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 b ehavior, but so far there has been little analysis of how it might encode t ime. We recorded the activity of 266 CA1 neurons, 51 CA3 neurons, and 219 e ntorhinal neurons from rats performing on a differential reinforcement of l ow rates (DRL) 15 sec schedule in which reinforcement was contingent on res ponses that occurred at least 15 sec after the preceding response. The unit data were analyzed using two different methods. First, each unit was subje cted to an ANOVA that examined the effects of the following: (1) the outcom e of the previous response (reward or nonreward); (2) the outcome of the re sponse 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 c ommon profile was an initial increase in unit activity at the beginning of the DRL interval, followed by a gradual decrease throughout the interval. W e suggest that this profile reflects temporal decay in circuits that may co de details of the previous trial and that could be used to "time" the DRL i nterval.