CORTICOCORTICAL AND THALAMOCORTICAL RESPONSES OF NEURONS IN THE MONKEY PRIMARY MOTOR CORTEX AND THEIR RELATION TO A TRAINED MOTOR TASK

1. We studied the responsiveness of neurons in the primary motor corte x (MI) of monkeys (Macaca fuscata) to electrical stimulation of the su pplementary motor area (SMA), primary sensory cortex (SI), and the ven tral subnucleus of the thalamus (VPLo) with chronically implanted elec trodes. 2. All neurons examined in this study were characterized by th eir relation to a motor task performed by the animals. They responded to stimulation of the cortical or thalamic area with excitation from o ne area alone (n = 128) or from multiple areas (n = 84) of all combina tions. In a majority of neurons, response latencies to both cortical a nd thalamic stimulation were within 5 ms. 3. A vast majority of neuron s (80%) that were active during a preparatory period for forthcoming r eaching movements were activated by SMA stimulation. They were activat ed only infrequently by SI or thalamic stimulation. 4. Movement-relate d neurons(active immediately before and during reaching movements) wer e activated by thalamic, SI, or SMA stimulation or by any combination of those stimuli. More than half of the movement-related neurons activ ated exclusively by either thalamic or SMA stimulation exhibited activ ity onset times earlier than those observed in the earliest muscles. B y contrast, most movement-related neurons that responded only to SI st imulation were late in their activity onset. 5. These findings suggest that the SMA input to MI is important in developing a preparatory typ e of activity in MI, whereas the thalamus (VPLo) provides substantial inputs in movement execution. The roles played by inputs from SI and S MA in relation to motor execution an debatable and are discussed here with reference to previous reports.