Hippocampus

The hippocampus is a seahorse-shaped structure (the name comes from the Greek for seahorse) located in the medial temporal lobe of each cerebral hemisphere. There are two hippocampi, one in each hemisphere, forming part of the limbic system. The hippocampus is essential for the formation and consolidation of new episodic memories (memories of specific events and experiences) and for spatial navigation.

Memory formation in the hippocampus involves a process called pattern completion and pattern separation — the ability to encode new experiences as distinct memories while linking them to relevant prior knowledge. The hippocampus binds together the different features of an experience (where, when, who, what) into a coherent memory trace, then consolidates that trace over time, partly during sleep, into longer-term cortical storage.

The hippocampus is also one of the few brain regions where neurogenesis — the birth of new neurons — continues into adulthood. Physical exercise, environmental enrichment, and learning are among the most potent stimulators of hippocampal neurogenesis. Stress, sleep deprivation, and chronic glucocorticoid exposure suppress it.

The hippocampus is among the earliest and most severely affected brain regions in Alzheimer's disease. Amyloid and tau pathology concentrate in the entorhinal cortex and hippocampus in early stages, which explains why episodic memory loss — forgetting recent events, conversations, and experiences — is typically the first clinical symptom. Hippocampal volume loss, measurable on MRI, is among the most useful imaging biomarkers for Alzheimer's disease progression.

Hippocampal volume naturally declines with age at a rate of approximately 0.5-1% per year in healthy older adults. This normal atrophy contributes to age-related declines in episodic memory. Factors that accelerate hippocampal atrophy include chronic stress (through cortisol-mediated damage), cardiovascular disease, diabetes, sleep deprivation, and physical inactivity. Conversely, aerobic exercise consistently shows neuroprotective effects on hippocampal volume.

For cognitive tracking, episodic memory performance — new learning and retention — is closely related to hippocampal function. Changes in this domain over time, particularly if progressive, are worth monitoring carefully.