Recordings from electrodes in people’s brains reveal that certain neurons in the hippocampus show a burst of activity to mark the boundary between different events
A newly discovered kind of brain cell involved in memory formation seems to mark the boundary between distinct events as we experience them.
The neurons, which have been called boundary cells, fire when new events happen, such as if we see someone walking into a room.
The cells were discovered in people with epilepsy who had electrodes put into their brain before surgery, by asking them to watch films showing sequences of events.
“It has long been appreciated in psychology that memory is not continuous, that it’s formed in chunks. But this has never been observed at the single neuron level,” says Ueli Rutishauser at Cedars-Sinai Medical Center in Los Angeles.
Much about how memories form is still unclear, but several insights have emerged from studying people who need electrodes put into their hippocampi – two curved structures on either side of the brain – because they have epileptic seizures that start in these regions. The hippocampi and surrounding areas of the brain are crucial for making new memories.
To work out exactly which part of the brain is malfunctioning, people with epilepsy may stay in hospital for several days with electrodes implanted to record brain activity. Each electrode has eight tiny wires protruding from it, and each wire can distinguish activity from up to five surrounding neurons.
In the latest study, Rutishauser’s team asked 19 such people to watch carefully constructed film sequences while the recording took place, listening to about 30 cells per person.
About 7 per cent of the neurons were boundary cells, whose firing peaked when new things happened. “They don’t say anything about the memory content, they just say there is a boundary,” says Rutishauser. He speculates that activity in these cells signals that the brain should begin to form a new memory, like starting a new folder.
When people were tested later by showing them pictures from the films, they were better at remembering scenes from straight after a boundary than those that had happened a few seconds later.
The findings “make a lot of sense”, says Rodrigo Quian Quiroga at the University of Leicester, UK. “They suggest a mechanism by which the hippocampus is signalling what scenes to put together and what scenes not to put together.”
Boundary cells haven’t been seen before because previous work in people with electrodes implanted before epilepsy surgery used memory tests involving words or still pictures, not films, says Rutishauser.
Journal reference: Nature Neuroscience, DOI: 10.1038/s41593-022-01020-w
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