During fixation, the eyes are not still, but often exhibit microsaccadic movements. The function of microsaccades is controversial, largely because the neural mechanisms responsible for their generation are unknown. Here we show that the superior colliculus (SC), a retinotopically organized structure involved in voluntary-saccade target selection, plays a causal role in microsaccade generation. Neurons in the foveal portion of the SC increase their activity before and during microsaccades with sizes of only a few minutes of arc, and exhibit selectivity for the direction and amplitude of these movements. Reversible inactivation of these neurons significantly reduces microsaccade rate without otherwise compromising fixation. These results, coupled with computational modeling of SC activity, demonstrate that microsaccades are controlled by the SC, and explain the link between microsaccades and visual attention.Microsaccades are the very small (typically <12 min arc), involuntary, fast eye movements that occur during fixation (1-3). The behavioral properties and functional significance of microsaccades have been extensively studied -and sometimes vigorously debated -for many years (1-14). However, the neural mechanisms responsible for their generation are unexplored. We now show that the superior colliculus (SC), a retinotopically organized structure known to be important for selecting and initiating voluntary eye movements (15)(16)(17), is also part of the neural mechanism that controls microsaccades.We analyzed SC activity associated with 15,205 microsaccades that occurred while monkeys fixated a small stationary spot (18). Each fixation trial lasted for 3,500 ms resulting in many microsaccades with a variety of directions and amplitudes (Fig. 1A,Supp. Fig. S1). These movements had dynamics like those of larger saccades (3) (Supp. Fig. S1A), consistent with evidence that pre-motor neurons (downstream from the SC) are active during movements as small as 12-15 min arc (19).We targeted neurons in the rostral pole of the SC, which represents foveal goal locations (18,20). Figure 1A shows the spiking activity of a neuron in the left SC during a single trial containing 9 microsaccades (highlighted in green). The neuron exhibited changes in activity that were correlated with microsaccades. For example, the microsaccades labeled 1 and 2 in Fig. 1A were predominantly downward and leftward, respectively, and both were associated with a reduction in the neuron's activity. In contrast, small, predominantly rightward † This manuscript has been accepted for publication in Science. This version has not undergone final editing. Please refer to the complete version of record at http://www.sciencemag.org/. The manuscript may not be reproduced or used in any manner that does not fall within the fair use provisions of the Copyright Act without the prior, written permission of AAAS.*To whom correspondence should be addressed. E-mail: zhafed@salk.edu. NIH Public Access Author ManuscriptScience. Author manuscript; available in P...

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.