Coordinate system representations of movement direction in the premotor cortex

Recently, we compared the encoding of movement direction in primary motor cortex (MI) under three coordinate systems, an extrinsic cartesian (CA), an intrinsic joint angle (JA), and an intermediate shoulder-centered (SC) and found that no single coordinate system was uniquely represented in MI. Here we extend this investigation to the premotor area which is regarded as a major source of input to the MI. Multiple single units were simultaneously recorded from the caudal portions of dorsal premotor (PMd) and ventral premotor (PMv) cortices as a behaving monkey moved its hand over the horizontal work space. We compared the mutual information between the neuronal firing rate of each unit and hand movement direction under each of the three coordinate systems. As in the MI study, the results showed that there was no evidence for a unique coordinate system representation. There was a bias towards the SC system in PMd, albeit weaker than the SC bias observed in MI. In PMv, there was a strong bias for both SC and JA coordinate systems as compared to a CA coordinate system. These results provided further evidence against a unique coordinate system for movement direction within the motor cortex. However, the directional information within both PMd and PMv was significantly less than that of MI suggesting that factors other than the detailed motion of the arm account for much of the variance of activity within the premotor cortex.