"Visual Closure is the ability to identify incomplete targets" (PTS). According to the PTS II manual, visual closure requires form perception, figure-ground perception, speed of information processing and simultaneous processing.
My art professor used to say: “Do not add unnecessary details to your paintings. Leave some room for the audience imagine things”. He used examples of paints with hands missing one finger, for example, and admires would just see a “hand” because their brain would fill in the missing finger.
While sight-reading, we fill in chords, scales, rhythms, etc. Visual Closure is a big part of the musicians' chunking system.
There are ways to improve our visual closure outside of the instrument.
The exercise I have been doing from the PTS requires you “to identify incomplete targets as fast as possible. The target is presented on the screen in an increasing degree of completeness ranging from 1% to 100%.” (PTS).
I confess I struggled with this exercise at first. It wast frustrating because it seemed very easy.
While sight-reading, we also use our "auditory closure"* to complete phrases and even make corrections. Good sight-readers and text readers are both known for making automatic corrections.
According to Lehman and Erichsson (1996) our expectations are powerful. Such expectations along with our previous knowledge, are a big part of filling in the score.
I recommend you to read Wolf (1976),Fine et al.(2006) and Sloboda (1976) on this subject. Kinsler and Carpenter (1995) concluded that sight-reading twice reduces ocular movements only because the first time reading add expectations.
From my experience, atonal and modern music are much harder to sight-read because they do not fulfill my expectation. There are no familiar rhythmic, harmonic and melodic patterns. Professionals also state that playing chamber music is easier since they have a feedback form the other partners (Wolf).
In the book Vision and Reading by Ralph P. Garzia, he writes: "Both visual and auditory perceptual skills subserve learning to read in very important ways...
Visual analysis skills influence strongly how easily a child learns to identify the letters of the manuscript alphabet with consistent accuracy; auditory analysis skills, in turn, have a powerful effect on how readily the child recognizes the letter-sound relationships that govern decoding."
Make sure you know your theory and do your ear training.
* I made up this term to represent our auditory expectations
