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Hippocampus & Entorhinal CortexHippocampus & Entorhinal Cortex

A small seahorse-shaped strip tucked deep in each temporal lobe — the brain's indexer and mapmaker. It doesn't store memories long-term, but at the moment of an event it staples together the fragments scattered across the brain, and later summons them back; it also draws the map inside your head that tells you "where I am and where I've been." Damage it, and a person is frozen at the last moment before — unable to lay down any new memory of the past.

Where it is, and the parts it's strung from

There's one hippocampus per hemisphere, curled deep in the medial temporal lobe. The real protagonist, though, is a circuit: incoming information first enters through the entorhinal cortex, the "gateway," then passes in turn through the hippocampus's little subfields, and finally exits back out through the entorhinal cortex again. This trisynaptic loop is the skeleton behind everything it can do:

Entorhinal cortex · EC

The hippocampus's gateway — nearly all traffic in and out of the hippocampus is relayed here. It also harbors its own navigation cells (grid cells, Nobel 2014), laying a hexagonal coordinate grid over space.

Dentate gyrus · DG

The workshop of pattern separation: it pries two similar experiences forcefully apart, coding them into two far-apart records. It's also one of the very few places the adult brain grows new neurons — those new cells may be what help you tell a new memory from old ones.

CA3

The core of pattern completion. Its neurons are massively recurrently connected, forming a self-linking web: feed in a broken cue and it auto-fills to the complete memory — the biological prototype of Hopfield associative memory.

CA1 / Subiculum

The circuit's exit stage: CA1 compares what CA3 completed against the raw input the EC delivered, then the subiculum ships the result back to the cortex for long-term storage.

What it does: memory's index, map, and replay

Bundle the hippocampus's work into a few jobs:

① Binding & indexing — at the moment of an event it rapidly notes "these fragments belong together" and clips the same bookmark on; later, following the bookmark, it summons the pieces scattered across regions and reassembles the scene. This is why it's irreplaceable for episodic memory ("that one time," with a time and a place).

② Consolidation & replay — in deep sleep the hippocampus replays the day's sequences, time-compressed, many times over (showing up as a burst of high-frequency firing called a sharp-wave ripple), "telling" the memory to the cortex bit by bit until the cortex holds it itself and no longer needs the hippocampus.

③ Cognitive maps — the hippocampus's place cells fire when you reach a particular spot, and a population of them strung together is a live map of "where you are." Space and memory are one thing here: to remember an event is, in a sense, to remember "where it happened."

H.M.: the man whose memory switch was cut

The most famous evidence about the hippocampus comes from a patient known as H.M. To treat severe epilepsy, both his hippocampi were surgically removed. The epilepsy improved — but the price was staggering: he could never form a new long-term memory again. Memories from before the surgery remained, but every day after, every meeting, every meal, was gone the moment he turned away; he lived forever in a present a few tens of seconds long. He could still learn new skills (like tracing a figure in a mirror, getting steadily better) yet had no memory of ever practicing — one cut cleanly split "remembering an event" from "learning a movement" into two systems, and all but single-handedly founded modern memory research.