Unit activity in the hippocampus and the parahippocampal temporobasal association cortex related to memory and complex behaviour in the awake monkey

Abstract

Monkeys ( Macaca fascicularis) were trained on a delayed match-to-sample (DMS) task using delays of upto 20 s. Unit activity was recorded from the hippocampus and the temporo-basal association cortex in the lateral parahippocampal region (partly corresponding to TF and TH) during the DMS task, as well as during a visual object discrimination task and some behavioural situations involving the experimenter. Units were encountered that gave visual responses which were sometimes context-dependent. Changes in discharge rate during the delay period of the DMS task were very rare and when present, very weak. On the other hand, many neurons, including some of those which were unresponsive during the DMS task fired vigorously (or were inhibited) during situations which involved attention, expectation or food consumption. For example, the neurones' firing rate was altered when the cage door was opened or closed, the experimenter entered or left the room or showed the monkey a piece of food before giving it to him. A variety of such responses in complex behavioural situations were seen, sometimes even in neurones which did not respond in the DMS task. Activity changes in neurons of the temporo-basal cortex thus appear to be related to the internal state associated with a stimulus and even some of the responses obtained in the DMS task can be interpreted as being related to changes in the behavioural state rather than to the mnemonic elements of the task.