Direction-specific activities of dorsolateral prefrontal and motor cortex pyramidal tract neurons during visual tracking.

Abstract

1. With two rhesus monkeys, activities of dorsolateral prefrontal cortex neurons (yt = 102) and rapidly conducting pyramidal tract neurons (PTNs) of the hand-arm motor area (n = 49) were compared for three kinds of single-step visual-tracking tasks. The manipulandum was a vertical handle, which the monkeys rotated 24’ from a central hold zone to a target hold zone by wrist flexion or extension. The first task was a delayed-response task (DR) in which the direction of the movement, that is, flexion or extension, was indicated by visual cues but was triggered, after a short delay period, by a visual go signal. The second task was a choice task (CT) in which wrist flexion or extension was triggered, after a short hold period, by both a go signal and a visual signal for direction of movement. The third task was a simple control task (ST) in which the same movement, either flexion or extension, was performed repeatedly after a go signal. 2. Only neurons with rate increase related to wrist movement were sampled. Two kinds of discharge patterns were differentiated, called bidirectional and unidirectional. When the rate increase was observed before and/ or during task movement, regardless of the direction of the movement, the pattern was bidirectional. When the rate increase occurred only for one direction (flexion or extension), with or without decrease for the other direction, the pattern was unidirectional. 3. During delayed responses 59 prefrontal neurons showed bidirectional activity and 43 neurons showed unidirectional activity before and during task movement. About half of these prefrontal neurons showed a gradually developing rate change in the later phase of the delay period, either bidirectionally or unidirectionally. It started about 0.31.2 s earlier than the go signal. PTNs showed similar changes. Thirty-two PTNs were activated unidirectionally and 18 PTNs were activated bidirectionally. About one-fourth of the PTNs showed a gradually developing rate change in the later phase of the delay period. Its temporal course was similar to that of prefrontal neurons. 4. During a choice task occurrences of activations of both prefrontal neurons and PTNs before and during movement were less frequent than during the delayed-response tasks. Gradually developing activation of prefrontal neurons also occurred with less frequency. Gradually developing activation of PTNs was not seen. 5. During simple control tasks gradually developing activation before the go signal persisted. 6. It is concluded that prior to voluntary movement there is neuronal activity specific to movement direction, in addition to nonspecific activity, in the prefrontal cortex. Movement-related activities may be involved in the effective execution of goal-directed voluntary movement.