Neuroscientists Have Discovered a Phenomenon That They Can’t Explain

This is, of course, just one study, of one brain region, in mice. But 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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It had already taken years for Schoonover and Fink to even confirm that representational drift exists in the piriform cortex. They needed to develop surgical techniques for implanting electrodes into a mouse’s brain and, crucially, keeping them in place for many weeks. Only then could they be sure that the drift they witnessed was really due to changes in the neurons, and not small movements of the electrodes themselves. They started working on this in 2014. By 2018, they were confident that they could get stable recordings. They then allowed implant-carrying mice to periodically inhale different odors.

The team showed that if a neuron in the piriform cortex reacts to a specific smell, the odds that it will still do so after a month are just one in 15. At any one time, the same number of neurons fires in response to each odor, but the identity of those neurons changes. Daily sniffs can slow the speed of that drift, but they don’t eliminate it. Nor, bizarrely, does learning: If the mice associated a smell with a mild electric shock, the neurons representing that scent would still completely change even though the mice continued to avoid it. “The prevailing notion in the field has been that neuronal responses in sensory areas are stable over time,” says Yaniv Ziv, a neurobiologist at the Weizmann Institute of Science who was not involved in the new study. “This shows that’s not the case.”

“There have been hints of this for at least 15 years,” across many parts of the brain, Schoonover told me. The hippocampus, for example, helps animals navigate their surroundings. It contains place cells—neurons that selectively fire when their owner enters specific locations. Walk from your bed to your front door, and different place cells will fire. But these preferences aren’t fixed: Ziv and others have now shown that the locations to which these cells are tuned can also drift over time.

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.

from Hacker News https://ift.tt/3vbkKay