Bits of your brain are falling asleep and waking up right now.

Science Daily looks at the neurons that rev up and those that spin down when different parts of your brain are used – a trick based on humans’ knack for “selective attention”:

When we are in a deep slumber our brain’s activity ebbs and flows in big, obvious waves, like watching a tide of human bodies rise up and sit down around a sports stadium. It’s hard to miss.

Now, Stanford researchers have found, those same cycles exist in wake as in sleep, but with only small sections sitting and standing in unison rather than the entire stadium. It’s as if tiny portions of the brain are independently falling asleep and waking back up all the time.

The team used what amounts to sets of very sensitive pins that can record activity from a column of neurons in the brain. In the past, people had known that individual neurons go through phases of being more or less active, but with this probe they saw for the first time that all the neurons in a given column cycled together between firing very rapidly then firing at a much slower rate, similar to coordinated cycles in sleep.

“During an on state the neurons all start firing rapidly,” said Kwabena Boahen, a professor of bioengineering and electrical engineering at Stanford and a senior author on the paper. “Then all of a sudden they just switch to a low firing rate. This on and off switching is happening all the time, as if the neurons are flipping a coin to decide if they are going to be on or off.”

Those cycles, which occur on the order of seconds or fractions of seconds, weren’t as visible when awake because the wave doesn’t propagate much beyond that column, unlike in sleep when the wave spreads across almost the entire brain and is easy to detect.

A question that comes out of this work is why the neurons cycle into a lower activity state when we’re awake. Why not just stay in the more active state all the time in case that’s when the saber tooth tiger attacks?

One answer could relate to energy. “There is a metabolic cost associated with neurons firing all the time,” Boahen said. The brain uses a lot of energy and maybe giving the cells a chance to do the energetic equivalent of sitting down allows the brain to save energy.

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