An opaque surface influences the depth from the Pulfrich phenomenon

When a pendulum, swinging in a frontoparallel plane, is viewed with a neutral density filter before one eye, the pendulum bob appears to rotate in an elliptical path. This illusion, known as the Pulfrich phenomenon, was used to examine depth cue interaction in a natural environment. An opaque surface was placed behind the pendulum. This intervention put the 'Pulfrich depth' of the pendulum in conflict with the information provided by other depth cues, such as interposition, and observers reported that the 'far' part of the pendulum's Pulfrich trajectory appeared to be flat. This was not the case when the surface was transparent. If the opaque surface was placed farther behind the pendulum's physical path, the far trajectory became increasingly elliptical. These results suggest that the opaque surface influences the pendulum's depth qualitatively and quantitatively. That is, the depth order between the surface and the pendulum is determined by interposition, but the absolute depth of the pendulum's Pulfrich trajectory is constrained by the surface's depth.