Cortical mechanisms of goal-directed motor acts in the rhesus monkey.

Multineuronal activity from the frontal cortex (s. principalis) and the motor cortex (g. precentralis) in the Rhesus monkey during the goal-directed performance was recorded. The experimental sequence comprised a chain of signals securing adequate behavioral performance. The sequence began with an anticipatory signal, then the conditioned signal was switched on, a lamp in either the right or the left key, and after a 5-10 s delay, a trigger signal opening a screen followed. Then the monkey had to press the key indicated by light and grasp the food from a box. Differences in the average firing rate and in the patterns of response were observed by comparing the unit activity during the CS expectancy with the trigger signal expectancy (delay). A mathematical treatment of the unit firing pattern allowed us to select specific firing pattern rearrangements likely to be involved in the short-term memory mechanisms, i.e. intracortical reverberation and successive recruitment of neuronal populations. Unit activity of s. principalis in one hemisphere depended on the CS right vs. left location. Similar differences were found for the delays. During the delay, information about the CS spatial location is probably stored as short-term traces in the units of s. principalis. The frontal cortex unitary responses elicited by the trigger signal depended on the direction of the subsequent movement. The motor cortex units provided less pronounced differences in .response to the trigger. In contrast to the motor cortex units, some frontal cortex units reacted only to the presence of food reward.