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While I like the discussion about day-dreamiing and not really "seeing" while we're distracted, I really think a good example of how the nervous system contructs "seeing" is dreaming.

When you go to sleep, your brain calms down and fires in a synchornous pattern for most of the night. However, every few hours, yours eyes began to dart back and forth and your brain appears neorologically awake "The dreaming brain employs all of the same systems and networks [as the waking brain]... but with a few differences. Input from the outside world is screened out. Selfawareness ceases. The body is paralyzed. And everything that the dreaming brain sees, hears or feels is generated from within." (1)

But how can you be "seeing" so much with your eyes closed?

Just before REM sleep, neurons in the pons begin to wake up and fire for no apparent reason, sending this information to the geniculate cortex and on to the occipital lobe, forming what is called a "PGO" wave or spike. This random information is, in turn, processed by the occipital lobe any way it can presumably leading to the bizarre, incoherent imagery of dreams. So your brain is actually quite capable of "seeing" with your eyes closed - of seeing even with no discernable input at all.

PS. PGO spikes released during "daytime" hours might be a neurological correlate of LSD-based hallucinations (2).

1. http://query.nytimes.com/gst/fullpage.html?res=9E05E7DD123FF931A25754C0A962958260&sec=&spon=&pagewanted=print

2.

http://www.erowid.org/references/refs_view.php?ID=3116