In the 2004 film "Eternal Sunshine of the Spotless Mind," the
characters played by Jim Carrey and Kate Winslet undergo a process to
erase all memories of their relationship from their minds. The film
uses a variety of methods to show how the memories disintegrate and
disappear, and it becomes a race against time when Carrey's character
decides he doesn't want to complete the process. He tries to protect
his memories about Winslet's character by hiding them in unrelated
memories.


Good luck, Jim. Not even scientists are completely sure how memories
are formed, how we retrieve them or how they disappear. There are
many types of memory; we humans are pack rats who file away
information ranging from how to make our grandmother's favorite pie
to how to solve algebra equations. But which things does the brain
decide to save? Where does it put this information, and why can't we
get to some of that information when we really want it?




Henrik Sorensen/Stone/Getty Images
This detective writes down all her clues so she won't forget them.
Scientists have been able to pinpoint where certain types of memory
are stored. They've also discovered how neurons fire and synapses are
strengthened when storing these memories. But they don't know exactly
what goes into that neuron to store the memory, or how to dissolve
that synaptic connection if you want to forget something. In
September 2008, new clues about memory emerged that may eventually
help us crack this cold case. In one study, researchers found that
the neurons activated in the recall of a memory are likely the ones
that fired when the event originally occurred [source: Carey]. So
when you talk about reliving old memories, you actually are, because
the brain is doing the exact same thing it did the first time.


But what if there wasn't a first time? One of the problems of getting
to the bottom of memory is that it seems to play tricks on the brain
sometimes. For example, we often create false memories. On July 7,
2005, London experienced a series of bombings. A follow-up study
found that four out of 10 people have false memories of the event
because they claimed to have seen nonexistent television footage
[source: Randerson]. If we're storing things that are unreliable,
does memory serve any purpose at all? According to thinkers as
ancient as Aristotle, we might only need memories as a way to predict
and anticipate the future [source: Eagleman].


The question of what we've experienced and how we experienced it is
leading into our next unsolved brain mystery -- the age-old question
of consciousness. Read more about this riddle of the brain on the
next page.