For most of us, it's tricky enough to remember what we were doing this time last week, let alone on some random day years ago. But for a blind 20-year-old man referred to by researchers as HK, every day of his life since the age of about eleven is recorded in his memory in detail. HK has a rare condition known as hyperthymesia and his is only the second case ever documented in the scientific literature (the first, a woman known as AJ, was reported in 2006; pdf).
Brandon Ally and his team have completed comprehensive tests with HK and they've scanned his brain and compared its structure with 30 age-matched controls. They found that HK has normal intelligence, that he performs normally on standard desktop tests of short and long-term recall, and that he has normal verbal learning skills. It's specifically his autobiographical memory that's phenomenal.
The researchers assessed HK's autobiographical memory by choosing four dates from each year of his life since his first memory (that was from 1993 when he was aged three and half), making 80 dates in total. For each of these dates, they gathered at least three facts from HK's family, medical records and the historical records for his neighbourhood in Nashville. HK was then interviewed about each of these 80 dates - for example, he was asked "Can you tell me what happened during your day on January 2nd, 2001". His answers, often detailed, were transcribed and fact-checked.
HK's recollection of days from his life between the ages of 9 and 12 grew dramatically more accurate and detailed, reaching nearly 90 per cent accuracy for memories at age 11, rising to near perfect accuracy thereafter. For some dates, HK was quizzed again at a second session - the consistency of his answers was 100 per cent.
What's it like to have hyperthymesia? HK told the researchers that his autobiographical memories are rich in sensory and emotional details and feel just as vivid regardless of whether they're from years ago or from yesterday. Ninety per cent of the time he experiences these memories in the first-person, compared with rates of approximately 66 per cent in the general population. HK said autobiographical memories frequently enter his consciousness, triggered by news, smells, sounds and emotions. Most days he wakes up thinking about what he's done on that day in previous years. Bad memories come to mind just as often as positive ones, but he is able to choose to focus more on the positive.
In terms of brain structure, overall HK's brain was smaller than average (likely related to his having been born prematurely at 27 weeks). By contrast, his right amygdala was larger, by about 20 per cent, than in the controls. He also has enhanced functional connectivity between his right amygdala and hippocampus and in other regions. The amygdala is a small subcortical structure and part of the limbic system, which is involved in emotional processing. The researchers think that HK's enlarged amygdala and its enhanced connectivity lends a deeper personal salience to his experiences than is normal, thus making them more memorable.
Ally and his team acknowledged that "unique case studies such as HK are not easily translated or generalisable to the normal population", and so should be interpreted with caution. That said, they argued their results provide further evidence for the role of the amygdala in autobiographical memory. "Further, perhaps the present findings can help guide future regions of brain stimulation in memory-disordered populations, with the goal of improving memory function," they speculated. "Indeed, brain stimulation to deep, subcortical memory-related structures has shown very early promise in patients with Alzheimer's Disease."
Ally, B., Hussey, E., and Donahue, M. (2012). A case of hyperthymesia: rethinking the role of the amygdala in autobiographical memory. Neurocase, 1-16 DOI: 10.1080/13554794.2011.654225
Post written by Christian Jarrett for the BPS Research Digest.