Spatial and temporal characteristics of perceptual completion at the blind spot

The present study examined perceptual completion, or filling-in, at the blind spot Perceptual completion is one of important functions of the visual system in order to restore the outer world Specifically, perceptual completion at the blind spot is representative of the restoration of physical space. Previous studies on perceptual completion at the blind spot were mainly qualitative, not quantitative (e.g., Ramachandran, 1992). In this study, we examined quantitatively perceptual completion at the blind spot by using a line stimulus with a gap. The test line was presented across the blind spot so that the gap fell exactly on the blind spot of observer's right eye. Observers were required to report that the test line appeared to be identical to the reference line, which was continuous and presented to the temporal retina with the same eccentricity of the test line. Perceptual completion was defined as that the gap of the test line appeared filled in as like as the reference line and the minimum length of the line segment for filling-in to occur was measured. The following results were obtained from the six experiments. (1)When the line segments in the opposite sides of the blind spot were very short, they appeared as the two line segments located separately in different positions. (2)The minimum length of the line segments for filling-in to occur was larger in the vertical orientation of the line than that in the horizontal orientation. Thus, there is anisotropy of perceptual completion at the blind spot. (3)The anisotropy of perceptual completion does not depend on the difference of the horizontal and vertical axis of the blind spot. (4)The width and exposure duration of the test line affected on perceptual completion at the blind spot. (5)The anisotropy we found was confirmed by another stimulus pattern, such as line grating. (6)Perceptual completion at the blind spot tolerated differences in misalignment, orientation, and contrast of the line segments.