Semantic memory refers to the memory of meanings, understandings, and other knowledge; in contrast to episodic memory.
The cognitive neuroscience of Semantic Memory
This is a controversial issue with two dominant views:
1) On the one hand, many researchers and clinicians believe that semantic memory is stored by the same brain systems involved in episodic memory, i.e. the medial temporal lobes (MTL). They see the MTL as a kind of 'declarative memory' machine.
2) Other researchers believe the hippocampus is only involved in episodic memory and spatial cognition. This then raises the question where semantic memory may be located. Some believe semantic memory lives in temporal neocortex. Others believe that semantic knowledge is widely distributed across all brain areas. To illustrate this latter view, consider your knowledge of dogs. Researchers holding the 'distributed semantic knowledge' view believe that your knowledge of the sound a dog makes exists in your auditory cortex, whilst your ability to recognize and imagine the visual features of a dog resides in your visual cortex. Perhaps all these representations are indexed by the left temporal pole, a region particularly vulnerable to damage in semantic dementia.
Semantic Dementia
SD is also known as the temporal variant of fronto-temporal dementia (tvFTD). Patients in the early stages of SD show a debilitating incapacity to recall and remember semantic knowledge. This is typically assessed by tasks such as picture naming, category listing (e.g. "list all the fruits") and non-verbal tasks where the patient is given three photos and asked to point to a semantically related pair (e.g. given a picture of a tree, a leaf and a cabbage). The early stages of SD typically involve the left temporal pole and anterior-ventral temporal cortex. As SD develops, the disease starts eat away at the orbito-frontal cortex. Both variants of FTD (fronal variant and temporal variant) eventually become full-blown FTD, involving both the frontal and temporal corticies.
