New Delhi: 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. New research with monkeys finds that 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.
What’s more, it appears that when the neurons have cycled into the more active, or “on,” state they are better at responding to the world. The neurons also spend more time in the on state when paying attention to a task. This finding suggests processes that regulate brain activity in sleep might also play a role in attention.
“Selective attention is similar to making small parts of your brain a little bit more awake,” says Tatiana Engel, a postdoctoral fellow at Stanford University and co-lead author of the research, which appears in Science. Former graduate student Nicholas Steinmetz was the other co-lead author, who carried out the neurophysiology experiments in the lab of Tirin Moore, a professor of neurobiology and one of the senior authors.
Pins and neurons—Understanding these newly discovered cycles requires knowing a bit about how the brain is organized. If you were to poke a pin directly into the brain, all the brain cells you’d hit would respond to the same types of things. In one column they might all be responding to objects in a particular part of the visual field-the upper right, for example.
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 and firing at a much slower rate, similar to coordinated cycles in sleep.
“During an on state the neurons all start firing rapidly,” says Kwabena Boahen, a professor of bioengineering and electrical engineering and a senior author of 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.
The team found that the higher and lower activity states relate to the ability to respond to the world. The group had their probe in a region of the brain in monkeys that specifically detects one part of the visual world.
The scientists said the findings also relate to previous work, which found that more alert animals and humans tend to have pupils that are more dilated. In the current work, when the brain cells were spending more time in an active state the monkey’s pupils were also more dilated. The findings demonstrate an interaction among synchronous oscillations in the brain, attention to a task, and external signs of alertness.