Memory, then & now
Memory is like a streaming video that is bookmarked by facts, scenes, characters and thoughts.Updated: Jul 09, 2011 22:47 IST
Researchers have long known that the brain links all kinds of facts, related or not, when they are learned about the same time. Just as the taste of a cookie and tea can start a cascade of childhood memories, as in Proust, so a recalled bit of history homework can bring to mind a math problem - or a new dessert - from that same night.
For the first time, scientists have recorded traces in the brain of that kind of contextual memory, the ever-shifting kaleidoscope of thoughts and emotions that surrounds every piece of newly learned information. The recordings, taken from the brains of people awaiting surgery for epilepsy, suggest that new memories of even abstract facts - an Italian verb, for example - are encoded in a brain-cell firing sequence that also contains information about what else was happening during and just before the memory was formed, whether a tropical daydream or frustration with the Mets.
Video of things past
The new study suggests that memory is like a streaming video that is bookmarked, both consciously and subconsciously, by facts, scenes, characters and thoughts. Experts cautioned that the new report falls well short of revealing how contextual memory and different cues interact; some words might throw the mind into a vivid reverie, while others do not. But the report does provide a glimpse into how the brain places memories in space and time.
"It's a demonstration of this very cool idea that you have remnants of previous thoughts still rattling around in your head, and you bind the representation of what's happening now to the fading embers of those old thoughts," said Ken Norman, a neuroscientist at Princeton who did not participate in the study. "I think they have very good evidence that this process is crucial to time-stamping your memories."
In the new study, in the current issue of the journal PNAS (Proceedings of the National Academy of Sciences), doctors from the University of Pennsylvania and Vanderbilt University took recordings from tiny electrodes implanted in the brains of 69 people with severe epilepsy. The implants are standard procedure in such cases, allowing doctors to pinpoint the location of the flash floods of brain activity that cause epileptic seizures.
The patients performed a simple memory task. They watched a series of nouns appear on a computer screen, one after another, and after a brief distraction recalled as many of the words as they could, in any order. Repeated trials, with different lists of words, showed a predictable effect: The participants tended to remember the words in clusters, beginning with one and recalling those that were just before or after.
This pattern, which scientists call the contiguity effect, is similar to what often happens in the card game concentration, in which players try to identify pairs in a grid of cards lying face-down. Pairs overturned close in time are often remembered together.
Recording from the electrodes, the researchers looked for a neural firing pattern that had a very distinct signature - it updated continually, like a news ticker tape. They found a strong signal in the temporal lobe of the brain, an area extending roughly between the temple and the ear. When participants recalled a word - "cat," for example - the pattern in this region looked identical to when "cat" was originally seen on the computer screen.
Moreover, the pattern was only slightly different when they recalled the words just before, and just after, "cat" on the list.
"Here we have shown, in effect, that the word before 'cat' - let's say it's 'tree' - has coloured or influenced the encoding for 'cat,' just as 'cat' has influenced the encoding of the next word, let's say 'flower,' " said Michael J Kahana, a neuroscientist at the University of Pennsylvania and an author of the paper. His co-authors were Jeremy R Manning, Dr Gordon Baltuch and Dr Brian Litt, all of Penn; and Sean M Polyn of Vanderbilt.
The way the process works, the authors say, is something like reconstructing a night's activities after a hangover: remembering a fact (a broken table) recalls a scene (dancing), which in turn brings to mind more facts - like the other people who were there - and so on. Sure enough, the people in the study whose neural updating signals were strongest showed the most striking pattern of remembering words in clusters.
"When you activate one memory, you are reactivating a little bit of what was happening around the time the memory was formed," Dr Kahana said, "and this process is what gives you that feeling of time travel."