Memory consolidation · Grey Matter

Memory consolidation is the process that turns a fresh, fragile memory into a lasting one: first by stabilising the synaptic changes that recorded it, then, over weeks to years, by reorganising it so that it depends less on the hippocampus and more on the cortex.

Memory consolidation. Memory consolidation is the process that turns a fresh, fragile memory into a lasting one: first by stabilising the synaptic changes that recorded it, then, over weeks to years, by reorganising it so that it depends less on the hippocampus and more on the cortex.

Two timescales are usually distinguished. Synaptic consolidation, within hours, needs new protein synthesis to fix the changes in synaptic strength made during learning. Systems consolidation is slower. At first the hippocampus holds the index that ties together the pieces of an episode stored in different cortical areas (the image in visual cortex, the sounds in auditory cortex, the emotion through the amygdala); then, as the episode is replayed, the cortical pieces become linked to each other directly. Recalling a memory reactivates much of the same cortical network that was active during the experience.

Sleep is when much of the replay happens. In rats, hippocampal cells that fired together during a run fire together again in the same order during the following sleep, in brief sharp-wave ripples, and interrupting those ripples impairs the memory.

The metaphor of a librarian and shelves fits the standard model: the hippocampus is the index, the cerebral cortex the shelves. It fits facts and general knowledge best; for vivid personal episodes, competing models hold that the hippocampus stays needed as long as the memory keeps its detail.

Recalled memories become changeable again. Reactivating a memory can make it labile for a while before it is stored once more (reconsolidation), which is one reason memories drift with retelling.

Questions: How does the hippocampus hand a memory over to the cortex? During an experience each feature is processed where it belongs (the face in visual cortex, the voice in auditory cortex, the feeling through the amygdala), and the hippocampus stores a compact index of which cortical patterns were active together. When a cue later reactivates the index, it reactivates the whole cortical pattern, and that is recall. Every reactivation, in waking rest and above all in sleep, also strengthens the direct links between the cortical pieces themselves. After weeks to years many memories can be retrieved from the cortex alone, the way a book can be found on its shelf once the librarian's card is no longer needed. What does sleep do for the transfer of memories to the cortex? In deep sleep the hippocampus replays the activity of the day: cells that fired together while a rat ran a track fire together again, in the same order and compressed in time, during brief bursts called sharp-wave ripples. These bursts tend to coincide with the slow waves and spindles of the cortex, a timing thought to let the cortex strengthen the links between the pieces the hippocampus is replaying. In rats, disrupting the ripples after learning impairs the memory, and in humans a night of sleep after learning improves retention compared with the same hours awake.