We all live our lives in a daily cycle of alertness and tiredness, but the brain functions that drive this are still mostly a mystery. Stanford scientists have now uncovered a strange new finding: different sections of the brain cycle in and out of sleep all day, while you're awake, suggesting that the ability (or inability) to pay attention may be linked to brain wave cycles while asleep.
While much research has focussed on mechanisms that guide us in and out of sleep, this new study recorded the activity of columns of neurons in the brain, and noticed, for the first time, that all the neurons in each column would cycle together between phases of high and low activity.
"During an on state the neurons all start firing rapidly," says Kwabena Boahen, 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."
These cycles are similar to those that occur during sleep, suggesting that bits of the brain are regularly falling asleep and waking back up – a phenomenon that's been seen in birds that rest with microsleeps during long-haul flights. We may not have observed these cycles in humans before because they're hard to detect, on account of occurring in a matter of seconds or less, and the brain waves don't spread out much beyond that column of neurons.
"Selective attention is similar to making small parts of your brain a little bit more awake," says Tatiana Engel, co-lead author on the study.
To test the effects of these alertness cycles on the ability to pay attention to a task, the team hooked the probes up to the brains of monkeys. The region of the brain they tracked was responsible for detecting a particular section of the animals' visual field, and the monkeys had been trained to respond to changes in that visual area. The researchers triggered a cue that told the monkeys that a change was about to take place in a certain area, and then rewarded them if they acknowledged that they'd seen the change.
In response to seeing a cue, the monkeys anticipated a change and paid more attention to the visual area. Inside the brain, neurons within a given column all continued cycling through the on and off phases, but tended to remain active for longer when the monkeys were paying attention.
Then, if that change occurred while the neurons were active, the animals were more likely to spot the stimulus, but even while paying close attention, if it flashed up while the neurons happened to be in the off part of the cycle, the monkeys tended to miss it.
"The monkey is very good at detecting stimulus changes when neurons in that column are in the on state but not in the off state," says Engel.
So if we're more effective when our neurons are switched on, why does the brain bother switching them off at all? The researchers suggest that it's probably a matter of efficiency. Given its power and complexity, the human brain requires an enormous amount of energy to run, so cycling through stand-by states allows the brain to direct energy elsewhere. A little downtime might also give cells the chance to clear out the neuronal waste that builds up when they're active.
"There is a metabolic cost associated with neurons firing all the time," says Boahen.
The research is published in the journal Science.
Source: Stanford University
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