Hitting the target: relatively easy, yet absolutely difficult

It is generally agreed that absolute-direction judgments require information about eye position, whereas relative-direction judgments do not. The source of this eye-position information, particularly during monocular viewing, is a matter of debate. It may be either binocular eye position, or the position of the viewing-eye only, that is crucial. Using more ecologically valid stimulus situations than the traditional LED in the dark, we performed two experiments. In experiment 1, observers threw darts at targets that were fixated either monocularly or binocularly. In experiment 2, observers aimed a laser gun at targets while fixating either the rear or the front gunsight monocularly, or the target either monocularly or binocularly. We measured the accuracy and precision of the observers' absolute- and relative-direction judgments. We found that (a) relative-direction judgments were precise and independent of phoria, and (b) monocular absolute-direction judgments were inaccurate, and the magnitude of the inaccuracy was predictable from the magnitude of phoria. These results confirm that relative-direction judgments do not require information about eye position. Moreover, they show that binocular eye-position information is crucial when judging the absolute direction of both monocular and binocular targets.