Hilary Boden explains the mind’s knack for forgetting
Memory is complicated, and it’s distinctly annoying: especially when the only PIN you can remember is that one you used years ago, or your new e-mail password seems to be constantly usurped by your out of date, completely useless “spicegirlsrule” one. It seems very easy to remember the wrong thing.
Researchers at Yale University have investigated how the brain juggles old and new information, when it selects the correct memory, and when and how the process fails. The team asked 24 healthy undergraduate volunteers to memorize a picture-word pair, and then to learn a contradictory one, before being asked to describe the most recent pair. The original memory was constructed by displaying a word above an unrelated picture: for example, ‘SWIM’ was presented above a photo of Al Gore. This pairing would be shown twice to the volunteers, before a new image altered the association: for example, a new pairing of ‘SWIM’ with an image of the Grand Canyon would be shown.
When the subjects were asked to remember the word ‘SWIM’, researchers were able to use data from brain scans in order to determine which association was most dominant. This is because specific areas of the brain respond to different categories of images, for example images of faces, scenes or objects. Levels of activity in these areas indicated the ‘strength’ of the memory. This distinction enabled the researchers to measure the strength of new and old memories separately. The example discussed earlier saw that the majority of the activity was in the region which responds most strongly to faces, even when the participants were asked to remember the Grand Canyon scene. The results, published in the 7 March issue of the Journal of Neuroscience, saw that when the pattern of brain activity for old memories was stronger, the volunteer was less likely to recall the newer pair correctly. Even when participants did report the correct, new memory, traces of the old memory were observed in the scans. Moreover, when new and old memory strengths were evenly matched, the volunteers were observed to take longer to answer the question. ‘We’re seeing evidence of things people are trying to not remember,’ Postdoctoral Fellow and co-author of the study Brice Kuhl explained. These results are perhaps intuitive, and scientists have long suspected that the interference of older memories plays a role in memory problems.
Another experiment, carried out by physicist Eric Lutz and his team at the Free University of Berlin, has confirmed that erasing memory really does require work. By measuring the heat released when a single bit of data is deleted, the link between erasing information and heat flow has been confirmed. This validates the prediction made in 1961 by IBM physicist Rold Landauer, who calculated that in erasing a bit of information, an extremely small amount of heat (approximately 3 billionths of a trillionth of a Joule) would be released.
Lutz went to the laboratory of Sergio Ciliberto, of the French National Centre for Scientific Research in order to test Landauer’s calculations. There, scientists created a one-bit memory system with a floating silica bead and a double well, where the bead could rest in either one well or the other. Through lowing and tilting the barrier between the two wells, representing state 1 and state 0, the bead could be encouraged to ‘jump’ from one well to the other. The system memory could be erased by ‘resetting’ it to state 1, regardless of which state the bead started in. The speed at which the bead was reset was carefully measured, allowing the researchers to calculate the heat produced during the shift. Subsequently, Lutz’s experiment is the first to show that Landauer’s prediction of a tiny release of heat energy was correct. These results link information and heat flow, whilst conforming to the second law of thermodynamics. Scientists now hope that this link may prove useful in building miniature computers in the future.
So perhaps the brain-computer metaphor is really very appropriate, it certainly goes towards explaining why trying to forget an embarrassing night out seems the hardest thing to do!
Image by kylemackie