motion, vergence angle and monocular cues – for sensing motion in depth and 3D object position is an area of active research in vision science and neighboring disciplines. Prevalence and impact of stereopsis in humans Not everyone has the same ability to see using stereopsis. One study shows that 97.3% are able to distinguish depth at horizontal disparities of 2.3 minutes of arc or smaller, and at least 80% could distinguish depth at horizontal differences of 30 seconds of arc. Stereopsis has a positive impact on exercising practical tasks such as needle-threading, ball-catching (especially in fast ball games), pouring liquids, and others. Professional activity may involve operating stereoscopic instruments such as a binocular microscope. While some of these tasks may profit from compensation of the visual system by means of other depth cues, there are some roles for which stereopsis is imperative. Occupations requiring the precise judgment of distance sometimes include a requirement to demonstrate some level of stereopsis; in particular, there is such a requirement for aeroplane pilots (even if the first pilot to fly around the world alone, Wiley Post, accomplished his feat with monocular vision only.) Also surgeons normally demonstrate high stereo acuity. As to car driving, a study found a positive impact of stereopsis in specific situations at intermediate distances only; furthermore, a study on elderly persons found that glare, visual field loss, and useful field of view were significant predictors of crash involvement, whereas the elderly persons' values of visual acuity, contrast sensitivity, and stereoacuity were not associated with crashes. Binocular vision has further advantages aside from stereopsis, in particular the enhancement of vision quality through binocular summation; persons with strabismus (even those who have no double vision) have lower scores of binocular summation, and this appears to incite persons with strabismus to close one eye in visually demanding situations. It has long been recognized that full binocular vision, including stereopsis, is an important factor in the stabilization of post-surgical outcome of strabismus corrections. Many persons lacking stereopsis have (or have had) visible strabismus, which is known to have a potential socioeconomic impact on children and adults. In particular, both large-angle and small-angle strabismus can negatively affect self-esteem, as it interferes with normal eye contact, often causing embarrassment, anger, and feelings of awkwardness. For further details on this, see psychosocial effects of strabismus. It has been noted that with the growing introduction of 3D display