Temporal Information Processing in Prefrontal-Basal Ganglia Circuit

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We have been conducting neurophysiological experiments to reveal neuronal mechanisms for temporal information processing in the prefrontal-basal ganglia circuit. Monkeys were trained to perform a duration discrimination task. In the task, two visual cues were presented successively in different duration (0.2〜1.6 sec). Each cue presentation was followed by a 1-sec delay period. The subjects were instructed to choose a longer presented cue after the second delay period. Single-unit activity was recorded from the prefrontal cortex and the striatum while the monkeys were performing the task.Prefrontal neurons showed phasic activity during the first cue period. The activity had peak time around 800ms that was close to the midpoint between long and short cue duration. This activity might have functioned for filtering cue duration to discriminate between long and short cues. In the first delay period prefrontal neurons exhibited differential activity following long and short cues. In the neurons with preference for long cues the delay activity after longer cues was uniformly high regardless of cue duration, but that after shorter cues was uniformly low. This binary change in the delay activity indicates that the prefrontal cortex could represent cue duration categorically as long or short. Striatal neurons phasically responded during the first and second delay periods. Responses during the first delay period changed depending on the first cue duration. Activity of populations of these neurons correlated with the first cue duration positively or negatively. Responses during the second delay period differed between long-short and short-long trials. These findings suggest that striatal neurons could encode cue duration with monotonically changing responses and discrimination results between the two cue durations.