Memory — write, retrieve, consolidate, allocate

Writing is NMDA coincidence detection (glutamate ∧ depolarisation ∧ Ca²⁺ → STDP); reading is attractor pattern-completion from a partial cue (capacity ~0.138N). Consolidation moves a trace hippocampus → cortex with sleep replay (a Ribot gradient), and recall reopens it for reconsolidation. Which neurons hold a memory is set by CREB excitability competition, not chance.

Writing is NMDA coincidence detection — glutamate ∧ depolarisation ∧ Ca²⁺ above threshold → STDP. Reading is attractor pattern-completion: a partial cue restores the whole pattern, with capacity ~0.138N for N units. The write threshold is gated by hypothalamic state, with an inverted-U in salience. Consolidation moves a trace hippocampus → cortex with sleep replay (a Ribot gradient; replay rescues catastrophic interference), and reconsolidation reopens a recalled trace (Nader). Which neurons hold a given memory is set by CREB excitability competition, not chance.

Write

A memory is written where activity coincides. The NMDA receptor is a coincidence detector: it admits Ca²⁺ only when glutamate binding and post-synaptic depolarisation occur together and the Ca²⁺ rise clears threshold — glutamate ∧ depolarisation ∧ Ca²⁺ > threshold. That conjunction triggers spike-timing-dependent plasticity (STDP), strengthening the synapse. The write is a logical AND on the switch substrate, not a continuous recording.

Retrieve

Reading is not playback; it is pattern-completion. A network of switches with recurrent wiring is an attractor: present a fragment of a stored pattern and the dynamics fall into the nearest basin, restoring the whole. The storage capacity of such a network scales with its size — on the order of 0.138N stored patterns for N units. Retrieval is the basin choice, which is why a partial cue suffices and why memories are reconstructive.

Threshold

What gets written is gated by state. The hypothalamic setpoint (§7) sets the write threshold, so emotional salience controls what consolidates — but with an inverted-U: too little and nothing is written, too much and the write is impaired. State decides the gate, and the gate is not monotone.

Consolidate and reconsolidate

A fresh trace is not yet permanent. Consolidation moves it from hippocampus toward cortex, replaying it during sleep; this produces the Ribot gradient (older memories more cortical, more robust) and lets replay rescue what would otherwise be catastrophic interference between new and old. Recall reopens the trace — reconsolidation (Nader) — making a stored memory briefly editable again before it re-stabilises.

Allocate

Which specific neurons carry a memory is not random. Cells with momentarily higher excitability — set by CREB — win a sparse competition to join the engram, which is how memories formed close in time get linked and why allocation is biased rather than uniform. The engram is selected, not scattered.