Microsaccades track location-based object rehearsal in visual working memory
Besides controlling eye movements, the brains oculomotor system has been implicated in the control of covert spatial attention and the rehearsal of spatial information in working memory. We investigated whether the oculomotor system also contributes to rehearsing visual objects in working memory when object location is never asked about. To address this, we tracked the incidental use of locations for mnemonic rehearsal via directional biases in microsaccades while participants maintained two visual objects (coloured oriented gratings) in working memory. By varying the stimulus configuration (horizontal, diagonal, and vertical) at encoding, we could quantify whether microsaccades were more aligned with the configurational axis of the memory contents, as opposed to the orthogonal axis. Experiment 1 revealed that microsaccades continued to be biased along the axis of the memory content several seconds into the working-memory delay. In Experiment 2, we confirmed that this directional microsaccade bias was specific to memory demands, ruling out lingering effects from passive and attentive encoding of the same visual objects in the same configurations. Thus, by studying microsaccade directions, we uncover oculomotor-driven rehearsal of visual objects in working memory through their associated locations. SIGNIFICANCE STATEMENTHow humans rehearse information in working memory is a foundational question in psychology and neuroscience. To provide insight into the cognitive and neural bases of working-memory rehearsal, we turned to microsaccades - small eye-movements produced by the brains oculomotor system. We reveal how microsaccades track the locations of visual objects during memory rehearsal, even when object locations are never asked about. This brings three advances. From a psychology standpoint, it demonstrates how memory rehearsal automatically engages object locations. From a neuroscience standpoint, it demonstrates how such location-based rehearsal relies on brain circuitry that also controls our eyes. Finally, from a practical standpoint, it demonstrates how microsaccades can be utilised to track the properties of working-memory rehearsal across space and time.