Motor cortical neuronal activity patterns in monkeys performing several force tasks at the ankle

Normal rhesus monkeys were conditioned at light signals to exert forces with both feet on fixed foot bars. The taks included small and large sustained forces in plantar and dorsiflexion and large phasic forces in both directions. The tasks were selected to relate to behaviors known to be impaired in the upper motor neuron syndrome. Extracellular recordings were made from 226 area 4 motor cortex units (MCUs) in the hindlimb region of area 4. Ninety percent of the MCUs showed increased activity temporally related to force and EMG changes; 10% showed decreased activity exclusively. The 20 MCUs showing increased activity differed in their directional preference: half were active only in relation to development of force in one direction (undirectional) and the remainder with forces in both directions (bidirectional). Only 28/101 of the bidirectional units were symmetric (equal activity with force in both directions). The majority were asymmetric with a greater degree of activity in one direction. Both unidirectional and bidirectional (symmetric and asymmetric) MCUs often developed increased activity not only with agonist force production but also with force relaxation in the antagonist muscles. Seventy-eight percent of the MCUs showing increased activity had phasic discharge qualities, lacking sustained activity during the prolonged force holds. Thirty-eight percent of the MCUs showing increased activity were more active with larger force, and 14% with smaller force; 48% had closely similar activity with both force levels. None of these characteristics was related to directionality. A spectrum of MCU behaviors was found that ranged in complexity from units which increased or decreased discharge with force in one direction to those responding with force production and force relaxation in both directions.