WHY do we spend about 10 per cent of our waking hours with our eyes closed, blinking far more often than is necessary to keep our eyeballs lubricated?
Scientists have found the answer to this mystery, determining that the human brain uses that tiny moment of shut-eye to power down.
The mental break can last anywhere from between a split second to a few seconds before attention is fully restored, researchers from Japan's Osaka University have found.
During that time, scans that tracked the ebb and flow of blood within the brain revealed that regions associated with paying close attention momentarily go offline. And in the brief break in attention, brain regions collectively identified as the ''default mode network'' power up.
The default mode network is the brain's ''idle'' setting. When our attention is not required by a cognitive task such as reading or speaking, this far-flung cluster of brain regions comes alive, and our thoughts wander freely.
But our thoughts seldom stray far from home. We contemplate our feelings, we wonder what a friend meant by a comment, we consider something we did last week, or imagine what we'll do tomorrow.
Most of us take between 15 and 20 such moments of downtime a minute, and scientists have observed that most blinking takes place near or at the point of an ''implicit stop''.
While reading or listening to another person, that stop generally comes at the end of a sentence; while watching a movie, we're most likely to blink when an actor leaves the scene or when the camera shifts to follow the dialogue.
The study, published this week in the journal Proceedings of the National Academies of Science, studied 20 healthy young people in a brain scanner as they watched snippets from the British comedy Mr. Bean.
An earlier study had shown what implicit stops in the Mr. Bean video most commonly elicited a spontaneous blink, so researchers knew when to look for changes in the brain's activity patterns.
Sure enough, when subjects blinked, the researchers detected a momentary stand-down within the brain's areas involved with processing visual stimuli and areas that governed attention. The circuitry of the default mode network stepped up to fill the momentary lapse in attention, and then yielded again as order, and attention, returned.
Los Angeles Times