Common Firing Patterns of Hippocampal Cells in a Differential Reinforcement of Low Rates of Response Schedule

Lesion studies show that the hippocampus is critically involved in timing behavior, but so far there has been little analysis of how it might encode time. We recorded the activity of 266 CA1 neurons, 51 CA3 neurons, and 219 entorhinal neurons from rats performing on a differential reinforcement of low rates (DRL) 15 sec schedule in which reinforcement was contingent on responses that occurred at least 15 sec after the preceding response. The unit data were analyzed using two different methods. First, each unit was subjected to an ANOVA that examined the effects of the following: (1) the outcome of the previous response (reward or nonreward); (2) the outcome of the response on which the firing of the cell was synchronized; and (3) time. This showed that, for CA1, CA3, and entorhinal cortex, changes in unit activity were related to all aspects of the task, with the firing of >90% of units recorded in each region being related to at least one of the three factors. Second, intercorrelations between the firing profiles of individual units revealed several functional categories of hippocampal neurons but no clear categories of entorhinal neurons. Of the hippocampal categories, the most common profile was an initial increase in unit activity at the beginning of the DRL interval, followed by a gradual decrease throughout the interval. We suggest that this profile reflects temporal decay in circuits that may code details of the previous trial and that could be used to "time" the DRL interval.