NEURONAL-ACTIVITY IN THE PRIMATE HIPPOCAMPAL-FORMATION DURING A CONDITIONAL ASSOCIATION TASK-BASED ON THE SUBJECTS LOCATION

The hippocampal formation (HF) functions in two domains of memory: spa tial and nonspatial associative memory. The HF includes the hippocampu s proper, the dentate gyrus, and the subicular complex. Studies of spa tial correlates of HF neuronal activity have revealed that a subject's location in space can impose critical constraints on patterns of neur onal activity in the HF. This report compares monkey HF neuronal respo nses in two kinds of stimulus-response association tasks (go/no-go tas ks with symmetrical reinforcement). In a place-dependent, conditional, stimulus-response association (PCA) task, the subject's location was the condition upon which stimulus (object)behavioral response associat ion depended. In a place-independent, simple, stimulus-response associ ation (ISA) task, the object-behavioral response contingency was indep endent of the subject's location. Of 329 neurons recorded, the activit y of 88 increased or decreased significantly in response to the presen tation of an object during the PCA task. Responses of 17 neurons depen ded differentially on specific combinations of object, place, and beha vior in the PCA task (specific-combination neurons). These specific-co mbination responses do not simply reflect object-behavioral response a ssociation in the PCA task, since neuronal responses in the same objec t-behavioral response association were not restored in the ISA task in which there was no dependence on the subject's location. This suggest s the influence of location on HF neuronal responses in object-behavio ral response association when the subject's location is imposed as a c ondition. Responses of 12 neurons differentiated the kind of object in the PCA task (object-differential neurons). In the ISA task, most obj ect-differential responses diminished or disappeared. Since the HF obj ect-differential responses occurred in all locations, these responses were probably elicited by the conditions imposed by the PCA task. Char acteristics of the two neuron types suggest that HF neurons encode bot h stimulus percept and attributes such as the place where the stimulus is presented and the conditional relation imposed in the task.