Semantic memory · Grey Matter

Semantic memory is the store of general knowledge about the world (what words mean, what a lemon tastes like, that Paris is in France) held without any memory of when or where it was learned.

Semantic memory. Semantic memory is the store of general knowledge about the world (what words mean, what a lemon tastes like, that Paris is in France) held without any memory of when or where it was learned.

It is declarative, like episodic memory, and it is distributed: the shape of a lemon is represented near visual cortex, its taste near taste cortex, the action of squeezing it near motor cortex. The leading model adds a hub in the anterior temporal lobe on both sides that binds these features into a single concept, so that a lemon is recognised from any of them. Lateral temporal cortex and the angular gyrus of the parietal lobe take part in retrieving and combining meanings, and the inferior prefrontal cortex selects among them when a context calls for one.

Its loss shows the hub at work. In semantic dementia, a degeneration that begins in the anterior temporal lobes, patients lose the meaning of words and objects across every modality while memory for recent events and everyday skills is relatively spared.

It survives damage that destroys episodic memory. Patients with hippocampal amnesia keep the vocabulary and facts they learned long before the injury, although learning new facts becomes slow and poor.

Many semantic facts begin as episodes. Repeated experiences of a thing leave their common core as knowledge after the individual episodes have faded.

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. What happens when the front of both temporal lobes degenerates? Semantic dementia, a form of frontotemporal degeneration, strikes the anterior temporal lobes first, and the result is a slow loss of meaning across every modality. Patients cannot name objects, then cannot say what they are for, then fail to recognise them from pictures, sounds or touch, with specific knowledge going before general (a dog becomes an animal, then a thing). Recent events and everyday routines are relatively spared, the opposite of hippocampal amnesia, which is the main evidence for a semantic hub in the anterior temporal lobes.