[The effect of location of a highlight in a shaded circle on depth perception]

Two experiments were performed to examine how the apparent depth of a shaded circle on a plane was affected by the direction and the distance of a highlight in the shaded area from the center of the circle. Fourteen undergraduates were asked to rate the concentrically shaded circle according to the apparent depth using the method of magnitude estimation. Experiment I showed that the rating was the highest in the upper-left, upper-right, and top conditions of the direction and the lowest in the bottom condition. Experiment 2 showed that the rating was the highest when the highlight was located at a third of the diameter of the circle apart from the center and the lowest when the highlight was on the edge of the circle or in the center. On the whole, the results support Ramachandran's "Assumption of Light From Above", which states that humans perceive a circle shaded bright in the top and dark in the bottom as convex due to their long experience on the earth since their birth.