Superior Long Term Memory

This post is based on an important book by Scott D. Slotnick titled “Cognitive Neuroscience of Memory.” Remember to consult the website
to see the anatomical information referred to in this post.

Perhaps the most famous research on superior memory, one that has been reported in previous healthy memory blog posts regards London taxi drivers. At one time they needed to memorize the layout of 25,000 city streets and the locations of thousands of city attractions. One study investigated whether there were differences in the size of brain regions between taxi drivers and control participants. They found that these taxi drivers had changes in the size of only their hippocampus, with 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 changes in both types of hippocampal gray matter size correlated with the length of time they had been taxi drivers, which ranged from 1.5 to 52 years (with the largest changes for those who had been taxi drivers the longest).

A follow-up study compared the brain region sizes between London taxi drivers and London bus drivers, who were a better matched control in terms of driving experience, stress, and other factors. The same results were obtained, where the taxi drivers had a relatively larger posterior hippocampus and a relatively smaller anterior hippocampus than bus drivers, and this correlated with the length of time they had been driving a taxi.

Another group of people who have superior memory are those who participate in the World Memory Championships and those who are known for extraordinary memory abilities. A study compared such individuals with control participants to asses whether there were differences in cognitive abilities, differences in the size of brain regions, and differences in the magnitude of fMRI activation during memory tasks. People defined as having superior memory did not differ from control participants in the cognitive abilities tested (IQ ranges were 95 to 119 and 98 to 119, respectively) or in the size of an brain regions. The fMRI task required superior memory for a sequence of digits (a task where those whose superior memory excelled), memory for a sequence of faces, or memory for a sequence of snowflakes. Across tasks, those with superior memory had greater activation in the posterior hippocampus, the retrosplenial cortex, and the medial parietal cortex, which are regions that have been associated with long term memory. Almost all of the participants with superior memory reported using a memory strategy called the method of loci. (entering method of loci into the search block of the healthy memory blog yields 11 hits).

Another case study investigated another individual with a superior memory, who is known as PI, was able to recall the digits of pi to more than 65,000 decimal places. His performance was similar to control participants on the large majority of cognitive tasks. Not surprisingly his working memory was in the 99.9th percentile. But it is conceivable that that might be the result of the extraordinary amount of time he spent memorizing pi. His general memory was average. He was impaired on test of visual memory (3rd percentile or below).

He also reports on individuals who are considered as having highly superior autobiographical memory or HSAMers. There have been eight previous posts on HSAMers. These are people who have detailed episodic 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, and public events that occurred on that day of the week. In one study of HSAMers their performance was normal on most standard cognitive tasks. A comparison of different brain regions between HSAMers and control participants revealed a number of differences including greater white matter coherence in the parahippocampal gyrus, which could reflect greater contextual processing associated with episodic retrieval, and a relatively smaller anterior temporal cortex. The decrease in size of the anterior temporal cortex, which has been associated with semantic memory, may reflect the disuse of this region because those with HSAM rely more on episodic retrieval. Much more research needs to be done with this interesting group.

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