Covariation of primate dorsal premotor cell activity with direction and amplitude during a memorized-delay reaching task

Extensive behavioral evidence suggests that the direction and amplitude of reaching movements are planned as two independent parameters by the motor system. However, whereas direction-related activity has been well documented by neurophysiological studies in many motor structures including the dorsal premotor cortex (PMd), there is much less concensus about the prominence and timing of amplitude-related premotor activity. We studied this issue using an instructed-delay task in which prior information about target location (direction and distance) must be memorized before movement initiation. The results show that prior information about distance is reflected in PMd activity during the delay period well before movement initiation, and begins to be expressed as early as 150 ms after presentation of target location. The prominence of neural correlates with direction is relatively constant throughout the trial, but distance correlates become gradually more prominent with time, both during and after the delay period. A small majority of cells were modulated only by direction during the delay period, but very few were modulated only by distance, and most of the rest were modulated by both. Therefore PMd neurons usually process information about distance only in conjunction with directional information. These results do not support a separate neuronal substrate for distance in PMd, but do not preclude its existence elsewhere. The results also support a progressive change in the nature of the movement-related representation in PMd with time in an instructed-delay paradigm.