REACHING MOVEMENTS WITH SIMILAR HAND PATHS BUT DIFFERENT ARM ORIENTATIONS .2. ACTIVITY OF INDIVIDUAL CELLS IN DORSAL PREMOTOR CORTEX AND PARIETAL AREA-5

Neuronal activity in primary motor cortex (MI) is altered when monkeys make reaching movements along similar handpaths at shoulder level wit h two different arm orientations, either in the natural orientation wi th the elbow positioned below the level of the shoulder and hand or in an abducted orientation with the elbow abducted nearly to shoulder le vel. The present study examines to what degree two other cortical area s, the dorsal premotor (PMd) and parietal area 5, also show modulation of cell activity related to arm geometry during reaching. The activit y of most (89%) of the 207 cells in PMd recorded while monkeys made re aching movements showed a statistically significant change in activity between orientations [analysis of variation (ANOVA), P < 0.01]. A com mon effect of arm orientation on cell activity was a change in the ove rall level of discharge either before, during, and/or after movement ( 67%, ANOVA, task main effect, P < 0.01). Many cells (76%) showed a sta tistical change in their response to movement direction (ANOVA, task x direction interaction term, P < 0.01), including changes in dynamic r ange and changes in the preferred direction of cells that were directi onally tuned in both arm orientations. Overall, these effects were sim ilar qualitatively but not as strong quantitatively as those observed in hn. A sample of cells was recorded in area 5 of one monkey. Most (9 5%) of the 79 area 5 cells showed a change in activity when reaching m ovements were performed using different arm orientations (ANOVA, P < 0 .01). As in PMd and MI, many area 5 cells (56, 71%) showed changes in their tonic discharge before, during, and/or after movement, and 70 ce lls (89%) showed changes in their response to movement direction (ANOV A, task x direction interaction term, P < 0.01). The observed changes in neuronal activity related to posture and movement in MI, PMd and ar ea 5 demonstrate that single-cell activity in these cortical areas is not simply related to the spatial attributes of hand trajectory but is also strongly influenced by attributes of movement related to arm geo metry.