Superior Memory Does Not Come Without a Cost
Written by: Scott D. Slotnick
Part 1 of the Memory series
Scott D. Slotnick author of Cognitive Neuroscience of Memory gives us an insight into superior memory research.
Long-term memory abilities naturally vary among people within the range of normal, but there are rare individuals that have truly superior memory. Surprisingly little is known about the brain basis of superior memory. Fortunately, there are a few sparse lines of research that have begun to shed light on the brain basis of superior memory.
One line of research on superior memory has focused on London taxi drivers who must learn the layout of 25,000 city streets and the locations of thousands of city attractions. In one study, the investigators assessed whether there were differences between the sizes of brain regions between taxi drivers and control participants who did not drive taxis (Maguire et al., 2000). The assumption was that if a brain region was used more (or less) in the taxi drivers, this region would grow (or shrink) in size. They found that taxi drivers only had changes in the size of their hippocampus, a region within the medial temporal lobe that is critical for episodic memory (i.e., detailed memory for previous events). Specifically, taxi drivers had a relative increase in the amount of gray matter within the posterior hippocampus and a relative decrease in the amount of gray matter within the anterior hippocampus. Moreover, the types of changes in both types of hippocampal gray matter size correlated with the length of time they had been taxi drivers, with the largest changes for those who were taxi drivers the longest. In a follow-up study (Maguire, Wollett, & Spiers, 2006), taxi drivers were found to be worse at copying a complex drawing from memory, and a subsequent study from the same group (Woollett, Spiers, & Maguire, 2009) observed the same result and also reported that taxi drivers were relatively worse than control participants at learning object-place pairs and word pairs. These behavioral memory deficits may be due to the relatively smaller size of the anterior hippocampus in taxi drivers. Thus, although the taxi drivers had superior memory for navigating London, it appears to have come at a cost for other forms of memory.
Another group of individuals who have superior memory are those who have participated in the World Memory Championships and those known for extraordinary memory abilities. One study compared such individuals with that of control participants to assess whether there were differences in differences in the magnitude of brain activation during memory tasks (Maguire, Valentine, Wilding, & Kapur, 2003). The tasks required memory for a sequence of digits (where those with superior memory excelled), memory for a sequence of faces, or memory for a sequence of snowflakes. Across tasks, those with superior memory had greater brain activation in the posterior hippocampus, which is consistent with the London taxi driver results above. A separate study investigated another individual with superior memory known as PI who was able to recall the digits of to over 65,000 decimal places (Raz et al., 2009). Although PI had superior ability on working memory (99.9th percentile), his general memory was average (50th percentile) and he was impaired at tests of visual memory (3rd percentile or below). Thus, as with the London taxi drivers, PI’s superior memory ability appears to have had a cost outside the domain of his expertise.
A final group of individuals with superior memory that will be considered have highly superior autobiographical memory. These individuals have episodic/detailed memory for every day of their later childhood and adult life. If they are given any date, they can recall the day of the week, the public events that occurred that day, and detailed autobiographical details from that day. In one study, a comparison of the different brain regions between participants with highly superior autobiographical memory and control participants revealed a number of differences including greater white matter coherence in the parahippocampal gyrus and a relatively smaller anterior temporal cortex (LePort et al., 2012). The decrease in size of the anterior temporal cortex, which has been associated with semantic memory (i.e., non-detailed memory for facts such as the definition of a word), may reflect the disuse of this region because they rely more on episodic memory. Although tests of semantic memory were not included in this study, their smaller anterior temporal cortex would suggest that those with superior autobiographical memory may be deficient on such tasks.
The previous findings indicate that individuals with superior memory in one domain often have inferior memory ability in another domain. One important issue is whether the brain changes in those with superior memory preceded their mental ability or whether practice with their mental ability resulted in the changes to their brain. In the case of London taxi drivers, it appears to be the latter as their memory abilities and brain structure returned to normal after they retired (Wollett et al., 2009). For the other groups, it is likely a mixture of nature and nurture that drives their modified mental processing and brain processing. Superior memory research is still in its infancy and promises to be an exciting new area of research.
Check out the rest of the memory series blogs:
Part 3: The Brain Basis of Forgetting
Part 4: Episodic Memory in Mammals
Part 5: The Consolidation Debate