This page represents the hippocampal head as a bilateral anatomical concept, although the displayed Human Reference Atlas mesh is the left-sided object. The atlas groups the hippocampus structurally with subcortical regions, as many MRI segmentation systems do, but developmentally and histologically it is allocortex: its components have a three- to four-layered organisation rather than the six-layered pattern of neocortex. The two hippocampi are homologous rather than a single midline structure. Clinical effects therefore depend on side, extent, a person’s typical language organisation, and whether neighbouring entorhinal, perirhinal, parahippocampal, amygdalar, or white-matter structures are also involved. The conspicuous digitations of the anterior head, called the pes hippocampi, should not be mistaken for separate functional compartments.

The entorhinal cortex is the major cortical gateway into and out of the hippocampal formation. The broader anterior-parahippocampal connection listed in this atlas is a gross regional proxy because the atlas has no standalone entorhinal entry; it should not be read as making the two labels synonymous. The trisynaptic sequence is a valuable teaching model, but it is not the only route through this circuitry. In addition to the entorhinal-to-dentate, dentate-to-CA3, and CA3-to-CA1 relays, entorhinal axons project directly to CA3 and CA1. CA1 and the subiculum provide important output toward deep entorhinal layers and wider cortical and subcortical targets. The head also contains folded and interlocking subfields, so a coarse surface mesh cannot show the cellular borders on which many physiological claims depend.

Evidence for long-axis specialisation is best described as graded. Anterior hippocampal territories are more often associated with broad contextual, motivational, and emotion-related representations, while posterior territories more often support fine spatial detail. These are population-level tendencies, not a rule that assigns one cognitive operation to the head and another to the tail. Functional connectivity also places the hippocampal formation within a medial-temporal subsystem of the default network; that network label does not imply that the hippocampus is active only during rest.

Henry Molaison’s profound amnesia after surgery was pivotal evidence for a medial-temporal memory system, but his bilateral operation affected more than the hippocampal heads. Later clinicopathological cases showed that bilateral damage largely limited to the hippocampal formation is sufficient to cause enduring anterograde memory impairment, with severity related to the amount of tissue involved. Immediate memory span and some forms of nondeclarative learning may remain relatively preserved, so “memory loss” is too broad a description of the syndrome.

Hippocampal sclerosis is strongly associated with drug-resistant temporal lobe epilepsy, yet its histological patterns and clinical presentations are not uniform. Neuronal loss and gliosis may predominate in different subfields. Mesial temporal seizures can include impaired awareness, behavioural arrest, automatisms, and psychic, sensory, or autonomic auras; no single aura establishes the diagnosis or reliably identifies one hippocampus without electroclinical correlation.