Contrasting properties of monkey somatosensory and motor cortex neurons activated during the control of force in precision grip

1. Single cell activity was investigated in the precentral motor (MI) and postcentral somatosensory (SI) cortex of the monkey to compare the neuronal activity related to the control of isometric force in the precision grip and to assess the participation of SI in motor control. 2. Three monkeys (Macaca fascicularis) were trained in a visual step-tracking paradigm to generate and precisely maintain force on a transducer held between thumb and index finger. Great care was taken to have the monkeys use only their fingers without moving the wrist or proximal joints. In two monkeys electromyographic (EMG) activity was checked in 23 muscles over several sessions. 3. Five similar classes of task-related firing patterns were found in both SI and MI cortical hand and finger representations, but their relative proportions differed. The majority of the SI neurons were phasically or phasic-tonically active (61%), whereas in MI the neurons that decreased their firing rate with force were most frequent (42%). 4. The timing of activity changes related to the onset of force increase from low to higher levels strongly differed in the two neuronal populations. In SI, only 14% of the task-related neurons increased or decreased their firing rate before the onset of force increase, in contrast to 56% in MI. Only 3% of the SI neurons showed changes before the earliest EMG activation. 5. In both SI and MI neurons with tonic and phasic-tonic, increasing or decreasing discharge patterns disclosed a relationship between neuronal activity and static force. Distinction was made between neurons modulating their activity in a monotonic way and those that were active only at one force level and had a kind of recruitment or deactivation threshold. The latter ones were more frequent in MI than in SI, and in the neuron population with decreasing firing patterns. For the neurons with increases in activity, statistically significant linear correlations between firing rate and force were found more frequently in MI than in SI, where the proportion of nonsignificant correlations was relatively high (35% vs. 15% in MI). In SI the indexes of force sensitivity, calculated from the slopes of the regression lines, covered a wider range than in MI; and their distribution was bimodal, with one mean of 30 Hz/N and the other of 155 Hz/N. In contrast, the mean rate-force slope in MI was 69 Hz/N.(ABSTRACT TRUNCATED AT 400 WORDS)