Semantic Memory

Semantic memory consists of all explicit memory that is not autobiographical. Examples of semantic memory is knowledge of historical events and figures; the ability to recognize friends and acquaintances; and information learned in school, such as specialized vocabularies and reading, writing and mathematics.

Semantic memory refers to the memory of meanings, understandings, and other concept-based knowledge unrelated to specific experiences. Semantic and episodic memory together make up the category of declarative memory, which is one of the two major divisions in memory. The counterpart to declarative, or explicit memory, is procedural memory, or implicit memory.

Semantic memory includes generalized knowledge that does not involve memory of a specific event. For instance, you can answer a question like "Are wrenches pets or tools?" without remembering any specific event in which you learned that wrenches are tools.

Location of semantic memory in the brain

The cognitive neuroscience of semantic memory is a somewhat controversial issue with two dominant views. On the one hand, many researchers and clinicians believe that semantic memory is stored by the same brain systems involved in episodic memory. These include the medial temporal lobes (MTL) and hippocampal formation. In this system, the hippocampal formation "encodes" memories, or makes it possible for memories to form at all, and the cortex stores memories after the initial encoding process is completed.

Recently, new evidence has been presented in support of a more precise interpretation of this hypothesis. The hippocampal formation includes, among other structures: the hippocampus itself, the entorhinal cortex, and the perirhinal cortex. These latter two make up the "parahippocampal cortices". Amnesics with damage to the hippocampus but some spared parahippocampal cortex were able to demonstrate some degree of intact semantic memory despite a total loss of episodic memory. This strongly suggests that encoding of information leading to semantic memory does not have its physiological basis in the hippocampus. (Vargha-Kadem et al.)

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.

The neural basis of episodic and semantic memory is not yet known today. However, some scientists suggest that episodic memory might be dependent on the right hemisphere, and semantic memory on the left hemisphere.

The relationship of episodic memory to semantic memory

Episodic memory is thought of as being a "one-shot" learning mechanism. You only need one exposure to an episode to remember it. Semantic memory, on the other hand, can take into consideration multiple exposures to each referent - the semantic representation is updated on each exposure.

Episodic memory can be thought of as a "map" that ties together items in semantic memory. For example, semantic memory will tell you what a "dog" looks and sounds like. All episodic memories concerning your dog will reference this single semantic representation of "dog" and, likewise, all new experiences with your dog will modify your single semantic representation of your dog.

Some researchers believe that episodic memories are refined into semantic memories over time. In this process, most of the episodic information about a particular event is generalized and the context of the specific events is lost. One modification of this view is that episodic memories which are recalled often are remembered as a kind of monologue. If you tell and re-tell a story repeatedly, you may feel that you no longer remember the event, but that what you're recalling is a kind of pre-written story.

Others believe that you always remember episodic memories as episodic memories. Of course, episodic memories do inform semantic knowledge and episodic memories are reliant upon semantic knowledge. The point is that some people do not believe that all episodic memories will inevitably distill away into semantic memory. Read More