Every morning since January 2004, Rebecca Sharrock crosses off the date on a calendar in her room. Like many people, the 31-year-old uses it to keep track of time, distinguishing the present day from the ones that came before. Unlike many, Sharrock can remember what happened on specific days five, 10, 15 years ago. What day was it on 21 July 2007? A Saturday, Sharrock can recall when asked. An avid Harry Potter fan, Sharrock remembers her stepdad going to the shops that day to buy a copy of the newly published Harry Potter and the Deathly Hallows. The sensation of a warm breeze evokes positive experiences from childhood, as does a vocal workshop Sharrock attended at school when she was 13. “Coincidentally, on that day in late October in 2003, that’s when the US president visited Australia for the first time,” she recalls. (After we speak, I check this: George Bush Jr arrived in Australia for his first presidential visit on 22 October 2003.) Sharrock remembers her mother watching the news that day. “Even though it didn’t mean anything to me, him coming here, that memory [of the vocal group] brings back that whole day,” she says. Sharrock, who lives in Brisbane, didn’t realise there was anything unusual about her memory, until – on 23 January 2011 – her parents showed her a TV news story about people with an extraordinary ability to recall events from their own lives. The segment featured Prof Craig Stark, a neurobiology and behaviour researcher at the University of California, Irvine. Stark’s lab studies a condition called highly superior autobiographical memory (HSAM), also known as hyperthymesia. Sharrock is one of about 60 known people in the world with the condition. In studies, Stark and his colleagues have asked people to recall memories from a particular day one week earlier, and also longer – one year ago, say, or a decade. People with HSAM are significantly better at recalling both personal and public events, and the exact days and dates on which they occurred. “They don’t remember everything,” Stark says. People with HSAM do forget things – but compared to people with ordinary episodic memory, “it’s very, very gradual”. Their extraordinary ability to recall lived experiences results from a type of remembering known as episodic memory. People with HSAM don’t, however, perform any better at standard laboratory memory tests such as rote memorisation tasks. HSAM was first recognised as a condition in 2006, after American woman Jill Price contacted the late Dr James McGaugh, a collaborator of Stark’s at UCI. “Whenever I see a date flash on the television (or anywhere else for that matter), I automatically go back to that day and remember where I was, what I was doing, what day it fell on,” Price wrote. “Most have called it a gift but I call it a burden. I run my entire life through my head every day and it drives me crazy!!!” Many people with HSAM describe a similar tendency to revisit days and test their recall of events, Stark says. “They describe things like, when being young, forgetting something and being really traumatised by it, not wanting to have that happen again.” Some structure their days using calendars, because they are able to remember the experience of marking off a specific date. “I’ll continue to have a calendar in my room until my last day of life,” Sharrock says. “I fear that I won’t know the exact date [otherwise] … Blurring days together – the possibility of that happening just scares me.” To a person with average memory, perhaps Sharrock’s fear feels something like the discombobulating sensation when a friend reminisces about a shared experience of which you have no recollection. “One of the things we don’t really know on these folks is how much of it is an inherent biological thing that makes their memory … better in this domain,” Stark says. His lab has looked at the brains of people with HSAM, but didn’t find major differences in structures important for memory, such as the hippocampus and amygdala. “We found some things in terms of the morphology and in terms of some functional conductivity [of the brain] that actually were more consistent with OCD than anything else,” Stark says, adding that people with HSAM also score highly on scales for obsessive-compulsive traits. While having HSAM can come in handy – Sharrock’s mum checks with her whether purchases are still under warranty – it also has its downsides. “I need to have distractions such as noise and light around me to get to sleep,” Sharrock says. “If everything’s quiet, memories just flash into my mind and that keeps me awake.” For Sharrock, who also has OCD, anxiety and autism, it makes bad memories difficult to deal with. “If I’m remembering something negative, my emotions of that experience will come back,” she says. “Sometimes people will say that I’m just deliberately not letting go, and I’m just like dwelling on the negatives in my life. “It’s awful to be a medical exception because very few people understand what you’re going through and there just aren’t many treatments designed for it. “Remembering this way just seems so normal to me.”