Illustration: Aïda Amer/Axios

Most humans can rely on some aspects of memory, but some live on the extremes: those who remember everything that happens to them — or have Highly Superior Autobiographical Memory (HSAM), and those who can’t remember events from their lives at all — and have Severely Deficient Autobiographical Memory (SDAM).

Why it matters: By looking at people with abnormalities in memory, neuroscientists encounter new ideas about what happens in the brain, James McGaugh, a neurobiologist at the University of California, Irvine, tells Axios.

“You can ask them about any day in their life,” McGaugh says about people with HSAM, which he discovered in 2000 when a woman named Jill Price wrote to him:

"Whenever I see a date flash on the television I automatically go back to that day and remember where I was, what I was doing, what day it fell on and on and on and on and on. It is non-stop, uncontrollable and totally exhausting.”

The neurological mechanisms underlying HSAM are still being studied.

  • Those with HSAM retrieve memories more rapidly and certain brain regions are more active in them than in adults who don’t have the condition.
  • Today, the total known number of people in the world with HSAM is around 120.

The flip side: Susie McKinnon doesn’t remember any events from her life. She doesn’t remember any vacations she’s taken. She doesn’t remember herself as a kid.

  • McKinnon emailed University of Toronto neuroscientist Brian Levine in 2006 about her condition, and is the first person identified with SDAM.
  • Many people with SDAM say they can’t form images in their heads, something Levine plans to investigate further with brain scans.
  • So far thousands of people who have taken Levine’s memory survey say they have SDAM. But “many people don’t even know they have it until adulthood,” he tells Axios.

What’s next: People with memory abnormalities suggest the possibility that we all have the potential ability for conditions like HSAM in our brains, but it may be impaired “because we have a brain that’s totally functioning,” McGaugh says.

  • “What happens if part of the brain is no longer effective? Does that allow another part of the brain to expand and do these marvelous things?" McGaugh asks. "That’s the future of brain research, exploring what the potential is of the brain, not just what it’s like for ordinary people.”

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