Strabby Looks to Break Through the 2D Wall

A couple of weeks ago I mentioned the joy of discovering a new legion of expressive bloggers who were influenced by the experiences of “Stereo Sue”, one of whom is Sally Rye, aka “Strabby“.  In anticipation of breaking through the wall of her 2D flatland, Strabby had a nice epiphany that she blogged about yesterday.

Sharing that she was alarmed this past November 2010 when she “flunked” the Stereo Fly test, Strabby cited evidence that stereopsis exists on a 2D to 3D continuum, rather than being an all or nothing phenomenon.  Strabby suggests that the Stereo Fly should be part of every child’s visual screening.

Strabby has really hit on something here.  Our field has gravitated toward Random Dot Stereopsis testing, such as the “E” plates above.  If a patient has strabismus and is incapable of bifoveal fixtion, the two test plates are indistinguishable from one another.  Through the polaroid glasses, the “E” plate looks the same as the blank plate, and the appearance is the same whether the patient uses one eye or both eyes.  But that’s not how vision works in the real world.  In the real world, there is a continuum between true 2D vision with one eye closed, as compared to the sense of depth that a patient with strabismus might have with both eyes open.  Most strabismus patients in therapy find themselves on the weak end of the continuum prior to vision therapy, and acquire a greater sense of depth through therapy.

Aside from the many procedures that we employ in vision therapy to encourage greater spatial awareness, projection and depth, one of the targets I have come to enjoy is the distance stereo slide that is part of our Marco projection system.

The target has four pairs of lines arranged around a central cross.  Looking through polaroid filters, the patient sees one line of each pair with the right eye and the other line with the left eye.  Patients with “normal” binocular vision and good stereopsis have no trouble merging or fusing each pair of lines into one line.  The pair with the greatest separation or disparity, which is the one on the left, will create maximum stereoscopic effect and be perceived as closest to the patient.  The pair with the least separation, which is the top line, will be perceived as further away.

Now here’s the kicker:  patients with compromised binocular vision will say that they can perceive that all the lines are floating off the screen, but they can’t differentiate the relative depth among the four lines.  As they improve their stereoscopic localization through vision therapy, they are able to attain a greater sense of relative depth among the four lines.  The reason I like to use this as a test target during progress evaluations is that this isn’t a target that the patient has practiced during therapy sessions.  True perceptual learning has occurred when the patient is able to transfer what has been practiced to a novel target.  And for the strabismic patient, the payoff is how these disparity cues can be used in real space rather than in a simulated sense in a 2D booklet at arm’s length.

This is the 2D to 3D continuum to which Strabby is alluding.  Nicely done!