Rare but precious: microsaccades are highly informative about attentional allocation

To clarify the relation between attention and microsaccades, we monitored microsaccades while observers performed tasks with different attentional demand. In four high-demand conditions, observers shifted attention covertly to a peripheral location, or focused attention at fixation. Three corresponding low-demand conditions on physically identical displays provided a basis for comparison. Our results revealed two distinct effects of attentional load: higher loads were associated consistently with lower microsaccade rates, but also with increased directional selectivity (up to 98% congruent). In short, when microsaccades were most rare, the direction of microsaccades proved most informative about the location of the attention focus. The detailed time-courses of the two effects differed, however, suggesting that they may reflect independent processes.