There's new evidence that the brain's activity during sleep isn't random. And the findings could help explain why the brain consumes so much energy even when it appears to be resting.
"There is something that's going on in a very structured manner during rest and during sleep," says Stanford neurologist Dr. Josef Parvizi, "and that will, of course, require energy consumption."
For a long time, scientists dismissed the brain's electrical activity during rest and sleep as meaningless "noise." But then studies using fMRI began to reveal patterns suggesting coordinated activity.
To take a closer look, Parvizi and a team of researchers studied three people awaiting surgery for epilepsy. These people spent several days with electrodes in their brains to help locate the source of their seizures. And that meant Parvizi's team was able to monitor the activity of small groups of brain cells in real time.
"We wanted to know exactly what's going on during rest," Parvizi says, "and whether or not it reflects what went on during the daytime when the subject was not resting."
In the study published online earlier this month in Neuron, the team first studied the volunteers while they were awake and answering simple questions like: Did you drive to work last week?
"In order to answer yes or no, you retrieve a lot of facts; you retrieve a lot of visualized memories," Parvizi says.
As expected, the team saw activity in two widely separated brain areas known to be involved in episodic memories. And the activity was highly coordinated — suggesting the different brain regions were working together to answer the questions.
Next, the volunteers were allowed to rest and even go to sleep while the researchers continued to monitor signals from the two brain areas. And the signals from the two regions remained coordinated, as if they were still working together, Parvizi says.
"What we found," he says, "was that the same nerve cells that were activated to retrieve memories ... have a very coordinated pattern of noise."
The brain may be working to maintain the relationship between regions that have cooperated recently, and may need to again, Parvizi says. This would help explain why the brain, unlike the body, consumes a lot of energy whether or not it has a specific job to do.
"Any brain is designed in such a way that it's using a lot of energy at what we call a 'resting state,' " Parvizi says. "So it's not really a resting brain."