Abstract
Selective attention can be directed to external visual sensations as well as to internal visual representations held in working memory. It has long been known that the covert shifting of attention to (anticipated) external sensations is associated with directional biases in microsaccades. We have recently demonstrated that directional biases in microsaccades also occur when covertly shifting attention to internal visual representations maintained within the spatial lay-out of working memory – when there is no incentive for overt shifts of gaze. In the current project, we aimed to directly compare spatial biases in microsaccades when attention was directed to either external visual stimuli or internal representations of those same stimuli in working memory. Non-spatial colour cues directed attention to one of two visual objects, presented to the left and right of fixation. Cues either appeared before or during stimulus presentation (externally directed attention) or after stimulus presentation, while retaining the visual objects in working memory (internally directed attention). Our data confirmed profound spatial microsaccade biases during both externally and internally directed covert shifts of attention. Interestingly, these biases were even more robust (in addition to being slightly delayed) when directing attention internally, when there was no incentive for overt shifts of gaze. While covert-attention demands resulted in a mixture of micro- and macro-saccades when directing attention in perception, gaze biases in working memory occurred exclusively in the microsaccade range. Together, these data (1) reinforce the utility of microsaccades biases as a powerful tool for tracking covert attention in both external and internal domains and (2) bring the paradoxical insight that eye-movement biases by covert attention may be even more reliable in cases where there is in fact nothing to look at.