Microsaccade directions are not correlated with cued locations in a spatial working memory task

Although we know the eye moves imperceptibly during fixation, there is still debate concerning a functional role for microsaccades. We investigated this oculomotor activity in a spatial working memory task that had produced results used to argue for covert attention shifts as a rehearsal mechanism for locations of stimuli. Participants were briefly cued to one of many locations surrounding a central fixation cross, and were instructed to remember it pending onset of a probe stimulus seconds later. 2AFC responses were made indicating whether the probe was in the same location as the cue, or shifted inward toward fixation. Discrimination of large displacements was at ceiling, whereas small displacements were at floor. With the goal of affecting attention during the retention interval, another 2AFC task was imposed while participants waited for probes: Shape discrimination in one condition took place on 2/3 of trials, and color discrimination in another condition on every trial within selected blocks. These “choice” stimuli interacted with memory responses in a way not reported previously. We replicated some findings from the recent literature, such as inhibition of microsaccades after stimulus onsets, and a following “rebound” phase in which they occur at greater than baseline rates. However, we did not replicate behavioral results that would indicate the presence of covert attention shifts in the task, nor did we find directional bias of microsaccades toward cued locations during rebound phases. Moreover, the distribution of our microsaccade trajectories is anisotropic, with most of them oriented in the horizontal plane. Individual differences and strategy effects are brought to light in explaining results and suggesting future directions for inquiry. If microsaccades can signal the presence of some covert attention shifts, as it has been suggested recently, they cannot do so for all of them, or for shifts in all directions.