GoKawiilIt is a dogma in neuroscience that certain brain cells respond in the same way to the same thing. Specific neurons always fire, for example, when we see particular shapes and colours; other neurons activate to swing an arm or wiggle a nose. The brain needs this stability, the theory goes, to respond to the outside world in a consistent way.
So, when neuroscientist Laura Driscoll began her doctoral research at Harvard University in Cambridge, Massachusetts in 2012, her first task was to establish this baseline by tracking the activity of individual mouse neurons over time.
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To Driscoll’s surprise, the baseline kept moving. Over the course of several days, many of the cells’ responses had shifted noticeably. Neurons that had fired when a mouse was in a specific location on day one were barely responding in the same spot after a few weeks. “It absolutely defied all of our expectations,” recalls Driscoll, who is now at the Allen Institute in Seattle, Washington. “This was so surprising that my whole project changed.”
In 2017, she and her colleagues reported findings from that project that flew in the face of neuroscience dogma. Over a single day, neurons in the parietal cortex, a hub for processing sensory information, fired predictably in response to specific things, such as the position of the mouse in a virtual maze. But over the course of a few weeks, even though the task of navigating the maze remained the same, these activity patterns underwent major reorganization1. Some of the neurons stopped firing in response to stimuli that had previously activated them; others did the reverse. In groups of cells, however, patterns of neuronal activity remained more consistent over time. The results suggested that individual neurons might not have fixed roles, and that the response of single cells might be less important than the activity of whole populations.
When Driscoll published that work, there had already been a handful of papers describing similar observations in different parts of the mouse brain. But many in the neuroscience community were sceptical: researchers questioned whether these findings might be the result of an experimental quirk, such as imprecise tracking of single cells or subtle changes in the animals’ behaviour that the experimenters hadn’t accounted for.
Since then, many more researchers have reported evidence for neurons changing how they respond to certain stimuli or behaviours over time, a phenomenon that neuroscientists have dubbed representational drift (see ‘How neuronal activity drifts over time’). Evidence has been found for it in various brain regions and when using several different techniques. Generally, the community is coming to accept that drift is real, but some scientists remain unconvinced — in part because of some studies that have failed to find this effect.
And debates swirl around other questions, such as: how is the brain able to generate stable behaviours when neuronal representations are in constant flux? What purpose, if any, does drift serve? And how does drift relate to plasticity, in which the brain changes its connections to learn new things?
Understanding drift could have far-reaching implications, from deciphering how memories are formed and updated to informing the design of brain–computer interfaces and neural networks for artificial-intelligence tools, say researchers.
“When people talk about this phenomenon, it’s exciting because it’s just so full of possibilities,” says Andrew Fink, a neuroscientist at Northwestern University in Evanston, Illinois. “It raises really deep questions about what’s going on in the brain.”
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