BINOCULAR DISPLACEMENT OF UNPAIRED REGION

Binocular displacement of binocularly unpaired parts of the stimulus w as examined by means of the Poggendorff figure. The Poggendorff figure can be used to investigate displacement since lateral displacement of the transversal may cause bias in judgments of its collinearity. In e xperiment 1, the transversal had a disparity, and thus binocularly unp aired parts, relative to the rectangle. The magnitude of the Poggendor ff illusion should not have changed by addition of binocular disparity if displacement occurred. There was no or slight change when the tran sversal was seen behind the rectangle, but there was significant decre ase when the transversal was seen in front of the rectangle, suggestin g absence of displacement in this case. There were two possible explan ations. One was that displacement dependend on the positional relation between the unpaired stimuli and the binocularly presented rectangle, ie the occlusion constraint, which the case with the transversal in f ront did not satisfy. The alternative was that the decrease was due to the perceived front depth of the transversal, and not related to bino cular displacement at all. In order to discriminate between these two possibilities, the transversal was reduced to only the unpaired parts, resulting in dichoptic stimulation in experiment 2. In this stimulus, the positional relation between the unpaired and the paired stimuli w as the same as in the previous experiment, yet no front depth could be perceived. The results showed similar asymmetry as in experiment 1. T hus we conclude that binocular displacement depends on the positional relation between the unpaired and the paired stimuli, regardless of th eir perceived depth. This may imply that binocular displacement is not symmetric about the sign of disparity, hence that it is not just aver aging but is a reconstruction of the spatial layout of objects in the outside world to keep the visual direction of the unsuppressed unpaire d region veridical by using explicit cues to depth discontinuity.