Dorsal premotor areas of nonhuman primate: functional flexibility in time domain.

A voluntary motor act requires recognition of the informational content of an instruction. An instruction may contain spatial and temporal information. The recently proved role of the monkey frontal cortex in time computation, as well as in motor preparation and motor learning, suggested that we investigate the relationship between premotor neuron discharges and the temporal feature of the visual instructions. To this purpose, we manipulated the duration of an instructional cue in a visuomotor task while recording unit activity. We found two types of premotor neurons characterised by a discharge varying in relation to the duration of the cue: (1) "motor-linked" neurons, with a specific premotor activity constantly bounded to the motor act; (2) "short-term encoders" neurons, with a premotor activity depending on the cue duration. The cue duration was the critical factor in determining the behaviour of the short-term encoders cells: when the cue ranged from 0.5 s to 1 s, they presented a preparatory activity; when the cue was longer, up to 2 s, they lost their preparatory activity; when the cue was blinked the cells anticipated their discharge. The activity changed in few trials. These data confirm and highlight the role of frontal cortex in encoding specific cues with a temporal flexibility, which may be the expression of temporal learning and represent an extended aspect of cortical plasticity in time domain.