Hippocampus

Intro | Amygdala | Mammillary Body | Olfactory Bulb | Cingulate Gyrus | Dentate Gyrus | Fornix | Hippocampus | Septum | Thalamus

Part 1: Image-Mapped Tutorial
Part 2: Matching Self-Test
Part 3: Multiple-Choice Self-Test

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The Hippocampus resembles in appearance a marine organism after which it was named (Latin: sea-horse). It is located deep within the temporal lobes just beneath the thalamus, and posterior to or behind the amygdala. Damage to the hippocampus or to any region that is connected to it, causes anterograde amnesia, or the inability to learn new information.

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The now famous case study of a patient known as H.M. led researchers to focus on the role of the hippocampus in behavior. In 1953, H.M.'s epileptic seizures became so debilitating and unresponsive to medication that neurosurgeons removed the hippocampus and surrounding structures (including the amygdala) from both hemispheres. Following this surgery H.M.'s seizure disorder improved. He was left, however, with a rather selective memory deficit. Although H.M.'s intellect and personality were unchanged, it was difficult for him to remember events from 1 - 3 years before the surgery. This retrograde amnesia was moderate; he was able to recall older memories without difficulty. He also suffered a severe loss of ability to form new memories, anterograde amnesia. H.M. can store new information briefly, but has difficulty recalling it especially when distracted. For example, it took H.M. eight years to learn the floor plan of his parent's new home after he moved in with them, but he was still not able to find his way home if more than two blocks away. In spite of H.M's tremendous trouble learning new facts and remembering events, he is able to learn new skills with ease although he does not remember learning these new skills. The hippocampus is, therefore, not the site of memory storage but rather involved in the storage of new memories.

Extensive research seeking a better understanding of the hippocampus has provided comprehensive information concerning both structure and function of this complicated region. The hippocampus is divided into 4 divisions called CA1-CA4. CA1 and CA3 play major roles in hippocampal processes; extensive study has clarified the function of these specific hippocampal cells in relation to connected structures. In summary, the latest research indicates that the hippocampus is involved in consolidation of long-term memories, in particular for spatial location. In addition, this learning may take place via a neuronal phenomenon called long-term potentiation (LTP). LTP is a long-term increase in the excitability of a neuron to a particular synaptic input that is caused by repeated high-frequency activity at that synaptic input. Learning experiences that involve the hippocampal formation also induce biochemical changes that underlie LTP.