MOTOR CORTICAL ACTIVITY DURING DRAWING MOVEMENTS - POPULATION REPRESENTATION DURING SINUSOID TRACING

1. Monkeys were trained to trace sinusoids with their index fingers on a planar surface. During this task, both the direction and speed of m ovement varied continuously. Activity of individual units in the prece ntral gyrus contralateral to the moving arm was recorded as the task w as performed. These cells responded to passive movement of the shoulde r and/or elbow. The relation between discharge rate and movement direc tion for these individual cells could be described with a cosine tunin g function. 2. Data recorded as the sinusoid was traced were divided i nto 100 bins as each cell was studied during the experiment. In each b in, the activity of a particular cell was represented by a vector. The vector (''cell vector'') pointed in the direction of finger movement that corresponded to the highest rate of neuronal discharge. This dire ction, referred to as the preferred direction, corresponded to the pea k of the cosine tuning function. The direction of the vector was const ant between bins, but the magnitude of this cell's vector was a functi on of the instantaneous discharge rate. 3. This cell vector is a hypot hetical contribution of a single cell to the population response compr ised of 554 similarly derived vectors from different cells. The popula tion response was represented as the vector that resulted from forming the sum of the vector contributions from the individual cells. A sepa rate calculation was made for each bin, resulting in 100 population ve ctors for each sinusoid. 4. Within a given time series of population v ectors, their lengths and directions varied in a consistent relation t o the tangential velocity of the drawing movement. Therefore these vec tors are representative of the movement trajectory. Each increment of a trajectory was predicted by a population vector that preceded it by approximately 120 ms. These findings suggest that trajectory informati on is encoded in an ongoing manner in the motor cortex using a relativ e coordinate system that moves in conjunction with the finger.