Glossary · Sinh học
Hippocampus
A small structure in the brain that acts as a memory indexer: it sends memories to the appropriate parts of the cerebral hemispheres for long-term storage and retrieves them when needed.
The hippocampus sits at the end of an arching tract of nerve cells leading from the hypothalamus and thalamus.
In mice, new neurons are continuously added throughout life to one part of the hippocampus, the dentate gyrus, and this continuous addition remodels existing circuits. Experiments in adult mice show that increasing neurogenesis after a memory forms is sufficient to induce forgetting of that memory. In infant mice, when neurogenesis is high and fresh memories tend to be forgotten quickly, decreasing neurogenesis after memory formation mitigated forgetting. In guinea pigs and degus — species in which most granule cells are generated before birth — infants did not show this forgetting, but increasing neurogenesis induced it.
Hippocampus-dependent, event-related memories formed in early infancy are rapidly forgotten in both humans and other animals — a phenomenon called infantile amnesia. On the basis of these animal experiments, high hippocampal neurogenesis has been proposed as a contributing factor. A follow-up study in mice showed that "lost" infant memories could be recovered in adulthood, up to three months after training, by optogenetically reactivating the dentate gyrus neurons that had been active when the memory was encoded.