TEMPORAL GATING OF NEURAL SIGNALS DURING PERFORMANCE OF A VISUAL-DISCRIMINATION TASK

The flow of neural signals within the cerebral cortex must be subject to multiple controls as behaviour unfolds in time. In a visual discrim ination task that includes a delay period, the transmission of sensory signals to circuitry that mediates memory, decision-making and motor- planning must be governed closely by 'filtering' or 'gating' mechanism s so that extraneous events occurring before, during or after presenta tion of the critical visual stimulus have little or no effect on the s ubject's behavioural responses. Here we study one such mechanism physi ologically by applying electrical microstimulation(1-3) to columns of directionally selective neurons in the middle temporal visual area(4-9 ) at varying times during single trials of a direction-discrimination task. The behavioural effects of microstimulation varied strikingly ac cording to the timing of delivery within the trial, indicating that si gnals produced by microstimulation may be subject to active 'gating'. Our results show several important features of this gating process: fi rst, signal flow is modulated upwards on onset of the visual stimulus and downwards, typically with a slower time course, after stimulus off set; second, gating efficacy can be modified by behavioural training; and third, gating is implemented primarily downstream of the middle te mporal visual area.