CAPTURE OF THE VISUAL DIRECTION OF MONOCULAR OBJECTS BY ADJACENT BINOCULAR OBJECTS

Investigations of binocular visual direction have concentrated mainly on stationary objects, Eye positions were generally not measured and b inocular fixation was assumed to be perfect, During the viewing of sta tionary objects, vergence errors are not negligible but small, During the viewing of moving objects, however, errors in binocular fixation a re much larger, Existing rules for binocular visual direction were exa mined under the latter, more demanding viewing conditions, Eye movemen ts were measured objectively by the scleral coil technique, Subjects v iewed a large stereogram in which the half-images oscillated in counte rphase. The stereogram contained two square random-dot patterns placed side by side with a gap in between, A vertical line, visible only to one eye, oscillated in the gap, Subjects were asked to adjust the ampl itude of line motion until the line was perceived to be stationary, In so doing, they set amplitudes equal to the amplitudes of half-image m otion if the gap between the patterns was narrow, They set amplitudes significantly smaller in wider gaps, Subjects made considerable fixati onal errors in following the oscillations of the line and the random-d ot patterns, The results of the settings and of the retinal errors tog ether refute existing rules for binocular visual direction of monocula r objects, Perceived directions of monocular objects cannot be specifi ed by geometrical rules that include only the positions of the objects and of the two eyes, The results suggest that perceived directions of monocular objects are captured by the binocular visual directions of adjacent binocular objects, Capture of binocular visual direction was found to be effective for gaps as wide as 8 deg between the binocular objects, The phenomenon of binocular capture has negative consequences for the general use of nonius lines as indicators of eye position. (C ) 1997 Elsevier Science Ltd.