Parallel updating and weighting of multiple spatial maps for visual stability during whole body motion.

It is known that the brain uses multiple reference frames to code spatial information, including eye-centered and body-centered frames. When we move our body in space, these internal representations are no longer in register with external space, unless they are actively updated. Whether the brain updates multiple spatial representations in parallel, or whether it restricts its updating mechanisms to a single reference frame from which other representations are constructed, remains an open question. We developed an optimal integration model to simulate the updating of visual space across body motion in multiple or single reference frames. To test this model, we designed an experiment in which participants had to remember the location of a briefly presented target while being translated sideways. The behavioral responses were in agreement with a model that uses a combination of eye- and body-centered representations, weighted according to the reliability in which the target location is stored and updated in each reference frame. Our findings suggest that the brain simultaneously updates multiple spatial representations across body motion. Because both representations are kept in sync, they can be optimally combined to provide a more precise estimate of visual locations in space than based on single-frame updating mechanisms.

Gaze is driven by an internal goal trajectory in a visuomotor task.

When we make hand movements to visual targets, gaze usually leads hand position by a series of saccades to task-relevant locations. Recent research suggests that the slow smooth pursuit eye movement system may interact with the saccadic system in complex tasks, suggesting that the smooth pursuit system can receive non-retinal input. We hypothesise that a combination of saccades and smooth pursuit guides the hand movements towards a goal in a complex environment, using an internal representation of future trajectories as input to the visuomotor system. This would imply that smooth pursuit leads hand position, which is remarkable, as the general idea is that smooth pursuit is driven by retinal slip. To test this hypothesis, we designed a video-game task in which human subjects used their thumbs to move two cursors to a common goal position while avoiding stationary obstacles. We found that gaze led the cursors by a series of saccades interleaved with ocular fixation or pursuit. Smooth pursuit was correlated with neither cursor position nor cursor velocity. We conclude that a combination of fast and slow eye movements, driven by an internal goal instead of a retinal goal, led the cursor movements, and that both saccades and pursuit are driven by an internal representation of future trajectories of the hand. The lead distance of gaze relative to the hand may reflect a compromise between exploring future hand (cursor) paths and verifying that the cursors move along the desired paths.