MOTOR CORTICAL ACTIVITY PRECEDING A MEMORIZED MOVEMENT TRAJECTORY WITH AN ORTHOGONAL BEND

Two monkeys were trained to make an arm movement with an orthogonal be nd, first up and then to the left (inverted left perpendicular), follo wing a waiting period. They held a two-dimensional manipulandum over a spot of light at the center of a planar working surface. When this li ght went off, the animals were required to hold the manipulandum there for 600-700 ms and then move the handle up and to the left to receive a liquid reward. There were no external signals concerning the ''go'' time or the trajectory of the movement. It was hypothesized that duri ng that period signs of directional processing relating to the upcomin g movement would be identified in the motor cortex. Following 20 trial s of the memorized movement trajectory, 40 trials of visually triggere d movements in radially arranged directions were performed. The activi ty of 137 single cells in the motor cortex was recorded extracellularl y during performance of the task. It was found that 62.8% of the cells changed activity during the memorized waiting period. During the wait ing period, the population vector (Georgopoulos ct al. 1983, 1984) beg an to grow approximately 130 ms after the center light was turned off; it pointed first in the direction of the second part of the memorized movement (<--) and then rotated clockwise towards the direction of th e initial part of the movement (up). These findings indicate processin g of directional information during the waiting period preceding the m emorized movement. This conclusion was supported by the results of exp eriments in ten human subjects, who performed the same memorized movem ent (inverted left perpendicular). In 10% of the trials a visual stimu lus was shown in radially arranged directions in which the subjects ha d to move; this stimulus was shown at 0, 200, and 400 ms from the time the center light was turned off. We found that as the interval increa sed the reaction time shortened for the visual stimulus that was in th e same direction as the upward component of the memorized movement.