Neuronal activity was recorded from the premotor cortex (PM) of Japanese monkeys while they performed hand movements with different amplitudes and directions. On each behavioral trial, two instructions were given sequentially: 1) an amplitude instruction (large or small) and 2) a direction instruction (flexion or extension). The onset of movement was triggered by a visual signal after a delay period. 2. Among various kinds of task-related neuronal activity recorded in the PM, two types were selected for study: 1) set-related activity, sustained activity change during the delay period that followed presentation of instruction signals (IS); and 2) movement-related activity, activity change immediately before and during movement, which followed the trigger signal (TS) presentation. 3. Thirty-two of 101 set-related neurons showed activity change after presentation of the first IS (Delay 1 set-related activity), when they were instructed in either amplitude or direction, but not both. All of the set-related neurons showed activity modulation after presentation of the second IS (Delay 2 set-related activity). When neurons showed both Delay 1 and Delay 2 set-related activity, they were usually more active during Delay 2, i.e., when the monkeys had received both amplitude and directional ISs. A majority of neurons with Delay 2 set-related activity (64%) showed relation to both movement amplitude and direction. Twenty-eight percent of the neurons showed relation to either amplitude or direction, but not both. These findings seem consistent with a view that serial, rather than parallel, processes of motor programming operate in preparation of intended movements. 4. A majority of PM neurons with movement-related activity (51%) showed activity change related to both the direction and amplitude of movement. Forty-two percent showed selective relation to either direction or amplitude. These findings support a view that PM contributes to the control of limb movements. 5. Histological reconstruction showed that a vast majority of PM set-related neurons were located in the dorsal aspect of the PM (PMd), medial to the arcuate spur and lateral to the superior precentral sulcus. In contrast, movement-related neurons were distributed in two distinct foci: one in the ventral aspect of the PM (PMv), immediately caudal to the genu of the arcuate sulcus and lateral to the spur of the sulcus; and the other in the PMd, overlap;ing the location of set-related neurons.(ABSTRACT TRUNCATED AT 400 WORDS)
