Adjacency and surroundedness in the depth effect on lightness

Using two perpendicular planes, one brightly and one dimly illuminated, A. L. Gilchrist (1977) showed that target lightness can change nearly from black to white by changing its perceived spatial position, with no change in the retinal image, if the target has an adjacent coplanar neighbor in each position. Earlier L. Kardos (1934) found a modest depth effect for a target that was not adjacent to its coplanar neighbor but surrounded by it. Using Kardos' experimental arrangement, but articulated planes and a between-subjects design, we obtained a large depth effect on lightness without adjacency. We then explored the role of adjacency and surroundedness using Gilchrist's perpendicular planes arrangement. We replicated the large depth effect when the target was adjacent to its coplanar neighbor. However, most of this depth effect was lost when adjacency was eliminated, by moving each target within its plane away from its coplanar neighbor. When we surrounded each target by extending its non-adjacent coplanar background, half the effect provided by adjacency was restored, but only in the brightly illuminated, not the dimly illuminated plane. Our findings support the view that, to compute surface lightness, the visual system groups surfaces in the image that seem to be equally illuminated.