The Hippocampus

A seahorse-shaped structure deep in the temporal lobe that is essential for forming new memories and for finding your way — and one of the first places damaged in Alzheimer's disease.

The hippocampus is a curled structure tucked into the inner fold of each temporal lobe. The 16th-century anatomist Julius Caesar Aranzi (Arantius) named it after the seahorse (Greek hippokampos), which its shape resembles in cross-section.

Structure

The hippocampus is a simpler, older type of cortex with three main layers. Information arrives from the entorhinal cortex and travels through a famous loop: to the dentate gyrus, then to region CA3, then CA1, and back out. ("CA" stands for cornu Ammonis, Ammon's horn — another historical name.) CA3 has many connections back onto itself, which may help it complete a whole memory from a fragment. The dentate gyrus is one of the few places where new neurons may be born in adult mammals.

Making memories

Patient H.M. showed that without the hippocampus on both sides, people cannot form new declarative memories — memories of events (episodic) and facts (semantic) — though old memories and skills largely survive. The hippocampus is thought to bind together the elements of an experience (who, what, where, when) stored across the cortex, and over time help transfer them into more independent cortical memory, especially during sleep. Long-term potentiation, the leading cellular model of learning, was discovered here in 1973.

Maps

The hippocampus also builds cognitive maps. Its place cells fire at particular locations; London taxi drivers, who memorise a vast street layout, have been found on average to have a larger posterior hippocampus than control groups.

Vulnerability

The hippocampus is sensitive to lack of oxygen, prolonged seizures, and — in animal studies — chronic high levels of stress hormones. It is one of the first regions affected in Alzheimer's disease, which is why forgetting recent events is often an early symptom.