Encoding of target direction and speed during visual instruction and arm tracking in dorsal premotor and primary motor cortical neurons

The encoding of direction and speed in the discharge of dorsal premotor (PM d) and primary motor (MI) neurons was studied during two-dimensional visual ly-instructed pursuit arm movements in which eight directions and four cons tant speeds were independently manipulated. Each trial consisted of equal d urations of visual observation of target movement without hand movement (cu e) and visual pursuit-tracking of the target with the hand (track). A total of 240 neurons was recorded from PMd and MI in two Macaca mulatta monkeys. Two classes of regression analyses were used to relate neuronal firing dur ing the cue and track periods to direction and speed. First, the average fi ring from each period was fitted to target direction or speed. Period-avera ged firing significantly correlated with direction more frequently in the t rack than in the cue period. Conversely, correlations with speed (with or w ithout direction) were more common in the cue than in the track period. Sec ondly, a binwise regression evaluated the temporal evolution of firing corr elations with direction and speed. Supporting the period-based results, sig nificant binwise correlations of the discharge with speed occurred preferen tially during the cue period when there was no hand movement. Prior to move ment, correlations of the firing with direction became significant and cont inued through the movement. Both analyses demonstrated a distinct tendency for neurons to be modulated by speed information early and by direction inf ormation later. This temporal parcellation reflects both the sequential dem ands of the task and constraints placed on the neural computations. The ear ly representation of target speed is hypothesized to reflect the need to ca lculate a 'go signal' for the initiation of movement.