Representation of accuracy in the dorsal premotor cortex

The endpoint accuracy of a reaching movement strongly affects kinematics, p articularly during the final phases of movement. However, where and how acc uracy is represented in the central nervous system remains unknown. In this study, the discharge of 150 neurons located primarily in the dorsal premot or cortex (PMd), were recorded from monkeys performing an instructed delay, centre-out reaching task in which movement direction and target size were systematically varied. Linear regression analyses were used to assess the d ependence of movement kinematics and cell discharge on target direction, si ze and tangential velocity (i.e. speed). The speed and timing of the moveme nt were dependent on both direction and target size. Initially direction wa s the dominant predictor whilst target size became more important as the ha nd reached the target. A temporal multiple linear regression analysis found significant correlations with target size in 99 of 150 cells. The discharg e of 134 cells was directionally tuned and 83 cells modulated with mean spe ed. Significant correlations of discharge with target size occurred through out the task as did correlations with direction. However, correlations with direction preferentially occurred early in the task, prior to movement ons et, whilst correlations with target size tended to occur late, well after m ovement onset. This temporal dependency of the firing in relationship to ta rget direction and size mirrored that observed for the kinematics. We concl ude that the discharge of PMd cells is highly correlated with the accuracy requirement of the movement. The timing of the correlations suggest that ac curacy information is available for the planning and for the on-line contro l of endpoint accuracy.