BINOCULAR VISUAL DIRECTION

We examine whether the rules of cyclopean visual direction, as express ed by Hering and others, correctly predict the percept of visual direc tion for structured visual stimuli. Theoretical inspection of the rule s of cyclopean visual direction shows a paradox for the binocular visu al directions of stimuli in which objects partly occlude each other. W e investigate how the rules of cyclopean direction are violated during viewing of structured random-dot stereograms with different depth pla nes. The directions of monocular and binocular visual elements are det ermined in an alignment task. Subjects align a monocular/binocular sli der with a monocular/binocular test line present in the random-dot ste reograms. The results show that the available rules of cyclopean direc tion are not sufficient for human vision in this general situation. Th e available rules can only be used for alignment of two binocularly vi sible lines or two monocular lines presented to the same eye. Alignmen t in these cases is predicted by almost any set of rules that transfor ms visible lines to a cyclopean eye. Stimulus conditions, in which eit her one line is presented to one eye and the other line to the other e ye or one line is presented to one eye and the other line to two eyes, provide a more critical test for validity of the cyclopean rules. Our results show that the rules of cyclopean direction fail to predict al ignment precisely in these conditions. Inspection of the data shows th at binocular alignment is achieved by alignment of two monocular lines presented to a single eye.