Types of long-term memory (LTM)
Research indicates several types of long-term memory (LTM), each with a separate function and associated with different brain areas. LTM sub-divides into explicit (easy to express in words), which requires conscious thought to be recalled, and implicit (difficult to express in words), which does not require conscious thought to be recalled.
One type of explicit LTM is episodic memory (EM), which gives an autobiographical record of personal experiences, like when your birthday is. Strength of EM relates to strength of emotions and the degree of processing at coding. EM helps us distinguish between real events and imagination.
A second type of explicit LTM is semantic memory (SM), which contains knowledge learned. Strength of SM also relates to the degree of processing at coding, with SMs generally longer lasting than EMs. SMs link to EMs, as new knowledge (SMs) is generally learned from specific experiences (EMs). Over time, such memories become less episodic and more semantic.
Procedural memory (PM) is a type of implicit LTM, allowing us to perform learned tasks with little conscious thought, for example surfing. Many PMs concern motor skills, like walking, and are learned early in life. PMs are also involved in language, helping individuals to speak and use grammar without thinking how to. As PMs do not need conscious thought, we can simultaneously perform other cognitive tasks requiring attention.
Fig 2.3 Recalling how to surf is an example of procedural LTM
Tulving (1989) investigated differences in the processing of episodic and semantic memory. Six participants performed 8 successive trials involving 4 SMs and 4 EMs. During a trial, participants attempted to retrieve a self-selected memory. EMs involved personally experienced events, like a holiday, while SMs involved knowledge acquired through learning, such as from reading a book. Radioactive gold was injected into the participant 60 seconds after retrieval began and their brain was scanned 8 seconds later (after the gold had arrived in the brain). In 3 participants there was greater activation in the frontal lobes of the brain during EM retrieval and in the posterior region of the cortex during SM retrieval (3 participants produced inconclusive data). This suggests SMs and EMs involve different brain areas and are therefore separate forms of LTM.
• Herlitz et al. (1997) assessed explicit LTM abilities in 1,000 Swedish participants and found that females consistently performed better than males on tasks requiring episodic LTM, although there were no differences in SM ability. This suggests there are gender differences in EM ability, possibly because females tend to have better verbal ability.
• Finke et al. (2012) reported the case study of ’PM’, a professional cellist who suffered severe amnesia due to damage in several brain areas through illness. His episodic and semantic LTM was so badly affected that he could not remember musical facts, but his ability to read and play music, including new pieces, was unaffected. This suggests different types of LTM are located in different brain areas.
• Van Gorp et al. (1999) found that abstinence from cocaine by heavy users led to a rapid increase in procedural memory ability. As abstinence from cocaine stimulates dopamine production, it suggests dopamine levels are linked to procedural LTM.
Episodic LTMs may differ from semantic LTMs in terms of different types of thinking and emotion, as EMs are associated with conscious awareness of events and emotional feelings related to them, while SMs are more associated with non-emotional, objective analysis of phenomena.
The frontal lobe brain area’s association with episodic LTMs is supported by case studies of amnesiacs with impaired episodic memories having damaged frontal lobes.
Procedural memories may take longer to learn than explicit LTMs, as they often involve motor functions and spatial abilities, while explicit memories tend to involve higher-level thought processes.
The extent to which episodic and semantic LTM systems are different is unclear. Although different brain areas are involved, there is also a lot of overlap between the 2 systems, with semantic LTMs often emerging from episodic LTMs. Therefore, it is not known whether or not the gradual transformation of an EM into a SM involves a change in memory systems.
As only 3 out of Tulving’s 6 participants showed differences in the processing of semantic and episodic LTMs, the findings cannot be generalised. Also, as 2 of the participants were Tulving and his wife, the findings may be prone to researcher bias.
Psychologists at Vanderbilt University have programmed EM into a robot, so that it can recall past experiences to help solve problems. Attempts are now being made to give it an episodic buffer (see page 27) so it can combine information from different sensory memory channels like a human does.