MEMORY REPRESENTATION WITHIN THE PARAHIPPOCAMPAL REGION

The activity of 378 single neurons was recorded from areas of the para hippocampal region (PHR), including the perirhinal and lateral entorhi nal cortex, as well as the subiculum, in rats performing an odor-guide d delayed nonmatching-to-sample task. Nearly every neuron fired in ass ociation with some trial event, and every identifiable trial event or behavior was encoded by neuronal activity in the PHR. The greatest pro portion of cells was active during odor sampling, and for many cells, activity during this period was odor selective. In addition, odor memo ry coding was reflected in two general ways. First, a substantial prop ortion of cells showed odor-selective activity throughout or at the en d of the memory delay period. Second, odor-responsive cells showed odo r-selective enhancement or suppression of activity during stimulus rep etition in the recognition phase of the task. These data, combined wit h evidence that the PHR is critical for maintaining odor memories in a nimals performing the same task, indicate that this cortical region me diates the encoding of specific memory cues, maintains stimulus repres entations, and supports specific match-nonmatch judgments critical to recognition memory. By contrast, hippocampal neurons do not demonstrat e evoked or maintained stimulus-specific codings, and hippocampal dama ge results in little if any decrement in performance on this task. Thu s it becomes increasingly clear that the parahippocampal cortex can su pport recognition memory independent of the distinct memory functions of the hippocampus itself.