Perspective vergence: oculomotor responses to line drawings.

When a perspective drawing is viewed monocularly, changes in fixation point are accompanied by changes in steady-state vergence; their direction is usually appropriate for the distance relationships implied in the illustration. The absolute magnitude of these responses varies appreciably among subjects; it can be consistently enhanced or reduced by modest changes in the drawing. Similar configurations of stimuli from three-dimensional objects would presumably also contribute to normal vergence movements during binocular viewing; it appears that their importance would increase with target distance. Corresponding changes in pupil diameter, as expected for the "near reflex", were not observed with perspective stimuli. Consistent, directionally appropriate vergence changes, paralleling perception, were also made by most subjects during monocular viewing of a Necker cube, but there, exceptionally large pupillary responses arose.