Life memories for most of us are like grains of sand, slipping through our fingers as we try to hold on to as many as we can. But for a rare few, emotional events stick like cobwebs and are much harder to shake off.
A new case study describes one such extraordinary individual: a female teenager, named TL for anonymity, who can recall an astonishing number of personal events from her life.
Fewer than 100 people worldwide are thought to have her condition, known as hyperthymesia, or highly superior autobiographical memory (HSAM).
It was only described for the first time in 2006, and TL's case study is the first comprehensive evaluation of how individuals with the condition retrieve personal events from the past and imagine personal events set in the future.
Her incredible ability to mentally 'time travel' through the days allows TL to feel as though she is re-experiencing what has come and pre-experiencing what is yet to be.
Her story could help scientists better understand how the human brain encodes, retrieves, or discards memories from our lives – thereby formulating a sense of identity and continuity.
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"In these individuals, known as hyperthymesics, memories are carefully indexed by date," explains neuropsychologist and lead author of the case study Valentina La Corte from Paris Cité University.
"Some will be able to describe in detail what they did on July 6, 2002, and experience again the emotions and sensations of that day."
Ever since TL was a kid, she's been able to mentally time travel to relive her memories in vivid detail, often from various angles, as if she were omnipresent at the scene.
TL revealed this ability to her friends when she was 8. Unable to imagine what she was talking about, they accused her of lying.
"She became aware that her mind functioned in an atypical way, and by fear of looking weird, she only mentioned it to her family at the age of 16," write La Corte and her colleagues.
At the age of 17, TL decided to share her story with the wider world.
In a memory test, La Corte and her team asked TL to recall four personally relevant events from five periods in her life. Her recollection for detail performed at the top of the normative average, consistent with today's accepted diagnosis of hyperthymesia.
TL's memories are apparently "rich in contextual and phenomenological detail" and characterized by a "powerful feeling of re-experience."
She can also 'travel' to the future. Her mental predictions can evoke a strong sense that events she hasn't previously personally experienced have happened before.
Perhaps most fascinating is TL's time-traveling 'machine'. Facts and scholarly knowledge that carry no emotional weight aren't stored alongside special visual imagery in her mind and require effort to memorize. She calls them "black memories".
By contrast, her personal memories, which do carry emotion, are more easily stored in her mind's eye in a rectangular, very large, white room with a low ceiling.
Like a library, this mental room contains the moments of her life, filed according to an intricate catalogue system. Each toy she's ever owned is there on display, with a tag dictating its name, and from whom and when it came from.
She can pore over family photos in the white room, as she's memorized every detail of them. If she wants to, TL can pull a much-loved book off the mental shelf for a comforting read.
Her memories are arranged chronologically, and the details are fuzzier the longer ago they happened.
"Roughly speaking," reads the case report, "TL could discriminate days of the past month, months of the past two years, and only years for older memories."
Unable to forget negative memories like the rest of us, TL stores them in a chest within the white room. That's where she keeps the death of her grandfather.
The white room is also adjoined to other rooms she can escape to when her emotions flare.
She has a cold "pack ice" room where she goes to cool down when she's angry. If she's working through a problem, she closes herself off in a small, empty "problems" room without distractions.
When her father left to pursue his military career, another unpleasant room appeared in her mind, inhabited by soldiers. TL finds herself there when she's feeling guilty.
TL's case study doesn't explore the difficulties of her exceptional memory, but others with HSAM have described the incessant and excessive stream of recollections as "non-stop, uncontrollable, and automatic".
Very few have shared their experiences in peer-reviewed papers.
"It is difficult to generalize findings about hyperthymesia, since they rely on only a few cases. Does aging affect the memories of these individuals? Do their mental time-travel abilities depend on age? Can they learn to control the accumulation of memories?" asks La Corte.
"We have many questions, and everything remains to be discovered. An exciting avenue of research lies ahead."
The study was published in Neurocase.