Absolute and Relative Visual Direction of Monocular and Binocular Targets

Seven experiments designed to measure the accuracy and precision of absolute and relative direction judgments under monocular and binocular viewing conditions are presented. The experiments assess the validity of claims in the literature that (a) the cyclopean eye (the centre from which visual direction judgments are made) is not fixed in the head, but moves along the interocular axis as a function of the stimulus situation and the eye(s) used to view the stimuli, and (b) absolute direction during monocular viewing is based on signals from the viewing eye only. The results of the experiments refute these claims and elucidate which types of visual direction tasks are germane to specifying the location of the cyclopean eye. Specifically, the results show that monocular relative direction judgments are highly accurate and precise, are independent of binocular eye position, and cannot be used as the basis from which to infer the position of the cyclopean eye. Absolute direction judgments, on the other hand, are less precise than relative direction judgments, and the accuracy of both monocular and binocular absolute direction judgments is dependent upon binocular eye position. When the eyes converge accurately on the target of interest, absolute direction judgments are accurate. When the eyes converge inaccurately on the target of interest, which is often the case during monocular fixation, absolute direction judgments are inaccurate. These results clarify the important distinction between relative and absolute visual direction and are discussed in terms of how visual directions specified from the cyclopean eye (perceptual variables) are derived from the inputs from the two eyes (physical variables).