The response of MSTd neurons to perturbations in target motion during ongoing smooth-pursuit eye movements

Several regions of the brain are involved in smooth-pursuit eye movement (SPEM) control, including the cortical areas MST (medial superior temporal) and FEF (frontal eye field). It has been shown that the eye-movement responses to a brief perturbation of the visual target during ongoing pursuit increases with higher pursuit velocities. To further investigate the underlying neuronal mechanism of this nonlinear dynamic gain control and the contributions of different cortical areas to it, we recorded from MSTd (dorsal division of the MST area) neurons in behaving monkeys (Macaca mulatta) during step-ramp SPEM (5-20 degrees /s) with and without superimposed target perturbation (one cycle, 5 Hz, +/-10 degrees /s). Smooth-pursuit-related MSTd neurons started to increase their activity on average 127 ms after eye-movement onset. Target perturbation consistently led to larger eye-movement responses and decreasing latencies with increasing ramp velocities, as predicted by dynamic gain control. For 36% of the smooth-pursuit-related MSTd neurons the eye-movement perturbation was accompanied by detectable changes in neuronal activity with a latency of 102 ms, with respect to the eye-movement response. The remaining smooth-pursuit-related MSTd neurons (64%) did not reflect the eye-movement perturbation. For the large majority of cases this finding could be predicted by the dynamic properties of the step-ramp responses. Almost all these MSTd neurons had large visual receptive fields responding to motion in preferred directions opposite to the optimal SPEM stimulus. Based on these findings it is unlikely that MSTd plays a major role for dynamic gain control and initiation of the perturbation response. However, neurons in MSTd could still participate in SPEM maintenance. Due to their visual field properties they could also play a role in other functions such as self-motion perception.