How do we make a memory? An idea gaining ground is that forming memories and recalling them involves brainwaves cycling several times a second in our hippocampi, two small curved structures on either side of the brain.
Evidence to support the idea is accumulating, including the first glimpse of subtle patterns in people’s actions that reflect these underlying rhythms. “You can make these brainwaves visible in behaviour,” says Maria Wimber at the University of Birmingham, UK.
The findings raise the possibility that one day we may be able to boost memory in people with Alzheimer’s disease by enhancing these brainwaves, says Aidan Horner at the University of York, UK, who wasn’t involved in the work.
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The hippocampi have long been thought to be central to memory because some people who have damage to this part of the brain become unable to form new memories of events. When rats are forming new spatial memories as they run around a maze, for instance, cells in their hippocampi start firing at between four and eight times a second, at the same frequency as we see in theta brainwaves.
Some studies have suggested humans also have such brainwaves associated with memory formation, although their frequency may be from three to eight times a second. To investigate further, Wimber’s team made use of the fact that people who need surgery to stop epileptic seizures originating in their hippocampi have recording electrodes put into these structures for several days to pinpoint the source of their seizures.
During this monitoring, people were asked to learn associations between unrelated words and pictures, and then were later shown the word alone and asked to recall the picture. Theta brainwaves could sometimes be seen in the hippocampi during the learning phase – but , suggesting they are necessary for forging memories.
It took a further step to see the brainwaves reflected in people’s actions, though. Healthy volunteers did a similar learning task, but after being prompted with the word cue they had to press a button the instant they remembered the picture, to record their reaction times over multiple trials. It typically took between one and three seconds for the image to come to mind.
When the results were analysed separately for each person in more than 100 trials each, their reaction times clustered at regular intervals of about one-third of a second, suggesting the memory surfaces at the same point in the theta cycle. “You can see an oscillation,” says Wimber. “Memories were more likely to pop into mind at regular intervals after the reminder, and not in between.”
This rhythmic pattern wasn’t present for those trials in which people gave wrong answers, or in a different task used as a control, where people answered questions about a picture that was present, rather than remembering it. “This isn’t something about just any engaging, cognitive task, it seems to be specific to [memory] encoding and retrieval,” says Horner. The research was presented at the Cognitive Neuroscience Society meeting held online this month.
Understanding this mechanism may eventually lead to new techniques for boosting fading memories. In previous research, people did in which they were shown movie clips where the brightness of the screen and loudness of the sound pulsed in synchrony four times a second.
It might be possible to show people with Alzheimer’s disease pictures and sound clips to help them learn the name of a new carer, for instance, says Horner. “It seems to work in highly controlled laboratory settings, but it’s unclear how it would pan out in the real world.”
In an alternative approach, a device held by the side of the head that produces a magnetic field pulsing at theta frequency boosted theta waves in the hippocampus and  in healthy volunteers. “If we can understand these mechanisms, we may be able to enhance memory more effectively,” says Joel Voss at Northwestern University in Illinois, who did that research.
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