Perceived visual direction near an occluder

When an opaque object occludes a more distant object, the two eyes often se e different parts of the distant object. Hering's laws of visual direction make an interesting prediction for this situation: the part seen by both ey es should be seen in a different direction than the part seen by one eye. W e examined whether this prediction holds by asking observers to align a ver tical monocular line segment with a nearby vertical binocular segment. We f ound it necessary to correct the alignment data for vergence errors, which were measured in a control experiment, and for monocular spatial distortion s, which were also measured in a control experiment. Settings were reasonab ly consistent with Hering's laws when the monocular and binocular targets w ere separated by 30 arcmin or more. Observers aligned the targets as if the y were viewing them from one eye only when they were separated by 2 arcmin; this behavior is consistent with an observation reported by Erkelens and c olleagues. The same behavior was observed when the segments were horizontal and when no visible occluder was present. Perceived visual direction when the two eyes see different parts of a distant target is assigned in a fashi on that minimizes, but does not eliminate, distortions of the shape of the occluded object. (C) 1999 Elsevier Science Ltd. All rights reserved.