Static versus dynamic effects in motor cortex and area 5: Comparison during movement time
We examined the directionality of static and dynamic motor effects on cell discharge recorded during arm movements in motor cortex and area 5. Movements of 8 directions were performed by rhesus monkeys in a two-dimensional space ('movement task')3. Static effects were determined using a static hold task in the same workspace ('static task'); they were adequately described as planar activity surfaces in 78124 (63%) and 63105 (60%) of motor cortical and area 5 cells, respectively, that showed statistically significant changes in steady-state activity associated with holding in different positions4. The frequency of discharge expected during the movement time on the basis of the static effect alone was calculated for every trial using the static plane equation and the movement path. The difference between this value and the discharge rate observed in the movement task during the same period of time was taken as an estimate of the contribution of non-static, i.e. dynamic factors. Thus static and dynamic tuning curves were generated. These curves were compared with respect to depth of tuning and spatial congruence. The former was estimated by taking the difference between the maximum and minimum of the curve, and the latter by measuring the correlation between the static and dynamic curves. We found the following. First, the dynamic tuning curves were more deeply tuned than the corresponding static ones, by 1.51 and 1.59 times in motor cortex and area 5, respectively. This result was statistically significant in both areas (paired t-test, P < 0.001). Second, the depth of dynamic and static tuning correlated significantly in both areas, but more strongly in the motor cortex (r = 0.667, n = 78, P < 0.001) than in area 5 (r = 0.38, n = 63, P < 0.01). Finally, the static and dynamic curves were significantly correlated (P < 0.05) in 3278 (41.0%) and 2863 (44.4%) of motor cortical and area 5 cells, respectively. However, both positive and negative correlations were observed.