WASHINGTON - Why do we spend 10 percent of our waking hours with our eyes closed - blinking far more often than necessary to keep our eyeballs lubricated?
Scientists have pried open the answer to this mystery, finding that the brain uses that tiny moment of shut-eye to power down.
The mental break can last anywhere from a split second to a few seconds before attention is fully restored, researchers from Japan's Osaka University found. During that time, scans that track 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.
Discovered less than a decade ago, the default mode network is the brain's "idle" setting. In times 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. In idle mode, however, our thoughts seldom stray far from home: We contemplate our feelings; we wonder what a friend meant by a recent comment; we consider something we did last week or imagine what we'll do tomorrow.
Most of us take 15 to 20 such moments of downtime per 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 generally comes at the end of a sentence. While watching a movie, for instance, we're most likely to blink when an actor turns to leave 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 subjects in a brain scanner as they watched "best bits" snippets from the British comedy "Mr. Bean." An earlier study had shown the researchers which implicit breakpoints in the "Mr. Bean" video most commonly elicited a spontaneous blink, so researchers knew when to look for changes in the brain's activation patterns.
Sure enough, when subjects blinked, the researchers detected a momentary stand-down within the brain's visual cortex and somatosensory cortex - both involved with processing visual stimuli - and in areas that govern attention. The circuitry of the default mode network stepped up to fill the momentary lapse in attention.