As noted earlier, the hippocampus was found to be involved in memory when H.M.'s hippocampi were removed and he exhibited anterograde amnesia. Figure 5 shows a view of the hippocampus and the amygdala using a transparent model of the human brain. The hippocampus was so named because it has a close resemblance to a sea horse ("hippo" means horse, and "kampos" is Greek for "sea monster"). The hippocampus is an elaborate infrastructure of computational systems. However, the complex basis on which
Figure 5 Studies from human and rodents show that the hippocampus mediates declarative information. Studies derived from rodents show that the amygdala is involved in classical conditioning (reproduced with permission from www.brainconnec-tion.com).
the hippocampus functions can be understood in a simpler way by describing its trisynaptic structure.
The hippocampal formation is composed of the hippocampus proper and is formed by layers of neurons that form areas CA1-CA3 (cornu ammonis 1-3), the dentate gyrus, the entorhinal cortex, and the subiculum. The hippocampal formation has an internal circuit known as the trisynaptic circuit. First projection is from the entorhinal cortex to the dentate gyrus. Second, the dentate gyrus neurons give rise to the mossy fibers (axons) that project to CA3 cell fields. Third, the CA3 neurons give rise to the Schaffer collaterals (axons), which project to the CA1 cell field. In addition, the hippocampal formation receives projections from many subcortical structures (e.g., amygdala). Neuroanatomy is replete with nerve tracts that run from one physical location in the brain to another, but the hippocampus can be thought of as a loop that returns its signals to a location close to their cortical origin.
The amygdala, derived from the Greek word for "almond," is a collection of nuclei located in the temporal pole of the cerebral hemisphere rostral to the hippocampal formation. The amygdala comprises the lateral, medial, basolateral, and central nuclei. These nuclei have a number of functional roles, from receiving and processing olfactory information to sending projections to areas involved in autonomic responses. Studies suggest that the amygdala is directly involved in classical conditioning associated with fear.
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