Stereopsis recovery

In cases of acquired strabismus with double vision (diplopia), it is long-established state of the art to aim at curing the double vision and at the same time recovering a patient's earlier ability for stereo vision. For example, a patient may have had full stereo vision but later had diplopia due to a medical condition, losing stereo vision. In this case, medical interventions, including vision therapy and strabismus surgery, may remove the double vision and recover the stereo vision which had temporarily been absent in the patient. Also when children with congenital (infantile) strabismus (e.g. infantile esotropia) receive strabismus surgery within the first few years or two of their life, this goes along with the hope that they may yet develop their full potential for binocular vision including stereopsis. In contrast, in a case where a child's eyes are straightened surgically after the age of about five or six years and the child had no opportunity to develop stereo vision in early childhood, normally the clinical expectation is that this intervention will lead to cosmetic improvements but not to stereo vision. Conventionally, no follow-up for stereopsis was performed in such cases. For instance, one author summarized the accepted scientific view of the time with the words: "Stereopsis will never be obtained unless amblyopia is treated, the eyes are aligned, and binocular fusion and function are achieved before the critical period for stereopsis ends. Clinical data suggest that this occurs before 24 months of age,[...] but we do not know exactly when it occurs, because crucial pieces of basic science information are missing." For purposes of illustration, reference is made to a book of doctors' handouts for patients, written for the general public and published in 2002, which summarizes the limitations in the terms in which they, at the time, were fully accepted as medical state of the art as follows: "If an adult has a childhood strabismus that was never treated, it is too late to improve any amblyopia or depth perception, so the goal may be simply cosmetic – to make the eyes appear to be properly aligned – though sometimes treatment does enlarge the extent of side vision." It has only been accepted very recently that the therapeutic approach was based on an unquestioned notion that has, since, been referred to as "myth" or "dogma". As one author states:The majority of adults will experience some improvement in binocular function after strabismus surgery even if the strabismus has been longstanding. Most commonly this takes the form of an expansion of binocular visual fields; however, some patients may also regain stereopsis.Scientific investigations on residual neural plasticity in adulthood now also include studies on the recovery of stereopsis. Now it is a matter of active scientific investigation under which conditions and to which degree binocular fusion and stereo vision can be acquired in adulthood, especially if the person is not known to have had any preceding experience of stereo vision, and how outcomes may depend on the patient's history of therapeutic interventions. == Examples and case studies ==

was the first to make the story of Susan Barry, whom he nicknamed "Stereo Sue", known to the general public. Stereopsis recovery has been reported to have occurred in a few adults as a result of either medical treatments including strabismus surgery and vision therapy, or spontaneously after a stereoscopic 3D cinema experience. Personal reports in Fixing my Gaze The most renowned case of regained stereopsis is that of neuroscientist Susan R. Barry, who had had alternating infantile esotropia with diplopia, but no amblyopia, underwent three surgical corrections in childhood without achieving binocular vision at the time, and recovered from stereoblindness in adult age after vision therapy with optometrist Theresa Ruggiero. Barry's case has been reported on by neurologist Oliver Sacks. Also David H. Hubel, winner of the 1981 Nobel Prize in Physiology or Medicine with Torsten Wiesel for their discoveries concerning information processing in the visual system, commented positively on her case. In 2009, Barry published a book ''Fixing My Gaze: A Scientist's Journey into Seeing in Three Dimensions'', reporting on her own and several other cases of stereopsis recovery. a woman who had been amblyopic before seeing in 3D described how empty space now "looks and feels palpable, tangible—alive!", a woman who had been strabismic since age two and saw in 3D after taking vision therapy and stated that "The coolest thing is the feeling you get being 'in the dimension, a woman who felt quite alarmed at the experience of suddenly seeing roadside trees and signs looming towards her, and two women who experienced an abrupt onset of stereo vision with a wide-angled view of the world, the first stating: "I was able to take in so much more of the room than I did before" and the second: "It was very dramatic as my peripheral vision suddenly filled in on both sides". Common to Barry and at least one person on whom she had reported is the finding that also their mental representation of space changed after having acquired stereo vision: that even with one eye closed the feeling is to see "more" than seeing with one eye closed before recovering stereopsis. Other first person accounts Michael Thomas has described the experience of instantaneous onset of three dimensional vision at the age of 69 in a Public Facebook post. == Recent scientific investigations ==

There is a growing recent body of scientific literature on investigations into the recovery of stereopsis in adults which started to appear shortly before Oliver Sacks' The New Yorker publication whereas other studies have investigated the effects of eye training procedures. Post-surgical stereopsis Certain conditions are known to be a prerequisite for stereo vision, for instance, that the amount of horizontal deviation, if any is present, needs to be small. In several studies it has been recognized that surgery to correct strabismus can have the effect of improving binocular function. One of these studies, published in 2003, explicitly concluded: "We found that improvement in binocularity, including stereopsis, can be obtained in a substantial portion of adults." Stereoacuity is limited by the visual acuity of the eyes, and in particular by the visual acuity of the weaker eye. That is, the more a patient's vision of any one of the two eyes is degraded compared 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: 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 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. == Health care policy matters ==

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. == See also ==