Introduction
Single unit activity was recorded chronically from motor and premotor cortical activity in rhesus monkeys trained to reach for targets and to draw in free space (3D). The activity patterns from many recorded cells were combined to form population vectors that closely matched the trajectory of the moving arm. The cortical activity reflected a number of behavioral invariants with a timing that suggests that these movement properties may result from constraints of CNS processing.
Methods
Cortical cells were recorded with microelectrodes inserted through the dura on a daily basis. Animals were trained to perform a variety of arm movements in a virtual reality environment. They viewed a stereoscopic display of 3D targets to reach for or tubular shapes to draw. The 3D positions of their hand and all segments of the arm were recorded with an optoelectric device. Reaching movements were to a set of 8 targets arranged at the corners of a cube from a center start position (center->out task). The objects drawn were circles, ellipses and figure-eights in different planes.
Results
Population vectors were constructed using directional tuning parameters derived from the center->out task applied to data collected in the drawing tasks. Activity characteristics were consistent and robust across tasks. The population vectors accurately predict the hand's velocity on a continuous and instantaneous basis. The prediction interval or time between cortical representation and movement varied inversely with the radius of curvature in the drawing.
Conclusions
Cortical activity is highly correlated with hand velocity in these tasks. The dynamics of this activity suggest that the characteristic invariants of arm movement may be due to constraints of cortical processing.
References
Schwartz, A.B. and Moran, D.W.: Arm trajectory and representation of movement processing in motor cortical activity. Eur. J. Neurosci. 12: 2000
Moran, D.W. and Schwartz, A.B.: Motor cortical representation of speed and direction during reaching. J. Neurophysiol. 82: 2676-2692, 1999.
Moran, D.W. and Schwartz, A.B.: Motor cortical activity during drawing movements: Population response during spiral tracing, J. Neurophysiol. 82: 2693-2704, 1999.
Schwartz, A.B. and Moran, D.W.: Motor cortical activity during drawing movements: Population response during lemniscate tracing, J. Neurophysiol 82: 2705-2718, 1999.