Evidence from brain imaging studies has indicated involvement of the prefrontal cortex (PFC) in time perception; however, the role of this area remains unclear. To address this issue, we recorded single neuronal activity from the PFC of two monkeys while they performed a duration-discrimination task. In the task, two visual cues (a blue or red square) were presented consecutively followed by delay periods and subjects then chose the cue presented for the longer duration. Durations of both cues, order of cue duration [long-short (LS) or short-long (SL)] and order of cue colour (blue-red or red-blue) were randomized on a trial-by-trial basis. We found that subjects responded differently between LS and SL trials and that most prefrontal neurones showed significantly different activity during either the first or the second delay period when comparing activity in LS and SL trials. The present result offers new insights into neural mechanisms of time perception. It appears that, during the delay periods, the PFC contributes to implement a strategic process in temporal processing associated with a trial type (LS or SL) such as representation of the trial type, retention of cue information and anticipation of the forthcoming cue.

To clarify the roles of the basal ganglia in time perception, single-unit activity was recorded from both sides of the striatum of a monkey performing a duration discrimination task. In the task, two visual cues were presented successively in different durations (0.2 approximately 1.6 s). Each cue presentation was followed by a 1-s delay period. The subject was instructed to choose a longer presented cue after the second delay period. There were two types of trials for sequence of cue duration, the long-short (LS) trials in which the first cue (C1) was longer than the second cue (C2) and the short-long (SL) trials in which the C1 was shorter than the C2. Striatal neurons phasically responded during the first delay (D1) and second delay (D2) periods. Responses during the D1 period changed depending on C1 duration. Activity of populations of D1-response neurons correlated with C1 duration positively or negatively. Responses during the D2 period differed between the LS and SL trials. Activity of population of D2-response neurons also changed depending on C2 duration. But the dependence on C2 duration was affected by the trial type, that is, whether the C2 was longer or shorter compared to the C1. These findings suggest that striatal neurons could encode cue durations with monotonically changing responses in the D1 period and discrimination results between the two cue durations in the D2 period.

Neural imaging studies have revealed that the prefrontal cortex (PFC) participates in time perception. However, actual functional roles remain unclear. We trained two monkeys to perform a duration-discrimination task, in which two visual cues were presented consecutively for different durations ranging from 0.2 to 2.0 s. The subjects were required to choose the longer cue. We recorded single-neuron activity from the PFC while the subjects were performing the task. Responsive neurons for the first cue period were extracted and classified through a cluster analysis of firing rate curves. The neuronal activity was categorized as phasic, ramping and sustained patterns. Among them, the phasic activity was the most prevailing. Peak time of the phasic activity was broadly distributed about 0.8 s after cue onset, leading to a natural assumption that the phasic activity was related to cognitive processes. The phasic activity with constant delay after cue onset might function to filter current cue duration with the peak time. The broad distribution of the peak time would indicate that various filtering durations had been prepared for estimating C1 duration. The most frequent peak time was close to the time separating cue durations into long and short. The activity with this peak time might have had a role of filtering in attempted duration discrimination. Our results suggest that the PFC contributes to duration discrimination with temporal filtering in the cue period.

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Functional imaging and lesion studies in humans and animals suggest that the basal ganglia are crucial for temporal information processing. To elucidate neuronal mechanisms of interval timing in the basal ganglia, we recorded single-unit activity from the striatum of two monkeys while they performed a visual duration discrimination task. In the task, blue and red cues of different durations (0.2–2.0 sec) were successively presented. Each of the two cues was followed by a 1.0 sec delay period. The animals were instructed to choose the longer presented colored stimulus after the second delay period. A total of 498 phasically active neurons were recorded from the striatum, and 269 neurons were defined as task related. Two types of neuronal activity were distinguished during the delay periods. First, the activity gradually changed depending on the duration of the cue presented just before. This activity may represent the signal duration for later comparison between two cue durations. The activity during the second cue period also represented duration of the first cue. Second, the activity changed differently depending on whether the first or second cue was presented longer. This activity may represent discrimination results after the comparison between the two cue durations. These findings support the assumption that striatal neurons represent timing information of sensory signals for duration discrimination.