Abstract
Microsaccades (MS, <1dva) that occur during fixation were studied extensively in response to transient stimuli, while participants had to fixate their gaze. In these conditions, MS are typically inhibited in response to transient stimulus presentation (Oculomotor Inhibition, OMI), and later released with a latency that depends on stimulus saliency, attention and expectations. However, it is yet unknown whether the same holds for free viewing, i.e. that MS obey a similar pattern of inhibition in response to the visual transient induced by large saccades in natural vision. Here, we investigated the hypothesis that every saccade provides a new stimulus that should result in OMI that depends on the post-saccadic image in a similar (but not necessarily identical) way as in flashed presentation. Participants (N=16), freely inspected static displays of randomly oriented Gabor texture images, with varied Contrast and Spatial Frequency (SF) for periods of 10sec each. Eye tracking recordings were divided into epochs triggered by saccade landing (>1dva) and MS reaction time (latency relative to fixation onset) was computed (msRT). We found that the msRT in free viewing was shorter for more salient stimuli (higher contrast or lower SF), as previously found for flashed stimuli. Moreover, we found that the msRT decreased across successive saccades, but only for the higher contrast, suggesting contrast dependent adaptation in free viewing. We also found a positive correlation between msRT and saccade size for high SF but not for low SF. Our results indicate that visual stimulus-dependent inhibition of microsaccades generalizes to free viewing, with latencies that resembles the pattern of inhibition in response to flashed-stimuli.