Abstract
The pattern of fixational eye movements is known to be altered in response to perceptual events, with a stereotypical decrease-increase-baseline modulation of microsaccade rate. This modulation is known to be affected by stimulus saliency and attention. However, the functional role and significance of this pattern as well as the hidden processes it may reflect are largely unknown. Here we explored the effect of task-induced cognitive processes on the pattern of microsaccade rate, while controlling stimulus properties. In the first experiment, observers viewed a sequence of 100 randomly ordered small (2 deg) red and blue circles and triangles briefly flashed at 1 Hz repetition rate. The observers silently counted the appearance of different target properties (shape, color) in separate runs. The modulation in the event-related microsaccade rate was analyzed for latency and amplitude. We found a significantly longer latency (prolonged microsaccade inhibition >100 ms, peak around 450 ms) for target stimuli as compared to distracters (e.g. for counted circles compared to ignored triangles) and even a longer latency for a conjunction condition (counting blue circles). In comparison, counting all of the items or not counting at all, produced another pattern with a large peak around 200ms, not observed in any of the selection conditions. In a second experiment, observers performed orientation discrimination of a briefly flashed Gabor patch, tilted right or left by 30-deg (easy) or 3-deg (difficult). We found a prolonged latency (>100 ms increase) for the difficult condition. Preliminary results indicate that a similar effect can also be observed in the auditory modality. These results demonstrate an oculomotor trace of task-dependent internal processes in which microsaccades are suppressed or activated according to processing load. Microsaccades thus provide an involuntary window into internal brain processes and can be used to explore these processes in healthy as well as non-communicative individuals.