Journal of experimental psychology: human perception and performance.

We examined the nonveridicality of visual direction produced by monocular viewing. In Experiment 1, 19 subjects pointed to a small light and moved a small light to their subjective median plane. The extent of constant error under monocular and binocular viewing conditions differed in both tasks (p less than .001). The monocular-binocular difference was larger when the viewing distance was 25 cm than when it was 50 cm (p less than .01). Also, correlations between phoria and monocular-binocular differences ranged from .58 to .77, depending on viewing distances and tasks. The effects of phoria within the context of Hering's .001). The monocular-binocular difference was larger when the viewing distance was 25 cm than when it was 50 cm (p less than .01). Also, correlations between phoria and monocular-binocular differences ranged from .58 to .77, depending on viewing distances and tasks. The effects of phoria within the context of Hering's .001). The monocular-binocular difference was larger when the viewing distance was 25 cm than when it was 50 cm (p less than .01). Also, correlations between phoria and monocular-binocular differences ranged from .58 to .77, depending on viewing distances and tasks. The effects of phoria within the context of Hering's principle of visual direction can account for these results. In Experiment 2, the same subjects adapted to phoria-induced error by placing a finger over a monocularly viewed target. The difference in their pointing responses before and after the task were reliable (p less than .005), and the correlations between phoria and the pre- to posttest differences were .45 and .77, depending on the number of adaptation trials. We argue that all monocular experiments dealing with visual direction should control for these effects.