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To put it simply, long-term memory (LTM) is a permanent storage facility that can hold a limitless quantity of information for long periods of time.
Endel Tulving was one of the first cognitive psychologists to point out that the unitary structure of Long-Term Memory (LTM) proposed in the Multi-store Model by Atkinson and Shiffrin's (1968) is too simple and inflexible. In 1972, Tulving suggested three types of LTM: episodic memory, semantic memory, and procedural memory. Later in 1980, Cohen and Squire further categorized LTM into explicit memory and implicit memory. The graph below shows the full structure of LTM:
Structure of long-term memory, StudySmarter Originals.
Explicit memory, also known as declarative memory, is the ‘knowing that’ memory. Explicit memory involves conscious efforts to be stored and recalled. Explicit memory includes semantic Memory and Episodic Memory.
Semantic memory is the memory for facts and knowledge, such as information about the world. As a metaphor, semantic memory might be compared to a combination of encyclopaedia and dictionaries. Semantic memory is stored in the form of meaning and concept.
London is the capital of the UK, the taste of an apple, the meaning of words, etc. Semantic memory is formed without time stamps. For example, we do not usually remember when we first tasted an apple. Semantic memory also requires conscious efforts to store and recall.
Episodic memory is the memory for specific events that we have experienced in our lives. As a metaphor, episodic memory might be compared to a daily diary. Episodic memory is time-stamped. Each episodic memory will include multiple elements: specific details of the event (eg time, place, people, etc.), the context, and the emotion are woven into one memory. Episodic memory also requires conscious effort to recall.
The most recent visit to a dentist, a first kiss, etc.
Implicit memory, also known as procedural memory, is the “knowing how” memory. Implicit memory does not involve conscious efforts to be stored or recalled.
Procedural memory is the memory of skills and behaviors. Procedural memory is formed without time stamps and is stored in the form of motor action. We can recall procedural memories without conscious awareness or effort, such as playing the piano or riding a bike. Since we can recall procedural memories without conscious awareness, procedural memories can include skills that we would find hard to explain to others, such as teaching others how to swim if we have already mastered swimming.
Let’s evaluate the evidence we have regarding LTM. Clinical studies of amnesia, such as HM and Clive Wearing, showed patients have difficulties recalling personal events in the past. Yet, their semantic memories were relatively unaffected.
In the case of HM, the episodic memory store was damaged, but the semantic memories were left unaffected after an accident damaged his brain. HM did not recall stroking a dog half an hour ago but was able to explain the concept of a dog.
In the case of Clive Wearing, the client was a professional musician and could play the piano without difficulty. However, he could not remember having learned to play after suffering a brain injury. This suggests an impaired episodic memory but functioning procedural memory. Both cases support the view that there are separated memory stores for LTM.
Brain scanning studies provide evidence to support the idea of different LTM stores. For example, Tulving et al. (1994) asked participants to perform different memory tests while having a brain scan with a PET scanner. The results seem to imply that episodic and semantic memories were located in the prefrontal cortex (PEC).
Semantic memory was stored on the left PFC, and episodic memory was on the right PFC. This shows a physical reality in the brain to the different types of LTM, confirmed in many research studies and supporting its validity.
Understanding different types of memory allow for the development of helpful real-world applications. Belleville et al. (2006) researched a group of older people suffering from mild cognitive impairment. The researcher compared participants who received memory training with that of a control group that did not. Findings show that participants in the experimental group outperformed the control on an episodic memory test. Researchers concluded that, by categorizing the different types of LTM, psychologists could have a better opportunity to improve peoples' life through devising appropriate treatments and interventions.
Supporting evidence drawn from clinical cases lacks control and has insufficient sample sizes. The use of clinical cases such as Clive Wear's allows researchers to examine memory in a way that would be impossible in laboratory settings. However, there are problems generalizing the findings of these clinical case studies with one, or a few, individuals to explain how memory works in the general population. Drawing firm conclusions from clinical case studies may overlook some unknown and specific issues which can explain the behavior of other individuals. This raises serious concerns about the generalisability of clinical findings.
The similarity between types of LTM suggests they may not be truly distinct enough to be categorized into three types. Also, there is a link between semantic and procedural memory. For example, we can produce automatic languages and talk fluently using semantic concepts, without having to recall the details of each semantic idea consciously. These studies revealed that a clear distinction between the three types of LTM is not a firm conclusion.
Cohen and Squire (1980) argued that episodic and semantic memories are stored together in one LTM store called declarative memory. Episodic memory will transfer and be encoded as semantic memory over time.
Endel Tulving suggested three types of LTM: episodic memory, semantic memory, and procedural memory.
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