Episodic memory · Grey Matter

Episodic memory is the memory of personal events, each tied to its own time and place, which lets a person mentally travel back to a moment and re-experience it: what happened at breakfast, where the keys were left, the last meeting with a friend.

Episodic memory. Episodic memory is the memory of personal events, each tied to its own time and place, which lets a person mentally travel back to a moment and re-experience it: what happened at breakfast, where the keys were left, the last meeting with a friend.

With semantic memory it forms declarative (explicit) memory, the kind that can be consciously recalled and put into words. It depends on the medial temporal lobe and above all on the hippocampus, which binds the who, what, where and when of an episode into a single trace. The content itself is distributed across the cortex that perceived it, the prefrontal cortex organises encoding and the search during recall, and the parietal and posterior cingulate cortex cortices are active when a remembered scene is relived.

Recall rebuilds the scene from its parts. Remembering or imagining a visual scene reactivates visual areas, in some studies even the primary visual cortex, which is why memory is a reconstruction more than a playback.

It is the most fragile kind of memory. It is the first to fail after hippocampal damage, as in anterograde amnesia, and in early Alzheimer's disease, when new events are not kept and the same question is asked again.

Episodes fade into facts. Over years, many episodes lose their details and leave only their gist, which behaves like semantic knowledge.

Questions: What separates knowing a fact from remembering an event? An episodic memory carries its own time and place (the day you learned that Paris is the capital of France), while a semantic memory is the fact itself, detached from the moment it was learned. Both are declarative, but episodes depend on the hippocampus to bind their details, while facts are held by the temporal cortex, with a hub in the anterior temporal lobes. The two dissociate in disease: hippocampal amnesia removes new episodes and spares old facts, and semantic dementia erodes facts and word meanings while recent events can still be recalled. Why does imagining or remembering a scene activate the visual cortex? A visual memory is stored largely in the visual areas that processed it, so recalling it means reactivating part of the same cortical pattern. Imaging found that imagined objects activate visual cortex, in some studies even the primary visual cortex, in a pattern that follows the size of the imagined object. The overlap is partial: some patients with damaged visual cortex still imagine well, so imagery relies on higher visual areas and memory more than on V1 itself. Why does Alzheimer's disease often first show as asking the same question again? The disease attacks the entorhinal cortex and the hippocampus early, the gateway through which new experiences become episodic memories. A person can still hold the answer in working memory for a few seconds and understand it perfectly, but it is not stored, so a few minutes later the question feels new and is asked again. Older memories and well-practised knowledge, already held by the cortex, last longer, which is why the early picture resembles a mild anterograde amnesia.