Microsaccades as an overt measure of covert attention shifts.

Microsaccades, or tiny eye movements that take place during periods of fixation, have long been thought to be random artifacts of the oculomotor system. Here we demonstrate a possible link between microsaccades and covert attention shifts. We designed two psychophysical tasks involving spatial cues that had identical sensory stimuli but differing patterns of attentional benefits and costs. We found that microsaccades, rather than being randomly distributed, had directions that were directly correlated with the directions of covert attention shifts in the two tasks. Our results suggest that microsaccades occur because of subliminal activation of the oculomotor system by covert attention.