The primate hippocampus is needed for spatial memory tasks in which the location of objects must be remembered, and in which the location of places where responses must be made are to be remembered. It is also involved in some non-spatial memory tasks such as recognition memory for visual stimuli. Some single neurons in the hippocampus of macaque monkeys performing these memory tasks respond to the positions in space of the stimuli; or to a combination of a non-spatial stimulus with a spatial response when the monkey must learn to make spatial responses to visual stimuli, with the responses of these neurons becoming modified during this type of learning; or to only novel visual stimuli in the recognition memory task. On the basis of these and related findings the hypothesis is suggested that the importance of the hippocampus in spatial and other memories is that it can rapidly form “episodic” representations of information originating from many different areas of the cerebral association cortex. These neurophysiological findings have been combined with information on the microanatomy of the hippocampus, and on long-term potentiation in the hippocampus to produce a neuronal network theory of the operation of the hippocampus. A key aspect of the theory is that the CA3 pyramidal cells with their 4% interconnectivity and Hebb-modifiable synapses implement an autoassociation memory which provides the basis for “episodic” memories which are required for many spatial and non-spatial memory functions.
