to the 20/20 vision standard, the lower are the prospects of improving or re-gaining stereo vision, unless visual acuity itself were improved by other means. strabismus surgery itself does not improve visual acuity. stereopsis following training procedures orthoptic exercises have proven to be effective for reducing symptoms in patients with convergence insufficiency and decompensating exophoria by improving the near-point convergence of the eyes that is necessary for binocular fusion. experiments on monkeys, published 2007, revealed improvements in stereoacuity in monkeys who, after having been raised with binocular deprivation through prisms for the first two years, were exposed to extensive psychophysical training. their stereo vision recovered in part, but remained far more limited than that of normally raised monkeys. scientists at the university of california, berkeley have stated that perceptual learning appears to play an important role. one investigation, published 2011, reported on a study on human stereopsis recovery using perceptual learning which was inspired by barry's work. in this study, a small number of stereoblind subjects who had initially been stereoblind or stereoanomalous recovered stereopsis using perceptual learning exercises. alongside the scientific assessment of the extent of recovery, also the subjective outcomes are described: "after achieving stereopsis, our observers reported that the depth “popped out,” which they found very helpful and joyful in their everyday life. the anisometropic observer gd noticed “a surge in depth” one day when shopping in a supermarket. while playing table tennis, she feels that she is able to track a ping-pong ball more accurately and therefore can play better. strabismic observer ab is more confident now when walking down stairs because she can judge the depth of the steps better. strabismics ab, dp, and lr, are able to enjoy 3d movies for the first time, and strabismic gj finds it easier to catch a fly ball while playing baseball." in a follow-up study, the authors of this study pointed out that the stereopsis that was recovered following perceptual learning was more limited in resolution and precision compared to normal subjects' stereopsis. dennis m. levi was awarded the 2011 charles f. prentice medal of the american academy of optometry for this work. tetris is a popular video game from the 1980s. a modified version of the game (not shown here) has been used for dichoptic training. there have been several attempts to make use of modern technology for enhanced binocular eye training, in particular for treating amblyopia and interocular suppression. in some cases these modern techniques have improved patients' stereoacuity. very early technology-enhanced vision therapy efforts have included the cheiroscope, which is a haploscope in which left- and or right-eye images can be blended into view over a drawing pad, and the subject may be given a task such as to reproduce a line image presented to one eye. however, historically these approaches were not developed much further and they were not put to widespread use. recent systems are based on dichoptic presentation of the elements of a video game or virtual reality such that each eye receives different signals of the virtual world that the player's brain must combine in order to play successfully. one of the earliest systems of this kind has been proposed by a research group in the university of nottingham with the aim of treating amblyopia, using virtual reality masks or commercially available 3d shutter glasses. the group also has worked to develop perceptual learning training protocols that specifically target the deficit in stereo acuity to allow the recovery of normal stereo function even in adulthood. another system of dichoptic presentation for binocular vision therapy has been proposed by researchers of the research institute of the mcgill university health centre. using a modified puzzle video game tetris, the interocular suppression of patients with amblyopia was successfully treated with dichotomic training in which certain parameters of the training material were systematically adapted during the course of four weeks. clinical supervision of such procedures is required to ensure that double vision does not occur. most of the patients who underwent this treatment gained improved visual acuity of the weaker eye, and some also showed increased stereoacuity. another study performed at the same institute showed that dichoptic training can be more effective in adults than the more conventional amblyopia treatment of an eye patch. for this investigation, 18 adults played the game tetris for one hour each day, half of the group wearing eye patches and the other half playing a dichoptic version of the game. after two weeks, the group who played dichoptically showed a significant improvement of vision in the weaker eye and in stereopsis acuity; the eye patch group had moderate improvements, which increased substantially after they, too, were given the dichoptic training afterwards. dichoptic-based perceptual learning therapy, presented by means of a head-mounted display, is amenable also to amblyopic children, as it improves both the amblyopic eye's visual acuity and the stereo function. the researchers at mcgill university have shown that one to three weeks of playing a dichoptic video game for one to two hours on a hand-held device "can improve acuity and restore binocular function, including stereopsis in adults". furthermore, it has been suggested that these effect can be enhanced by anodal transcranial direct current stimulation (tdcs). together with levi of the university of california, berkeley, scientists at the university of rochester have made further developments in terms of virtual reality computer games which have shown some promise in improving both monocular and binocular vision in human subjects. game developer james blaha, who developed his own crowd-funded version of a dichoptic vr game for the oculus rift together with manish gupta and is continuing to experiment with the game, experienced stereopsis for the first time using his game. in 2011, two cases of adults with anisometropic amblyopia were reported whose visual acuity and stereoacuity improved due to learning-based therapies. there are indications that the suppression of binocularity in amblyopic subjects is due to a suppression mechanism that prevents the amblyopic brain from learning to see. it has been suggested that desuppression and neuroplasticity may be favored by specific conditions that are commonly associated with perceptual learning tasks and video game playing such as a heightened requirement of attention, a prospect of reward, a feeling of enjoyment and a sense of flow. health care policy matters prism lenses (here unusually thick) are used for pre-operative prism adaptation. health insurances always review therapies in terms of clinical effectiveness in view of existing scientific literature, benefit, risk and cost. even if individual cases of recovery exist, a treatment is only considered effective under this point of view if there is sufficient likelihood that it will predictably improve outcomes. in this context, medical coverage policy of the global health services organization cigna "does not cover vision therapy, optometric training, eye exercises or orthoptics because they are considered experimental, investigational or unproven for any indication including the management of visual disorders and learning disabilities" based on a bibliographic review published by cigna which concludes that "insufficient evidence exists in the published, peer-reviewed literature to conclude that vision therapy is effective for the treatment of any of the strabismic disorders except preoperative prism adaptation for acquired esotropia". similarly, the u.s. managed health care company aetna offers vision therapy only in contracts with supplemental coverage and limits its prescriptions to a number of conditions that are explicitly specified in a list of vision disorders.