A commonly used assessment tool in vision screenings to identify the presence or absence of strabismus in pediatric populations is the RDE. The Random Dot “E” test consists of a demonstration plate containing a raised capital latter “E” requiring no binocular perception, and two test plates. One of the test plates contains the letter “E” which is identifiable only through stereoscopic perception when viewed through polaroid filters, and the other test plate is blank. The child is asked to identify which of the two plates contains the letter in a forced choice paradigm.
It is widely assumed that reliable identification of random dot stereopsis targets devoid of monocular cues is a foolproof way to rule out microstrabismus. After all, we don’t need RDS to identify a larger angle strabismus. It’s the smaller angles that escape detection, particularly when visual acuity through the strabismic eye is relatively good. But what if we can get fooled? What if there is evidence that a child with microstrabismus might be able to reliably to differentiate a random dot target as compared to a blank stimulus?
A recent article by Pageau, de Guise, and Saint-Amour in Optometry and Vision Science showed that when a random dot stimulus is made large enough in size, six of the nine microstrabismic children in their study were able to reliably identify the target. This possibility was raised nearly 30 years ago by two of my mentors, Ralph Garzia and Jack Richman, and has influenced my approach since then.
I do not view the possibility of a child with microstrabismus identifying the correct RDE plate as a design flaw, but as a reminder that the original purpose of RDS was to aid the breaking of camouflage by aerial reconnaissance photos viewed in stereopsis during WWII. Bela Julesz, the brilliant scientist at Bell Labs who spearheaded many applications of RDS, was careful to note that binocular stimuli devoid of any monocular cues do not exist in the real world. They are carefully contrived illusions, and nearly impossible to generate without computerized design. The children with microstrabismus who are able to differentiate the RDE plate from the blank plate during forced choice testing are likely picking up on the stereoscopic cues that fall outside of their suppression zone.
While the RDE remains a very valuable screening tool, passing the test should never be considered a guarantee that a child does not have strabismus. But we can take that a step further. When a child is engaged in vision therapy for strabismus, developing the perceptual abilities required to pass the test signals the development of higher order integration between the two eyes.