The Dual Nature of Stereopsis – Part 3

Professsor Selwyn Super is an esteemed colleague who has a wide range of interests.  Included in his impressive repertoire is a gift for illustration and authorship of children’s stories, under his nom de plume, Uncle Selwyn.  Our interest here however centers on his co-authorship of a significant paper in the Journal of Behavioral Optometry 20 years ago of which I had lost sight.  The article on stereoscopic testing  addresses two important issues.  One is the significance of testing stereopsis that occurs behind the plane of regard as well as the common practice of testing stereo in front of the plane (receding stereo as well as pop out stereo).  The other issue is the speed of stereopsis, or the time that it takes for the patient to perceive and localize the depth effect.

The paper paved the way for the development of the Super Stereoacuity Timed Tester, at one time sold through OEPF.  The test is pictured here in a reference from Bruce Evans in the UK.  There is scant mention of Super’s work in this area by other researchers, authors, lecturers, or clinicians, and I am as guilty of the oversight as anyone.  My intent here is to resurrect the importance of Professor Super’s insight, and to extend it from testing to therapy.

In a previous blog post I made reference to the Marco projection stereo slide pictured here.

To review, the patient is directed to look toward the “plus sign” in the center of the screen.  Around it are four pairs of lines, one above, below, to the left, and to the right.  In each pair one line is seen by the right eye and one by the left eye so that when the patient is fused, each of the four lines is seen singly and in depth.  The top pair has the least disparity and is therefore seen the furthest away; the left pair has the widest separation and therefore the greatest disaprity, hence it is seen the closest to the patient.  The interesting apsect to the me is that, as Professor Super would predict, for any patient with a binocular problem, the stereoscopic effect takes time to build.

So as the patient looks at the target, here are the three phases of perceptual emergence:

1.  The patient perceives that the lines are not in the same plane as the plus sign.

2. The patient perceives that the left line is the closest, but has trouble differentiating which is the furthest.  This is similar to any stereoscopic judgment, where the greater disparities are easier to identify of localize than the smaller or more subtle disparities.

3. The gestalt of the spiral effect emerges, so that the patient perceives the depth in clockwise fashion, with the top line the furtheast away, then the right, botom ,and left (12, 3, 6, and 9 o’clock).

When working with patients, particularly young adult or adult patients who are transitioning from monocular to binocular cues, the first phase of simply being able to perceive that the center plus and the surrounding lines are not in the same plane is a big accomplishment.  I like to cross check this by having the patient cover one eye – cover the right eye and the plus sign shits to the left of center; cover the left eye and the picture shifts to the right of center.

A strabismic patient’s capture of stereopsis proceeds on a continuum.  At first, as binocular cues register, it is a revelation to the visual cortices that something is different about the fused percept than the monocular percept.  We are pleased when the individual notices simple that there is something different about the binocular view.  What is it?  Hopefully it begins with phase one above – that the “plus” isn’t in the same plane as the lines.  We can cross-check this by comparing the binocular view to the view when covering one eye.  This may be the only noticeable difference.  Then as stereopsis builds, it may register that the lines are at differing depth planes even though it’s tough to tell which is where in space.  Ultimately, and ideally, then emege in clockwise spiral fashion.

What about the stereo-challenged individuals without strabismus?  If Professor Super is correct, and I believe he is, then the speed with which a patient can proceed from phase one throgh phase three is an important index of binocular processing speed.  This is one of the advantages of doing progress evaluations monthly, where a patient has the opportunity to view a unique target that she hasn’t “practiced getting better at”.  In that sense it’s a true measure of perceptual learning and transfer.

– Leonard J. Press, O.D., FCOVD, FAAO

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