A new random-dot stereo illusion and its application to the Anstis-Howard-Rogers effect.

When the dynamic visual noise of an untuned television set is viewed with image defocusing (positive lenses) and with a narrow vertical obstruction partially blocking the pupil of one eye, the video 'snow' seems to separate into two stable surfaces at different depths, divided by a vertical discontinuity. The main features of this illusion can be quantitatively accounted for in terms of the optics of defocused images and the retinal disparities predicted from blur circles. A residual component of the illusion, however, which was perceived by a majority of subjects, cannot be readily explained by geometrical optics; it apparently reflects a more subtle aspect in the processing of visual images, corresponding to the Anstis-Howard-Rogers stereo-effect, in which local depth configurations can bias global stereopsis. Several novel aspects of that effect are described, based on use of this obstructed-pupil illusion as the evoking stimulus.