Neural representations of speed during smooth pursuit eye movements

To compute object motion during a smooth pursuit eye movement, retinal image motion signals must be combined with information about movements of the eye. Cells in area MST respond to retinal image motion and pursuit eye movements alone; some cells combine motion and movement signals (Erickson and Thier, 1991). We wished to examine the combination of these signals and compare the data with models of visuomotor integration. We recorded units from presumptive MST in 2 rhesus macaques trained to fixate or pursue small visual targets. We measured speed tuning along the preferred-null axis of direction-selective MST neurons for random texture stimuli presented during both fixation and pursuit. Receptive field stimulation was kept constant across eye movement conditions. Most cells' visual responses were modified during pursuit. We fit speed tuning curves with descriptive functions that allowed us to compare different models of response change during pursuit. Most cells changed the gain of their responses during pursuit, but their selectivity for retinal speed was unchanged. Some cells showed both a gain change and a shift in speed tuning, but the shifts in speed tuning were usually not simply related to the speed of pursuit movements. We found no cells showing pure shifts in speed tuning that exactly compensated for the pursuit movements. Our findings are consistent with other reports of gain changes involved in coordinate transformations (Salinas and Thier, 2000). We suggest that area MST is involved in combining visual and movement information to support the perception of motion under natural viewing conditions.