Abstract
The memory for where in the environment a particular visual stimulus has been seen is one of the types of memory relatively specifically impaired by hippocampal damage in primates including man. In order to investigate what processing might be performed by the hippocampus related to this type of memory, the activity of hippocampal neurons was recorded while monkeys performed an object-place memory task. In this task, the monkey was shown a sample stimulus in one position on a video screen, there was a delay of 2 s, and then the same or a different stimulus was shown in the same or in a different position. The monkey remembered the sample and its position, and if both matched the delayed stimulus, he licked to obtain fruit juice. Of the 600 neurons analysed in this task, 3.8% responded differently for the different spatial positions, with some of these responding differentially during the sample presentation, some in the delay period, and some in the match period. Thus some hippocampal neurons respond differently for stimuli shown in different positions in space, and some respond differently when the monkey is remembering different positions in space. In addition some of the neurons responded to a combination of object and place information, in that they responded only to a novel object in a particular place. These neuronal responses were not due to any response being made or prepared by the monkey, for information about which behavioral response was required was not available until the match stimulus was shown. This is the first demonstration that some hippocampal neurons in the primate have activity related to the spatial position of stimuli. The activity of these neurons was also measured in a delayed spatial response task, in which the monkey was shown a stimulus in one position, and, after a 2 s delay when two identical stimuli were shown, had to reach to touch the stimulus which was in the position in which it had previously been seen. It was found that the majority of the neurons which responded in the object-place memory task did not respond in the delayed response task. Instead, a different population of neurons (5.7% of the total) responded in the delayed spatial response task, with differential left-right responses in the sample, delay, or match periods.