Read about this some time ago and seem to recall its to do with maintaining balance.
Apparently the body is constantly swaying from side to side (very minutely). The eyes pick up this swaying motion, and cancel it out, so that balance can be maintained.
As the swaying is so minute the eyes can only make the necessary corrections when glancing at something relatively near by (e.g. objects ahead when standing, or the canopy, wing etc. when sitting and flying).
Hence the problem when looking over a cliff or up a ladder...there is nothing close enough for the eyes to dectect the minute swaying ...thus the uneasy feeling...
UV