Temporal relation of population activity in visual areas MT/MST and in primary motor cortex during visually guided tracking movements.

There is growing evidence that in primate cerebral cortex the areas along the 'dorsal pathway' are involved in the transformation of visual motion information towards a motor command. To pursue this cortical flow of information from visual motion areas to the motor cortex, single-cell activity was recorded from visual areas MT/MST (middle temporal area/medial superior temporal area) and from primary motor cortex (M1) while monkeys tracked moving targets with their right hand. Spike activity of 353 directionally tuned motor cortex cells was combined to a time-varying population vector, and similarly a time-resolved visual population vector was calculated from 252 MT/MST cells. Both population vectors code faithfully for the direction of the collinear motion of target and hand. For a given direction, the length of the population vectors varied over time during the performance of the task. The temporal evolution of both population responses reflects the different relationship between the early visual responses to the moving target and the directional motor command controlling the hand movement. The results indicate that during the visual tracking task visual and motor populations which code for similar directions of movement are co-activated with considerable temporal overlap. Despite this co-activation in both modalities, we failed to observe any significant synchronization between areas MT/MST and M1.