I’ve mentioned our artistic patient with strabismus, Greg Voth, and the perspective he brings to VT as an artist who has represented 3D in 2D motifs. Yesterday was the first day for our new Resident, Dr. Hilary Gesford, and I was probing physiological diplopia with Greg. We were revisiting the concept of how esotropic projection tends to consolidate binocular space within a relatively near sphere, and results in suppression of one eye at relative distances beyond a near fixation point or region.
If I hold two Wolff Wands at midline along the line of sight, and ask Greg to fixate the far bead, he has no trouble seeing the space nearer to him in physiological diplopia. Look at the far bead, and the near bead appears double. However when we ask him to look at the near bead, it is very difficult for him to attain physiological diplopia.
We previously introduced this concept in talking about the relative regions of visual space when using a Brock String. For someone with esotropia, like Greg, there is often a region of fusion where the two eyes share the same visual space that is relatively close or near. When the amount of esotropia or inward eye turn is large, this region might be a finite point within only several inches from one’s nose, which we term a centration point. If the angle of turn is relatively small, the area of common space where the two eyes can align is more of a zone or region of points, and we term than a centration range. As far as the brain is concerned, normal binocular vision occurs at the centration point, and therefore physiological diplopia can occur anywhere between the face and the centration point or range.
All well and good. But if suppression instead of fusion occurs beyond the centration point or range, how might we change that? One way to help patient’s like Greg become aware of fusion is to use touch as a feedback cue to help his brain influence and localize where his eyes are meeting.
We can achieve this by having Greg hold the far wand while I hold the near one. In this instance while he’s looking at the bead closer to him, it becomes easier to gain awareness of the far bead as being double because touch gives him another clue that it’s located beyond the point where he’s aiming his eyes. In some instances it helps if the patient holds both wands, thereby feeling by touch that one is closer and the other is further.
Ultimately this can be done without the need for touch as a feedback cue, with vision operating on its own to localize space. As the French phenomenologist Maurice Merleau-Ponty famously said, vision is the brain’s way of touching. Physiological diplopia becomes an awareness of regions of space where the two eyes coincide but do not meet foveally. Ideally in a 3D (or more) world, patients like Greg substitute stereopsis or binocular depth as a clue to regions of space where both eyes share common ground. Yet another reminder that vision is an active process.
– Leonard J. Press, O.D., FCOVD, FAAO