At every moment, neurons whisper, shout, sputter and sing, filling the brain with a dizzying cacophony of voices. Yet many of those voices don’t seem to be saying anything meaningful at all. They register as habitual echoes of noise, not signal; as static, not discourse.
Since scientists first became capable of recording from single neurons 60 years ago, they’ve known that brain activity is highly variable, even when there’s no obvious reason it should be. An animal’s neural response to a repeated stimulus changes considerably from trial to trial, fluctuating in a way that seems almost random. Even in the total absence of a stimulus, “you would just record spontaneous activity, and you would see that it seemed to have a mind of its own,” according to David McCormick, a neuroscientist at the University of Oregon.
“This gave rise to a view of the brain as being somehow either very [noisy], or using some type of high-level statistics to get over this noisiness,” he said.
But over the past decade, that view has changed. It’s become apparent that this purported randomness and variability relates not just to messiness in the brain’s neural mechanics, but also to behavioral states like arousal and stress — states that seem to affect perception and decision-making as well. There’s more to all the noise, scientists realized, than they had assumed.
Now, by analyzing both the neural activity and the behavior of mice in unprecedented detail, researchers have revealed a surprising explanation for much of that variability: Throughout the brain, even in low-level sensory areas like the visual cortex, neurons encode information about far more than their immediately relevant task. They also babble about whatever other behaviors the animal happens to be engaging in, even trivial ones — the twitch of a whisker, the flick of a hind leg.
Those simple gestures aren’t just present in the neural activity. They dominate it.
The findings are changing how scientists interpret brain activity, and how they design experiments to study it.
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