The USUAL form of the rod test for distance discrimination has been designed to exclude all cues of distance except retinal disparity. In spite of this intention, the size cue has not been eliminated. In the Howard test, widely used in the armed forces and in industry, the size cue acts in harmony with, and supplements, retinal disparity. In other tests, of which the ``telebinocular'' may be taken as an example, size becomes not a secondary, but a confusion, factor. With the ``telebinocular" judgments based on retinal disparity are scored as correct responses, while size of the objects enters only to confound the observer. In a normal situation an object looks larger as it approaches the eye; in the ``telebinocular'' the smallest letter may be, on the basis of binocular parallax, the nearest. This pitting of the size cue against the disparity cue is also a feature of the Verhoeff
HIRSCH MJ, HOROWITZ MW, WEYMOUTH FW. DISTANCE DISCRIMINATIONIII. Effect of Rod Width on Threshold. Arch Ophthalmol. 1948;39(3):325–332. doi:10.1001/archopht.1948.00900020332007
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