Hippocampus

The hippocampus is a specialized part of the cerebral cortex, folded into the temporal horn of the lateral ventricle about the hippocampal fissure (Figs. 1 and 4). It is composed of several subregions, including (in order) the entorhinal cortex (in the parahippocampal gyrus), the parasubiculum, presubiculum, subiculum, fields CA1-CA3, and the dentate gyrus.

Like the amygdala, in rats the hippocampus receives strong olfactory inputs, but in primates it is dominated by multisensory inputs, mostly from higher order

Figure 4 Photomicrograph of the hippocampal formation and the mammillary nuclei in a macaque monkey (stained with the Nissl method). Ca, caudate nucleus; EC, entorhinal cortex; DG, dentate gyrus; P, putamen; OT, optic tract; Sub, subiculum.

sensory association cortex. These inputs reach the hippocampus through a relay in the perirhinal and entorhinal cortex, which in turn receive inputs from multisensory association cortical areas. There is a relatively unidirectional set of connections through the hippocampus, beginning from the entorhinal cortex to the dentate gyrus, and then in turn to CA3, CA1, the subiculum, and back to the entorhinal cortex (Fig. 5).

Outputs to other parts of the brain mainly arise in the subiculum and entorhinal cortex. These resemble those of the amygdala:

1. Projections back to many sensory association cortical areas, including the visual and auditory areas of the inferior and superior temporal cortex.

2. Projections to the hypothalamus (but not to brain stem autonomic nuclei).

3. Connections with prefrontal and cingulate cortical areas, via direct projections and also via projections to the anterior and mediodorsal thalamic nuclei, and the mammillary nuclei, and projections to the ventral part of the basal ganglia.

The hippocampus and surrounding areas of the parahippocampal gyrus are critically involved in memory processing in general and spatial orientation in particular. Recordings in the hippocampus have demonstrated cells that fire when the animal is in a particular spatial location, as defined by characteristic sensory and other experiential stimuli. This suggests that the hippocampus provides a neural mechanism for association of different parameters that are necessary for the moment-to-moment incorporation of experience into memory. Strikingly, bilateral lesions of the hippocampus and parahippocampal cortical areas produce amnesia, an inability to form new memories (although older memories may be intact). By comparison, amygdaloid lesions that produce emotional disturbances do not produce amnesia.

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