How the intention to act results in movement is a fundamental question
of brain organization. Recent work has shown that this operation invo
lves the cooperative interaction of large neuronal populations. A popu
lation vector method, by transforming neuronal activity to the spatial
domain, was used to visualize the motor cortical representation of th
e hand's trajectory made by rhesus monkeys as they drew spirals. Hand
path was accurately reflected by a series of population vectors calcul
ated throughout the task. A psychophysical rule relating speed to curv
ature, the ''power law,'' was found in this cortical representation. T
he relative timing between each population vector and the correspondin
g portion of the movement was variable. The population vectors only pr
eceded the movement in a predictive manner in portions of the spiral w
here the radius of curvature was greater than 6 centimeters. These res
ults show that the movement trajectory is an important determinant of
motor cortical activity and that this aspect of motor cortical activit
y may contribute only to discrete portions of the drawing movement.