To fuse the random-dot stereogram converge or diverge your eyes until the two circles become superimposed. You will see a vertical sinusoidal modulation of depth called "disparity grating" or "disparity corrugation." In psychophysical experiments we use corrugations whose orientation is close to horizontal, rather than vertical, because of the orientation anisotropy of human stereopsis: The human visual system is more sensitive to vertical, rather than horizontal, gradients of binocular disparity. The cause of this anisotropy is unknown.