No abstract

The laws of visual direction proposed by Wells some 200 years ago and Hering over 100 years ago are restated as three laws with the terminology of these two pioneers. This restatement serves both to honour their work and to strengthen some of the weaknesses in their original treatises. Second, two simple experiments, or tests of the laws, which the readers can readily perform by using their fingers as stimuli, are illustrated. Third, one of the three laws is modified to incorporate recent findings. Fourth, two recent experiments relevant to these new laws are described briefly, and their theoretical and methodological implications are discussed.

Three experiments, using two sets of Nonius lines placed in a random-dot stereogram, indicated that Nonius alignment does not always reflect binocular eye position and, thus, a caveat is necessary when Nonius alignment is used to monitor binocular eye position. We found that: (a) two Nonius lines with visual line values that differed by up to 7.6 min of arc can appear aligned; (b) the two lines of each of the two Nonius sets continued to appear aligned despite a change in vergence angle of 5.9 min of arc; and (c) the Nonius alignment reflected eye position better, when the binocular dots near the Nonius lines were eliminated.

Wells-Hering's laws summarize how we process direction and predict that monocular stimuli appear displaced with respect to the viewer, but not with respect to other seen objects [Erkelens, C. J.,& van Ee, R. (2002). The role of the cyclopean eye in vision: sometimes inappropriate, always irrelevant. Vision Research 42, 1157-1163] criticized this view and claimed that there is no perceptual displacement of these stimuli. We challenge their claim and improve on shortcomings of past studies. LEDs were monocularly presented to the observers, without their knowledge of which eye was being stimulated. Viewing distance was 9-10 cm; fixation distance was 30 cm. Observers reported the perceived relative and absolute directions of monocular stimuli. Our results are consistent with Wells-Hering's laws.

Studies of vision have informed theories first in philosophy and then in psychology. Over the centuries, an increasing number of phenomena have been enlisted to refute or reinforce particular theories. Nowhere has this been more evident than in binocular vision. How we see a single world with two eyes is one of the oldest and most consistently studied topics in vision research. It has been discussed at least since the time of Aristotle and it has been examined experimentally since the second century, when Ptolemy defined lines of visual correspondence for the two eyes. Prior to Wheatstone's invention of the stereoscope in the 1830s, binocular vision had been studied in terms of visual directions. The stereoscope established distance (or depth) as well as direction as dimensions of binocular vision. Subsequently, depth rather than direction has been the principal concern of students of vision, and texts in English devoted to analyses of direction rather than depth have been neglected. We examine the experiments on binocular visual direction conducted by Wells before Wheatstone, and by Towne and LeConte after him, and discuss the reasons for their neglect.