Blind Spot

In monocular viewing there is a region in the peripheral visual field that is blind due to the absence of photoreceptors at the site where the optic nerve exits the eye. This region, like certain other blind spots, nonetheless appears filled in (Walls 1954). Ramachandran (1992a,b) has recently reported several novel demonstrations of filling-in at the blind spot. In this essay we critically reevaluate the implications of many of these effects. Specifically, we argue that many blind spot phenomena taken to support early filling in (eg pop-out and alteration in apparent motion reported by Ramachandran 1992a, b) are actually consistent with the thesis that the visual blind spot is treated by early perceptual processing as a region of reduced or absent information. In support of this, we show that many perceptual effects observed in blind spot completion are similar in detail to the amodally perceived completion (Kanizsa 1979) of partly occluded objects viewed somewhat peripherally. Our goals in this essay are (1) to point out striking similarities between blind spot completion and the amodal completion of occluded parts of surfaces, and (2) to provide a common theoretical framework for understanding these phenomena in the context of surface segregation and perceptual interpolation.