The above suggested that the effect of foreground occlusions on vection may be due to the ability of foreground occlusions to alter the perceived visual background. It appears that another factor, depth perception, may also be involved in the effect of monocular foreground occlusions on the perception of static, two-dimensional (2D) pictures. The literature on monocular foreground occlusions for viewing static, 2D pictures is reviewed by Rogers .
It has long been observed that the appearance of three dimensions in a picture is more striking under certain viewing conditions, such as viewing the picture with one eye only and by using a peephole or a lens in a reduction screen or viewbox. A number of old viewing devices make use of one method or another to produce their sometimes powerful effects. The usual explanation is that the restricted view enhances the effectiveness of pictorial-depth information by reducing the conflicting flatness information that specifies a picture's objective surface...
Peephole viewing is monocular and head motion is prevented, and this should enhance perceived depth as described above. A peephole or a lens also restricts the observer's view of the picture itself, hiding the frame and surrounding surfaces. Loss of the visible frame and discontinuous surrounding surfaces reduce information for the picture as a flat object (perhaps even for the presence of a surface at all), potentially enhancing the illusion of depth in the picture. This is the oft-cited reason for the success of the various picture-viewing devices.
The following is a personal observation, in keeping with Rogers' discussion of the effect of monocular foreground occlusions on depth perception in 2D pictures. When viewing Claude Monet's Boats at Argenteuil through a monocular foreground occlusion, the components of the image appear larger than when viewed monocularly from the same position without the foreground occlusion. A possible explanation is that sizes are difficult to judge monocularly; that surrounding cues, for instance from the picture frame, would ordinarily affect monocular size perception; and that with these cues removed, the perceived size of the picture components increases. The overall scaling-up of the image components may stretch the perceived depth.
Two possible mechanisms for the effect of foreground occlusions have been presented in the last two sections. For vection, literature involving binocular foreground occlusions suggested that their effect may be due to an alteration of the perceived visual background. For monocular foreground occlusions applied to 2D, static images, the alteration of perceived depth was also mentioned. One might ask: ``is the alteration of perceived depth a significant factor in the effect of binocular foreground occlusions?''
I suggest that depth perception is not a significant factor for binocular foreground occlusions. The reason is that strong stereoscopic depth cues are present both with and without the binocular foreground occlusion. Consistent with this, I do not remember any of the participants run in the experiments described in Chapter 5 spontaneously mentioning that depth perception was affected by a binocular foreground occlusion. Nor did I have that impression myself.