Effects of Optic Flow in Motor Cortex and Area 7a
Moving visual stimuli were presented to behaving monkeys who fixated their eyes and did not move their arm. The stimuli consisted of random dots moving coherently in eight different kinds of motion (right, left, up, downward, expansion, contraction, clockwise, and counterclockwise) and were presented in 25 square patches on a liquid crystal display projection screen. Neuronal activity in the arm area of the motor cortex and area 7a was significantly influenced by the visual stimulation, as assessed using an ANOVA. The percentage of cells with a statistically significant effect of visual stimulation was 3 times greater in area 7a (370/587, 63%) than in motor cortex (148/693, 21.4%). With respect to stimulus properties, its location and kind of motion had differential effects on cell activity in the two areas. Specifically, the percentage of cells with a significant stimulus location effect was ∼2.5 times higher in area 7a (311/370, 84%) than in motor cortex (48/148, 32.4%), whereas the percentage of cells with a significant stimulus motion effect was ∼2 times higher in the motor cortex (79/148, 53.4%) than in area 7a (102/370, 27.6%). We also assessed the selectivity of responses to particular stimulus motions using a Poisson train analysis and determined the percentage of cells that showed activation in only one stimulus condition. This percentage was 2 times higher in the motor cortex (73.7%) than in area 7a (37.7%). Of all kinds of stimulus motion tested, responses to expanding optic flow were the strongest in both cortical areas. Finally, we compared the activation of motor cortical cells during visual stimulation to that observed during force exertion in a center→ out task. Of 514 cells analyzed for both the motor and visual tasks, 388 (75.5%) showed a significant relation to either or both tasks, as follows: 284/388 (73.2%) cells showed a significant relation only to the motor task, 27/388 (7%) cells showed a significant relation only to the visual task, whereas the remaining 77/388 (19.8%) cells showed significant relations to both tasks. Therefore a total of 361/514 (70.2%) cells were related to the motor task and 104/514 (20.2%) were related to the visual task. Finally, with respect to receptive fields (RFs), there was no clear visual receptive field structure in the motor cortical neuronal responses, in contrast to area 7a where RFs were present and could be modulated by the type of optic flow stimulus.