These types of memories are branded on most brains.
But not all brains, scientists in Toronto and elsewhere are discovering.
For most people, the mere mention of a key life event propels them back in time. They can picture where they were, who they were with, sometimes even the clothes they were wearing. With that flood of visual memories comes an echo of the sensations the event triggered.
Susie McKinnon does not register and relive memories in this way.
McKinnon is one of three people researchers at Toronto's Baycrest Health Sciences Centre have found who are not able to form and revisit memories of episodes in their past.
Brian Levine, the leader of the Baycrest team, named the condition lifelong severely deficient autobiographical memory, or SDAM.
He and his colleagues recently reported on their findings in the journal Neuropsychologia. Theirs is only the second report in the medical literature of this memory condition and the first to involve using brain imaging techniques to test people with the condition.
Finding more people with SDAM may change the understanding of how people with it function and cope. But from what has been seen so far, the condition does not appear to affect semantic memory — the ability to learn and retain facts. All three people the Baycrest team studied have careers and function well in their lives. One, in fact, has a PhD.
If they are married, they would know when they got married, and may recall details of the ceremony from photographs they've studied or stories others have told.
But they cannot recollect the event or revisit it in their mind's eye. Ask them a question about the big day that they haven't learned from studying photos or hearing stories and they'll draw a big blank.
Take McKinnon's wedding, for example, an event she describes as "a wonderful occasion, I'm sure."
"I know very colourful stories. But they could have just as easily happened to somebody else," admits McKinnon, 60, who lives in Tumwater, Wash.
"I can certainly assume that I was very happy and overjoyed.... But I don't have memories of me actually going through that. It's just a very well-rehearsed story that I've thought about a lot."
The condition is the inverse of something known as HSAM — highly superior autobiographical memory. That condition is often associated with actress Marilu Henner, best known for her role on the late 1970s sitcom Taxi.
People with HSAM remember their lives in extraordinary detail. Show them a pair of shoes and they can tell you precisely when and where they bought the footwear and how much they paid.
Reports of highly superior autobiographical memory started to emerge about a decade ago. Around the same time, McKinnon reached out to Levine, a cognitive neuroscientist at Baycrest's Rotman Research Institute.
McKinnon recognized her memory was atypical, though she was able to build a successful career in professional association management. She had long sought an explanation for her memory deficit and found Baycrest and the work its scientists do on memory.
Later, two other individuals contacted Baycrest as well, another American and a person who lives in Britain.
Daniela Palombo, first author of the paper in Neuropsychologia, says all three realized in early adulthood that they didn't experience memories in the way others around them did.
Palombo, now doing post-doctoral research at Boston University School of Medicine, says the three have come up with techniques to overcome their memory issue, such as relying on day planners and memorizing facts about events based on pictures and the accounts of others.
None of the three experienced brain damage or trauma that would explain the condition, though brain imaging shows slightly reduced volume in the part of the brain responsible for episodic memory, the hippocampus.
And a couple of imaging experiments the researchers did showed the brains of these people responded differently to tests that would draw on episodic memory — proof, the scientists say, that while their semantic memories are fine, they lack episodic memory.
"I believe them. But we're also getting brain signals that correspond to what they're saying. So it gives us a little more confidence," Levine says.Suggest a correction