Saturday, 8 March 2014

Neuropsychological Evidence for Differences Between STM and LTM

Some evidence for the difference between STM and LTM comes from case studies of people who have suffered brain damage. In these cases memory is selective; it affects one type of memory but not the other. 

Clive Wearing 
Wearing has an impaired short term memory and has a poor duration that can only hold information for 7 seconds (the average person has a duration of 18-30 seconds). Since his illness he cannot create any new memories and constantly feels like he has awoke from unconsciousness. Wearing contracted a form of amnesia after a herpes simplex (cold sore) spread to areas of his temporal lobes. 

Henry Molaison 
HM suffered from epilepsy and as a result underwent brain surgery to remove part of his temporal lobes and hippocampus. This alleviated his epilepsy but left him with severe memory deficits although his IQ remained above average. He was able to recall events in his early life but was unable to remember events for about 10 years before the surgery and could not learn or retain new information. He could remember approximately six numbers in the order they were presented suggesting his STM was okay. However, he would read the same magazine over and over again and not process the information into LTM, not realizing he had read it before. He was also unable to recognise psychologists who sent long periods of time with him. HM suffered from anterograde amnesia (the loss of ability to create new memories).

In both of these cases, episodic memory (the memory of autobiographical events times, places, associated emotions, and other contextual who, what, when, where, why knowledge that can be explicitly state) is lost. However Semantic memory ( which refers to the memory of meanings, understandings, and other concept-based knowledge, and underlies the conscious recollection of factual information and general knowledge about the world) is largely unaffected 

KF sustained brain injuries after a motorcycle accident. He appeared to have an intact LTM store in that he was able to learn new information and recall stored information. However, his STM was affected so that he had a recency effect of only one item. Also, although his visual information was on a normal level, he struggled with auditory and verbal information making conversation difficult. 

Alzeihmer's Patients
Patients with Alzheimer's disease have been found to have  low levels of a neurotransmitter acetylcholine. Drachman and Sahakian (1979) investigated this by administering a drug to a group of participants which blocks the action of acetylcholine in the brain. They then gave participants various memory tasks that tested either STM or LTM and compared their performance with a control group. They found that the experimental group performed normally on STM tasks but significantly more poorly in the LTM task.

Squire et al. found that the hippocampus is active in LTM tasks whereas areas in the prefrontal cortex are activated for STM tasks. 

Conclusions: what does this tell us?
-Amnesiacs tend to lose only one store suggesting that LTM and STM are separate
-However, it is more complicated as LTM seems to consist of more than one component (semantic and episodic). Plus, KF's case seems to suggest that STM is more complex than a single store.

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