Consistent features in the forelimb representation of primary motor cortexin rhesus macaques

The purpose of this study was to systematically map the forelimb area of pr imary motor cortex (M1) in rhesus macaques in an effort to investigate furt her the organization of motor output to distal and proximal muscles. We use d stimulus-triggered averaging (StTAing) of electromyographic activity to m ap the cortical representation of 24 simultaneously recorded forelimb muscl es. StTAs were obtained by applying 15 muA stimuli to M1 sites while the mo nkey performed a reach and prehension task. Motor output to body regions ot her than the forelimb (e.g., face, trunk, and hindlimb) was identified usin g repetitive intracortical microstimulation to evoke movements. Detailed, m uscle-based maps of M1 revealed a central core of distal (wrist, digit, and intrinsic hand) muscle representation surrounded by a "horseshoe"-shaped z one of proximal (shoulder and elbow) muscle representation. The core distal and proximal zones were separated by a relatively large region representin g combinations of both distal and proximal muscles. On the basis of its siz e and characteristics, we argue that this zone is not simply the result of stimulus-current spread, but rather a distinct zone within the forelimb rep resentation containing cells that specify functional synergies of distal an d proximal muscles. Electrode tracks extending medially from the medial arm of the proximal muscle representation evoked trunk and hindlimb responses. No distal or proximal muscle poststimulus effects were found in this regio n. These results argue against the existence of a second, major noncontiguo us distal or proximal forelimb representation located medially within the m acaque M1 representation.