ScienceDaily has an item (here) reporting on a paper, Out of Mind, Out of Sight: Eye Blinking as Indicator and Embodiment of Mind Wandering by D. Smilek and group in Psychological Science.
When your mind wanders, you’re not paying attention to what’s going in front of you. A new study suggests that it’s not just the mind, it’s the body, too; when subjects’ minds wandered, they blinked more, setting up a tiny physical barrier between themselves and the outside world….
… participants blinked more when their minds were wandering than when they were on task. …”What we suggest is that when you start to mind-wander, you start to gate the information even at the sensory endings — you basically close your eyelid so there’s less information coming into the brain,” says Smilek.
This is part of a shift in how scientists are thinking about the mind, he says. Psychologists are realizing that “you can’t think about these mental processes, like attention, separately from the fact that the individual’s brain is in a body, and the body’s acting in the world.” The mind doesn’t ignore the world all by itself; the eyelids help.
I suspect that the process that turns down the volume of auditory signals (for instance when we ourselves are speaking) may also be at work in this situation. As attention focus is the result of both a top-down and a bottom-up control, in order for the top-down to have more control, it may have to interfere with normal perception and bottom-up signals.