I’m like a kid in a candy shop today. Last week we re-carpeted the office, and in the process some books were moved around. One of them that I re-discovered was Seeing Spatial From edited by Michael Jenkin and Laurence Harris. The co-editors are affiliated with the Centre for Vision Research at York University in Toronto. If the facility sounds familiar, it may be because we’ve mentioned it as the site of a recent international conference on the plasticity of vision. The book, published way back in 2006, is actually a festschrift to David Regan, a pioneer in vision research whose body of work pertains intimately to developmental vision and vision therapy. Looking at my non-Kindle marginalia notes and highlighting, I forgot how immensely I enjoyed the book, so here’s hoping that you find some of my shared observations interesting and useful.
A discussion that captures the essence of the concept of seeing spatial form occurs in the second chapter, entitled Pictorial Relief, akin to some of the insights I’ve gleaned from one of our current adult VT patients, Greg Voth, an artist exploring his vision as he develops it. I love the introduction of the chapter by Koenderink, van Doorn and Kappers, which begins: “Look at a photograph: You see a flat piece of paper. Look into a photograph: You are aware of an all-but-flat pictorial space.”
This taps into a discussion we previously had on the dual nature of stereopsis, and specifically the difference between monocular and binocular cues to stereovision. Koenderink et al in chapter 2 pick this up in their discussion on the Influence of Viewing Mode, which changes your perception of apparent depth. This works both when looking into pictures and when looking into a real scene, though the effects are actually opposite. Here’s a key statement: Such viewing modes typically have to be learned. It is one part of an artist’s training to learn how to look. This phrase is evocative enough, but captured even better by something that Strabby wrote yesterday about her VT home therapy:
“I am so intent on doing my homework and crossing the homework items off my to-do list and trying to get to my goal but somehow there has to be a space for letting my eyes do what they can do. Mouths taste; it’s practically passive, except you have to get the food in the mouth and chew it. How can I chew with my eyes so that they can taste all there is to see?”
Koenderink et al suggest an experiment, which is almost the inverse of Strabby’s attempt to transition from monocular to binocular cues. It’s an exercise for those of us bin0cular folks who want to toggle between binocular and monocular cues (though as Sue Barry has noted well, it’s only a simulation). Stand in front of a painting and close one eye. Feel free to look around the painting using only eye movements. Now wait, simply look intently. Don’t think, look. If you have never experienced monouclar stereopsis it may take you half a minute or a minute to acquire it. Even if you are experienced it will take a few seconds. Don’t worry, you will know for sure when monocular stereopsis occurs because the whole scene changes on you. You are no longer looking at the painting, but into the painting. The depth becomes real.
Here’s the explanation. When you look at a picture frontally, with a single eye, you experience a certain pictorial relief. If you switch from monocular to binocular viewing, the relief collapses by a factor than depends on your binocular stereo vision. For typical observes the depth range decreases roughly in half; for so-called stereo blind observers there is little effect. If one uses a synopter, nulling the disparity field, the relief becomes much deeper than for binocular vision. Koenderink’s work is beautifully illustrated by Joost Van Kasteren who notes that seeing “depth” in pictures develops in children around age 3.
So here’s the bottom line regarding an application to VT. With every binocular activity, we want the patient to be able to discern the difference between looking under monocular vs. binocular conditions. If it’s a stereo projection, what changes when you cover one eye? I’m less interested in so-called suppression checks than I am in the qualitative perception of the space. Does it feel like you’re looking within or into the volume of space, or does it feel like you’re looking at the space. At some level, that has to factor into your appreciation of what’s different about the monocular feeling vs. the binocular feeling.
Once the patient registers what that feeling is like, work on making the experience more of a reflex. The simplest way to do this is to cover one eye, then time how long it takes to re-acquire the sense of binocularity. If you’re using a space fixator, for example, compare your performance of looking and localizing first with the left eye covered, then the right eye covered, then with both eyes together. If you’re doing the pointer-in-straw procedure, how is your accuracy in spatial localization and your feeling about the volume of space feeling different with one eye vs. the other eye vs. both eyes together? When you uncover one eye after a count of 10, does your ability to recapture the binocular view and feeling take longer than when you uncover one eye after a count of 3? When the difference is minimal, you’ve attained good reflex fusion. Now you’re building automaticity into visual attention to the transition between the monocular and binocular experience.
– Leonard J. Press, O.D., FCOVD, FAAO