Functional comparison of neuronal properties in the primate posterior hippocampus and parahippocampus (area TF/TH) during different behavioural paradigms involving memory and selective attention.

Monkeys were trained on a delayed match-to-sample (DMS) task. In addition a standardized behavioural trial was performed which involved an experimenter approaching the animal in certain sequence and presenting edible or other objects ('raisin trial'). Neuronal activity of 617 units was recorded in the posterior parahippocampus (PH) and in the posterior hippocampus (H). In many cases, we compared the activity of the same neuron in different tasks. 32.7% of the 455 PH neurons and 28.5% of the 130 H cells responded during the presentation of the visual stimuli in the DMS task. These responses were only mildly influenced by the physical dimensions of the visual stimulus, but often depended on the context in which the stimuli were presented. There was no differential response to the second stimulus that clearly depended on the nature of the first stimulus. 6.2% of the PH units, but none in H, responded in relation to the reward. 4.4% of the PH neurons, but none in H, showed a mild response during the interstimulus interval. 38.1% of 215 PH neurons and 37.8% of 45 H cells responded during one or more phases of the raisin trial. These responses were not related to the physical dimensions of the sensory stimuli. 210 PH and 41 H units were investigated during the DMS task as well as during the raisin trial. 18.1% (PH) and 12.2% (H) of the units responded during the DMS task, but not during the raisin trial; 17.1% (PH) and 36.6% (H) responded vice versa. A response in both trials was found in 17.1% of the PH neurons, but in none of the H cells. There were also other PH unit types showing responses during different aspects of the DMS task and even in other control paradigms, while no such overlap was encountered in H. Our results suggest a function of H and PH in the evaluation of the behavioural significance of sensory information. It may be this aspect which leads to anterograde memory disturbances after lesion of these areas. Since representation of neuronal information was found to be more specific in H, a possible function as an 'evaluation index' is discussed.