To the Editor.
—Stereo-pair photographs and diagrams can be viewed as three-dimensional images by chiastopic (ie, cross-eyed) observation without using any stereoscopic viewing devices. We found that the majority of a test group of adults could learn this skill in less than 30 minutes in supervised practice sessions. The acquisition of this skill should have practical application in viewing stereo-pairs of fundus photographs. It is well known that stereopsis can be attained by viewing a pair of pictures of a solid object at two slightly different angles. In the common application of this phenomenon, one of the two pictures is presented exclusively to the right eye and the other exclusively to the left eye by one of several available techniques (eg, red/green glasses, polarizing filters, prisms). It is not widely known, however, that it is possible to achieve stereopsis by looking directly at stereo-pairs without resorting to the use
Roy D. Brod, Andrew J. Packer. Learning the Skill of Chiastopic Direct Stereopsis. Arch Ophthalmol. 1988;106(1):15–16. doi:10.1001/archopht.1988.01060130017008