WHEN TIME LORDS FORGET

The Doctor: There was only one way to keep Clara safe. I had to wipe some of her memory.
Waitress: Of what?
The Doctor: Of me.

So Season 9 of Doctor Who ended with the Doctor (Peter Capaldi) getting his mind wiped, forgetting all about Clara Oswald (Jenna Louise Coleman), except for the hole left in his memories. It was an elegant climax, a nice echo of Season 4's downbeat conclusion, where Donna Noble (Catherine Tate) lost all her memories of being the Doctor's companion. This time round, the Doctor is the one with amnesia, giving Clara a much more optimistic and generous send-off in a TARDIS of her own - sorry to all the Clara-haters who wanted to see her stay dead; you're just going to have to accept it.

The exact mechanic by which the memory-wipe happens is a "neuro blocker" but it's really what film and TV writers call a Maguffin - an object that moves the plot along; it doesn't matter how it works. Nonetheless, "neuro" suggests it works by altering brain structures and we know that altering brain structures removes memories. The tragic case of H.M. came about when surgery on H.M.'s brain removed his hippocampus, taking with it memories of the last decade of his life and the ability to create new ones. A brain virus did something similar to Clive Wearing and Sir Colin Blakemore's case study on Clive Wearing reveals that he too suffered damage to the hippocampus.

I know her name was Clara. I know we travelled together. I know that there was an Ice Warrior on a submarine and a mummy on the Orient Express... but I’ve no idea what she said. Or what she looked like. Or how she talked. Or laughed. There’s nothing there. Just nothing.

The Doctor's amnesia is much more specific than H.M. or Clive Wearing - or the blanket memory wipe he gave to Donna Noble. The Doctor remembers that there was a Clara, he just can't remember the details. This sort of distinction was identified by Endel Tulving as episodic and semantic memory. Semantic memory is the memory of meanings and relationships: the Doctor remembers he had a companion, he remembers how she fitted into his life. What he lacks are the episodic memories, the particular details of her appearance and actions.

(With delicate irony, writer Stephen Moffat has the Doctor explain this to Clara herself, who is disguised as a waitress and talks to him and laughs at what he says.)

A similar thing happened to Clive Wearing, who remembers that he is a father, but cannot remember his children's names. He remembers that he loves Deborah, his wife, but cannot remember where she is.

I call myself 'Me'. All the other names I chose died with whoever knew me. 'Me' is who I am now. No one's mother, daughter, wife. My own companion - singular, unattached, alone.
- 'Me' / Ashildr

Season 9 introduced another aspect of the psychology of memory with the character of Ashildr (Maisie Williams), a Viking girl who becomes immortal. Ashildr outlives her husband and children and everything she every knew. Over the centuries, she masters every skill but is consumed with boredom and loneliness. She forgets most of her past, her old name and her family. She keeps a journal to remind herself of things, but tears out the pages when the memories they bring back are too painful. The only pages she leaves intact are the ones describing her long-dead children - but that is so that she will remember not to have any more.

Students studying Long Term Memory are told that its duration and capacity are "unlimited". However, we only think this because very few humans build up more than a century of memories. There might be a liit to LTM's duration/capacity, it's just that no one has reached it yet. But what might happen if you did?

Maybe LTM does have a duration - say, 150 years? If that were the case, then when someone passed their 151st birthday, they'd lose memories from their first year of life. No great loss, you might say. But when you hit 160 and you can't remember anything before the age of 10, that's a bit more alarming. Someone reaching 170 wouldn't remember their schooldays, at 180 you'd have forgotten your wedding, by 200 you might have forgotten your children and your parents. Scary.

If LTM has a limited capacity, then eventually it becomes "full" and you just can't add more. This is worse, because you'd be left in the same position as H.M., unable to make new memories and forgetting everything 30 seconds after it happens.

Ashildr doesn't seem to be as badly off as that. She seems to be able to choose to forget stuff, either because it's painful (like memories of family) or burdensome.

The idea of selective amnesia - choosing which stuff to forget and which to remember - is part of a big debate in the psychology of memory. Sigmund Freud pioneered the idea of motivated forgetting (he called it "repression") where we forget episodes that are disturbing, frightening or shameful. However, repressed memories aren't gone forever: they can turn up again, years down the line, as painful "flashbacks". This is a controversial idea because psychologists like Elizabeth Loftus argue that these "recovered memories" are really just imaginative reconstructions - it's quite possible they never really happened.

10,000 hours is all it takes to master any skill. Over 100,000 hours and you're the best there's ever been. I don't need to be indestructible - I'm superb.
- 'Me' / Ashildr

It is interesting that 'Me'/Ashildr doesn't forget skills. Using her immortal life to perfect her abilities, she doesn't forget how to fight or speak anguages or build reality-bubbles.

Psychologists call this procedural memory and it does indeed seem to operate separately from episodic and semantic memory. For example, Clive Wearing still remembers how to conduct music and play the piano. H.M. remembered how to play tennis, even though he had forgotten learning it and (because he learned after his brain damage) didn't know that he knew how to do it.

That's a perk to being immortal - you can get really good at stuff. Obviously, you can get to the final boss fights on all your favourite video games. You can learn to play electric guitar like Hendrix or surf or speak Finnish or become Tenth Dan in karate.

In Greek mythology, Tithonus was given eternal life by the gods, but they fogot to give him eternal youth as well. Silly gods, but poor Tithonus, who aged into a rotting zombie but could never die. The psychology of memory gives me a great idea for a short Science Fiction or Horror story. Imagine if a character did become immortal, through drugs or by becoming a vampire or whatnot. They would spend a long time enjoying themselves and become awesome at everything... but then, the sting in the tail! Their LTM is still finite and runs out. They carry on living (or being a vampire) but forget everything they ever knew - or turn into a creature with H.M.'s problem, unable to remember any new stuff and just living moment by moment. Imagine a vampire like that, that knows that it is hungry for blood and has forgotten it was ever a person - and forgets 30 seconds after you try to remind it! Creepy.