Abstract
If viewed peripherally, a single white disk presented at an intersection of the gray alleys in a grid of black squares is not visible (McAnany and Levine, 2002). This “blanking phenomenon” also applies to black disks and white squares. To determine the origins of this phenomenon, we used stereoscopic stimulus presentations. Patterns were presented on a computer monitor and viewed through a set of first-surface mirrors. To assure proper convergence, the subject fixated on a target that appeared 3D. The task was to determine which of three intersections contained a disk; disk threshold was derived by a staircase search.
Experiment I: Black squares on a medium gray field were presented to one eye, and a light disk on a gray field to the other. Upon fusion, the disk appeared centered in an intersection. The fused image should induce blanking; however, the disk remained visible. These results implicate a contribution to the phenomenon from a pre-fusion locus (retina or LGN). The possible confound of improper alignment of the image in each eye was addressed by introducing a grid of outline squares in the “disk” eye. While this increased threshold, the disk remained visible. Finally, as a control for the effectively reduced contrasts, black squares and a light disk were presented to one eye and a gray field was presented to the other eye. Blanking was present in this control arrangement.
Experiment II: Black squares and a light disk were presented to one eye while the other received white squares and a dark disk (thus, each eye received a stimulus capable of blanking). The disk was not detected. However, when black squares and a dark disk were presented to one eye and white squares and a light disk to the other (so neither eye received a stimulus capable of blanking), the disk was visible. These results also implicate a significant contribution to the phenomenon from a pre-fusion locus.