A considerable amount of prior research has been devoted to the influence of aging on stereoacuity in the general population.
Table 1 summarizes the results of studies published since 1960. Most, but not all these studies conclude that stereoacuity decreases with age. Before we conducted the present research, there was reason to believe that all prior studies shared at least one of the following limitations in the assessment of stereoacuity. First, all tests used for stereoacuity that failed to eliminate nonstereoscopic depth cues (for description of these tasks, see Howard and Rogers
15 ). The listed dioptic stereoacuity tasks (the Howard-Dolman apparatus, the Frisby Stereo Test [Richmond Products, Boca Raton, FL], and the Flashlight Diastereo Test) provide several monocular cues for depth, including accommodation, motion parallax resulting from head movements, perspective, and/or the relative widths of stimuli. This is also a criticism of stereoacuity tests traditionally used in clinical practice,
16 which is a likely reason that such tests are rarely used for research purposes, particularly when the question of anomalous stereopsis is an issue (i.e., because many individuals with stereo difficulties are not aware of the fact, a depth discrimination based on monocular cues might be made without any intention of doing so). Julesz’s statement that not only can stereopsis “occur in the total absence of monocular form, but also… if monocular form exists, stereopsis precedes perception of form and can scramble it”
17 exemplifies the need to control for monocular cues when assessing stereopsis. This criticism does not apply to the tests that present separate stimuli to the two eyes stereoscopically (the TNO [Richmond Products], Randot [Stereo Optical, Chicago, IL], and Titmus [Occulus, Wetzlar, Germany] tests). However, the stereoscopic as well as the dioptric tasks fail to eliminate vergence, which can be a potent cue for depth.
18 Consequently, the studies listed in
Table 1 might more appropriately be designated as investigations of age-related changes in depth discrimination.