BINOCULAR CORRESPONDENCE AND VISUAL DIRECTION

Two classic theories of direction vision, one by Hering, the other by Wells, are expressed in mathematical form and compared. The Hering dis parity field differs considerably from the Wells disparity field, but if both are scaled for the change of acuity with eccentricity their di fferences are much more subtle. This explains why it is hard to determ ine which theory predicts direction perception best, although the test s favour Hering's theory. It is proved that Wells's construction (his rule 3) follows directly from his first two rules and Aguillonius's as sumption that the horopter in the fixation plane is a frontoparallel l ine. Wells's theory is clearly outdated and does not mesh well with mo dern three-dimensional geometry of binocular vision, which Hering's th eory does. Moreover, Wells inextricably mixes distance and direction v ision right from the start, whereas Hering properly treats the two-dim ensional manifold of directions and the depth-gauging principles separ ately. The use of terms such as 'Wells-Hering' rules should be discour aged and both Wells and Hering should be remembered separately for the ir clearly distinct and independent contributions. The work of Hering is still relevant to modern theory and praxis of binocular vision. The extension of Hering's approach to vertical disparities is treated for stimuli in frontoparallel planes. It is shown that acuity-scaled vert ical-disparity information sampled at a single glance is below resolut ion beyond about arm's length. It can only be used if eye movements ar e allowed. Throughout, the simplest derivations of the geometrical rel ations that it was possible to find are given, so that the review of b inocular geometry might also be of some didactical use. Finally it is indicated in which direction it might be necessary to modernise the co ncept of binocular correspondence.