Automatic comparison of stimulus durations in the primate prefrontal cortex: the neural basis of across-task interference

Prefrontal cortex represents task relevant information, however several studies have shown that some stimulus features can modulate the neuronal activity even when the task doesn’t not require their processing. Also, as we have shown in a recent paper, previous goal information, but not other non-goal information, can modulate the cell activity even when irrelevant. We tested here whether also a comparison process could be activated in the prefrontal cortex neurons when the task doesn’t require any stimulus comparison. We trained two monkeys in two tasks. In the first task the monkeys were required to evaluate which of two stimuli sequentially presented, one blue and the other red, of different durations was the longest by selecting it when the two stimuli were presented again after a delay (D2) following the presentation of the second stimulus. In the second task, a matching-to-sample task (MTS) the monkeys’ task was to choose the target stimulus, either red or blue, presented twice in that trial. In both tasks the stimulus duration varied between 0.2 and 1.2 s, but in the MTS task the durations and their comparison were irrelevant. Nevertheless, we found 11% of neurons that encoded the relative duration of the stimuli (one-way ANOVA) in the D2 period after the presentation of the second stimulus. This result indicates that even a comparison process can take place at the level of the prefrontal cortex neurons when it is not relevant to the task.