in microsaccade-free trials (baseline condition), stimulus contrast was then maintained at threshold throughout the task. Only trials in which the gaze was maintained close to the center of the display before microsaccade onset were examined. Our results show that, before the onset of a spontaneous microsaccade, visual discrimination is improved with respect to baseline when microsaccades are directed toward the stimulus that changed orientation (Δd’=0.73±0.46, p<0.05). This modulation appears to be stronger for lower SF (Δd’=1.1±0.56 vs. Δd’=0.25±0.71, p< 0.05). On the other hand, performance was impaired when microsaccades were directed away from the stimulus that changed orientation. These findings show that in everyday settings, when observers view complex foveal stimuli, discrimination for a wide range of SFs across the central fovea is systematically modulated prior to an impending microsaccade.