The Neural Control of Vision
L. The Processing of Depth

When 61C is looked at directly all the figures are seen are receding because of they all have the same shading to create that impression. However, when fused, the display is more difficult to resolve because the disparity cues are set in conflict with the shading cues. The disparity cues provide protruding images. Individual figures may oscillate in the percept. When viewed for a more prolonged time, the stereo cues may prevail, at which time all figures will protrude. The bottom one will be seen as protruding more because this figure has greater disparity.