Introducing the brain: neurons in constant flux (Introduction)

by David Turell , Thursday, June 10, 2021, 18:38 (1050 days ago) @ David Turell

Nothing seems stationary with neurons changing as experience continues:

https://www.theatlantic.com/science/archive/2021/06/the-brain-isnt-supposed-to-change-t...

"Schoonover, Fink, and their colleagues from Columbia University allowed mice to sniff the same odors over several days and weeks, and recorded the activity of neurons in the rodents’ piriform cortex—a brain region involved in identifying smells. At a given moment, each odor caused a distinctive group of neurons in this region to fire. But as time went on, the makeup of these groups slowly changed. Some neurons stopped responding to the smells; others started. After a month, each group was almost completely different. Put it this way: The neurons that represented the smell of an apple in May and those that represented the same smell in June were as different from each other as those that represent the smells of apples and grass at any one time.

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"...other scientists have shown that the same phenomenon, called representational drift, occurs in a variety of brain regions besides the piriform cortex. Its existence is clear; everything else is a mystery. Schoonover and Fink told me that they don’t know why it happens, what it means, how the brain copes, or how much of the brain behaves in this way. How can animals possibly make any lasting sense of the world if their neural responses to that world are constantly in flux? If such flux is common, “there must be mechanisms in the brain that are undiscovered and even unimagined that allow it to keep up,” Schoonover said. “Scientists are meant to know what’s going on, but in this particular case, we are deeply confused. We expect it to take many years to iron out.”

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"In another experiment, Laura Driscoll, a neuroscientist who is now at Stanford, placed mice in a virtual T-shaped maze, and trained them to go either left or right. This simple task depends on the posterior parietal cortex, a brain region involved in spatial reasoning. Driscoll and her colleagues found that activity in this area also drifted: The neurons that fired when the mice ran the maze gradually changed, even though the rodents’ choices remained the same.

"These results were surprising, but not overly so. The hippocampus is also involved in learning and short-term memory. You’d expect it to overwrite itself, and thus to continuously drift. “Up until now, observations of representational drift were confined to brain regions where we could tolerate it,” Schoonover said. The piriform cortex is different. It’s a sensory hub—a region that allows the brain to make sense of the stimuli around it. It ought to be stable: How else would smells ever be familiar? If representational drift can happen in the piriform cortex, it may be common throughout the brain.

"It might be less common in other sensory hubs, such as the visual cortex, which processes information from the eyes. The neurons that respond to the smell of grass might change from month to month, but the ones that respond to the sight of grass seem to mostly stay the same. That might be because the visual cortex is highly organized. Adjacent groups of neurons tend to represent adjacent parts of the visual space in front of us, and this orderly mapping could constrain neural responses from drifting too far. But that might be true only for simple visual stimuli, such as lines or bars. Even in the visual cortex, Ziv found evidence of representational drift when mice watched the same movies over many days."

Comment: I'm not surprised the brain is so confusing. It could not have been designed by chance.