Memory-guided microsaccades

Microsaccades are overwhelmingly described as involuntary eye movements. Here we show in both human subjects and monkeys that individual microsaccades of any direction can easily be triggered: (1) “on demand”, based on an arbitrary instruction, (2) without any special training, (3) without visual guidance by a stimulus, and (4) in a spatially and temporally accurate manner. Subjects voluntarily generated instructed “memory-guided” microsaccades readily, and similarly to how they made normal visually-guided ones. In two monkeys, we also observed midbrain superior colliculus neurons that exhibited movement-related activity bursts exclusively for memory-guided microsaccades, but not for similarly-sized visually-guided movements. Our results demonstrate behavioral and neural evidence for voluntary control over individual microsaccades, supporting recently discovered functional contributions of individual microsaccade generation to visual performance alterations and covert visual selection.

Reference Frame of Human Medial Intraparietal Cortex in Visually Guided Movements

Visually guided reaching involves the transformation of a spatial position of a target into a body-centered reference frame. Although involvement of the posterior parietal cortex (PPC) has been proposed in this visuomotor transformation, it is unclear whether human PPC uses visual or body-centered coordinates in visually guided movements. We used a delayed visually guided reaching task, together with an fMRI multivoxel pattern analysis, to reveal the reference frame used in the human PPC. In experiments, a target was first presented either to the left or to the right of a fixation point. After a delay period, subjects moved a cursor to the position where the target had previously been displayed using either a normal or a left–right reversed mouse. The activation patterns of normal sessions were first used to train the classifier to predict movement directions. The activity patterns of the reversed sessions were then used as inputs to the decoder to test whether predicted directions correspond to actual movement directions in either visual or body-centered coordinates. When the target was presented before actual movement, the predicted direction in the medial intraparietal cortex was congruent with the actual movement in the body-centered coordinates, although the averaged signal intensities were not significantly different between two movement directions. Our results indicate that the human medial intraparietal cortex uses body-centered coordinates to encode target position or movement directions, which are crucial for visually guided movements.