Microsaccades while aiming are not just limited by gaze relocation demands

We try to keep our eyes still while threading a needle or aiming to shoot a still target since these tasks require maximum attention and fine motor control. Nevertheless, we make small saccades which are largely outside of our conscious control. The frequency of these microsaccades drops during the end of the task. Does this mean that they are a hindrance to the ongoing task and thus get suppressed? Or do microsaccades relocate gaze between closely situated regions of interest (ROIs) for gathering accurate visual information in the beginning and then are no longer required as ROIs finally overlap? In this study, we first used a simulated shooting task under normal and eccentric viewing conditions that allowed us to study microsaccadic activity under different gaze-localization demands. In the normal viewing condition, microsaccade amplitudes were tuned to the separation between the ROIs as opposed to the eccentric viewing condition in which they were independent of the ROI separation. Thus microsaccades served the function of gaze reorientation only in normal viewing condition. Surprisingly, microsaccade rates dropped in both the conditions at the end of the trial indicating that this change is not dependent on the ongoing gaze relocalization demands of the task. Another reason for the observed drop in microsaccade rate could be that attention is disengaged from the task location during the time at which microsaccade rate drops. In the second experiment, we tested this hypothesis by asking subjects to perform two tasks simultaneously: shooting and detection of a target letter from a rapid stream of letters (RSVP task). Attentional disengagement from the shooting location should be reflected as improvement in performance in RSVP task. But we did not see any improvement in RSVP task performance at the time of drop in microsaccade rate. We conclude that the change in microsaccade activity during aiming task cannot be explained by either the on-going gaze relocalization demands of the task or an attentional disengagement from the task location.