Semantic memory

 

How do we know that a lemon is usually yellow, sour and edible? That Paris is in France? Does a plant carry out photosynthesis?

Psychologists and neuroscientists use the term semantic memory when referring to this type of knowledge. In his seminal article "Episodic and Semantic Memory," Endel Tulving borrowed the term semantics from linguists to refer to the system of memory for "words and other verbal symbols, their meanings and references, the relationships between them, and the rules, formulas, and algorithms for manipulating them" (Tulving, 1972).

Today, most use the term semantic memory more broadly - to refer to all kinds of general knowledge about the world, be it words or concepts, facts or beliefs. What these types of knowledge about the world have in common is that they are composed of knowledge that is independent of specific experiences.

Semantic memory stores general information or knowledge that we can recall regardless of the circumstances in which it was originally acquired.

Semantic memory stores general information or knowledge that we can recall regardless of the circumstances in which it was originally acquired..

• The question arises whether there are individual differences in semantic representations?

Individual differences

Semantic representations appear to vary as a consequence of individual differences in sensorimotor experience.

For example, engagement of the left parietal cortex (a region activated during object actions) during object shape retrieval is modulated by the amount of lifetime tactile experience associated with the objects. Similarly, right-handers and left-handers, who use their hands differently to perform different actions with manipulable objects, use homologous but contralateral brain regions to represent those objects.

When we name tools, our dexterity (using tools with our hands) affects the lateralization of premotor activity; but not when we name non-tools, e.g. animals.

While reading action verbs (e.g. write, throw), right-handed people primarily activate the left regions of the premotor cortex, while left-handed people primarily activate the right. No such difference was observed for verbs of armless actions (e.g. kneel, giggle).

Analogous findings were observed for long-term experience in sports. Professional hockey players, when reading sentences describing ice hockey, activate premotor regions more than non-players (Beilock et al., 2008).

Further, such differences are not limited to motor experience. When professional musicians identified images of musical instruments, they activated the associated auditory cortex and neighboring areas more than non-musicians (Hoenig et al., 2011).

Intriguingly, changes in individual experience can also lead to changes in the representation of abstract concepts. The tendency of right-handed native English speakers to associate "good" with "right" and "bad" with "left" can be reversed when right-hand dominance is compromised due to a stroke or temporary induced handicap.

At first glance, individual differences may seem surprising, but the data described above suggest that they do occur.

If our concept of a lemon is determined by experience, then no two individuals' concepts of a lemon will be exactly the same.

Also, an individual's concept of a lemon is likely to change subtly over time, probably unconsciously. Incredibly.

• How is semantic memory formed? And is it possible to forget.. what is a lemon?

Links between semantic and episodic memory

Knowing that lemons are shaped like a rugby ball would be considered part of semantic memory, while knowing when we last touched a lemon would be considered part of episodic memory.

This division is reflected in the prominent taxonomy of long-term memory, in which semantic and episodic memory are characterized as distinct components of a declarative memory system for facts (semantic knowledge) and events (episodic knowledge).

How is semantic memory acquired?

Children who develop amnesia in early childhood (a consequence of bilateral hippocampal damage) are relevant to the question of whether the acquisition of semantic information depends on episodic memory. If semantic knowledge is acquired through episodic memory, then since these children had limited time to acquire semantic knowledge before developing amnesia, they should have limited semantic knowledge. Interestingly, despite their episodic memory impairment, the semantic knowledge of children with amnesia appears relatively intact (Bindschaedler et al., 2011; Gardiner et al., 2008). Also, even the famous H.M. managed to acquire some semantic knowledge after the operation due to the accident that led to his amnesia.

This suggests that semantic knowledge can be acquired independently of the episodic memory system.

However, semantic knowledge in these amnesic patients is not typical and is often acquired very slowly and laboriously. It is therefore possible that the acquisition of semantic memory normally depends on the episodic system, but other points of entry may be used when the episodic system is impaired. Alternatively, these patients may have enough residual episodic memory to allow the acquisition of semantic knowledge.

Conceptual knowledge about concrete objects is acquired through experience with them, whereby knowledge is based on distributed representations across brain regions involved in perceiving or acting on them. These distributed representations result in flexible concepts that can vary depending on the task and context, as well as individual experience.

Can semantic memories be "forgotten"?

Everyone occasionally has difficulty recalling episodic events (what did I have for dinner last night?), but can people lose knowledge of what things are? Let's imagine that we are walking in an orchard with a friend, who has no problem moving among the trees, but after passing under a lemon tree, he picks up a lemon and asks: "What is that?"

This syndrome is called semantic dementia (also known as the temporal variant of fronto-temporal dementia). This neurodegenerative disease causes gradual and selective atrophy of the anterior temporal cortex (predominantly on the left side).

Although patients with semantic dementia usually speak fluently and correctly, as the disease progresses they show severe word-finding difficulties and marked deficits in identifying objects, concepts, and people regardless of modality (eg, pictures, written or written words). Also, they have relatively preserved episodic memories, providing further evidence that the neural bases of episodic memory are at least partially independent of those underlying semantic memory.
Understanding the relationship between semantic and episodic memory is complicated by the fact that there are different types of semantic knowledge. It may be that for sensorimotor aspects of semantic knowledge (eg, knowledge about the shape, size, or smell of things), new information enters semantic memory through perceptual systems rather than through episodic memory, whereas semantic knowledge about information that does not enter directly through our senses (eg "encyclopedic knowledge", such as the fact that trees carry out photosynthesis) depends more on contextual information.

Moreover, sensorimotor and non-sensorimotor components of semantic knowledge may be stored in different areas of the cortex. It should be noted that even encyclopedic knowledge is often acquired indirectly. For example, knowing that apple trees carry out photosynthesis allows us to conclude that lemon trees also carry out photosynthesis.

Semantic knowledge can support basic abilities to make such generalizations.

Next: Abstract knowledge >>

Reference

- Beilock, S. L., Lyons, I. M., Mattarella-Micke, A., Nusbaum, H. C., & Small, S. L. (2008). Sports experience changes the neural processing of action language. Proceedings of the National Academy of Sciences, 105, 13269–13273.

- Bindschaedler, C., Peter-Favre, C., Maeder, P., Hirsbrunner, T., & Clarke, S. (2011). Grow­ing up with bilateral hippocampal atrophy: From childhood to teenage. Cortex, 47, 931–944. 

- Gardiner, J. M., Brandt, K. R., Baddeley, A. D., Vargha-Khadem, F., & Mishkin, M. (2008). Charting the acquisition of semantic knowledge in a case of developmental amnesia. Neuropsychologia. 46, 2865–2868.

- Hoenig, K., Müller, C., Herrnberger, B., Spitzer, M., Ehret, G., & Kiefer, M. (2011). Neuroplasticity of semantic maps for musical instruments in professional musicians. NeuroImage, 56, 1714–1725.

- Tulving, E. (1972). Episodic and semantic memory. In E. Tulving & W. Donaldson (Eds.), Organization of memory (pp. 381–403). New York: Academic Press.