Dr. Jim Sheedy and the Vision Performance Institute at the Pacific University College of Optometry are at the cutting edge of stereoscopic 3D (S3D) issues. Over 20 years ago Dr. Selwyn Super suggested that testing the speed of S3D responses was a simple and valuable clinical measure, and his patent on a clinical device lays out his rationale in detail. With the advent of S3D movies and learning systems to be used for distance classroom projections including surgical monitors, the speed and flexibility of distance stereo can and should be measured. The AO Vectographic Slide is a common one that contains Wirt Circles, and I’ve blogged before about the Marco 690 projector target that I’ve used for many years to look qualitatively at the speed of stereopsis response. Marco refers to this as their unique Minute Stereo Test. and is included in their next generation CP 770 projector target.
As you can see in the photo above, the screen is polarized in crossed disparity, with the line on the left of each pair viewed by the right eye and the line on the right viewed by the left eye. The left pair has the greatest disparity at 10 minutes of arc and it’s therefore always easiest for patients to detect that the left line is floating off the screen closest to them. The bottom pair has 4 minutes of arc disparity, the right pair has 2 minutes of arc disparity and the top pair 1 minute of arc. It’s fascinating to see how different people are in their sensitivity to the relative disparity of the targets. Here’s what typically happens.
At first the patient notices that all four lines are floating in the air, but can’t differentiate which one is closest and which is furthest. As separation begins to build between the lines, almost like an accordion unfolding, it is easiest to tell that the left is the closest and the top is the furthest. There is perceptual learning involved, which has a speed element to it. Once the patient is able to localize the relative difference between the four targets, the re-appearance of accurate S3D occurs much faster when one eye is covered and then uncovered.
This idea of sorting out multiple planes of depth simultaneously is the basis for the Chicago Skyline Vectogram used regularly in vision therapy, and the same points can be converted from convergent to divergent disparity by separating the targets. The ability to project the target overhead onto a polarized screen at distance allows you to simulate what the patient has to deal with while watching an S3D movie. The same type of binocular and accommodative interaction and projection can be used in an S3D environment using the therapy system of Computer Orthoptics and VTS-3.
Bernell has introduced distance 3D DVD targets for viewing on a flat panel screen so as you can see our field is moving toward a greater emphasis on binocular localization at distance, in extrapersonal viewing space well outside arm’s length – and in particular, in an S3D viewing environment.