We ended with Part 4 last week with this teaser: Would it be surprising if patients with unstable vergence and/or poor integration in visual-vestibular pathways had difficulty managing their way? And if they do, what can we do to help? The answer to the first question is no, it would not be surprising – and here is why (as reviewed by Miles and colleagues). There are three reflexes that function in parallel to serve as a pre-attentive filter for disturbances in motion as we interact with the environment, These reflexes serve as filters tuned to different features of binocular image flow:
Reflex 1: Radial-Flow Vergence – radial flow patterns experienced by the moving observer who looks in the direction of heading generate vergence eye movements that help maintain binocular alignment on objects ahead.
Reflex 2: Ocular Following Response – motion parallax experienced by the moving observer who looks off to one side taps into binocular stereo cues to help selectively stabilize retinal images in the plane of fixation.
Reflex 3: Disparity Vergence – changes in binocular alignment through vergence eye movements are used to reinforce the radial-flow vergence (Reflex 1).
Susan R. Barry, Ph.D. has helped me considerably to understand the linkage here, and we’ll be elaborating this during our presentation at the end of this week at the ICBO conference in the United Kingdom. As Sue explains, OFR is triggered if an object in the foreground moves in one direction while the background moves in the opposite direction, that is, as you move forward, the object close to you moves against your motion while the background moves with you. You respond with an OFR that tracks the foreground object. This is what happens when you’re tracking road signs while driving. This helps explain, in essence, why unstable vergence and/or strabismus can disrupt the OFR, with cascading effects disruptive to visual-vestiublar integration.
Sue has made the point that for patients with strabismus, the signals coming from each eye can be confusing at times, much different than when someone is completely monocular. There are also adaptations that occur in strabismus to deal with visual instability or confusion, but think how often we relate to this the static environment. The world of ecological optics opens up an entirely different dimension involving movement, and the type of adaptations required. So let’s begin with a simple example: Thumb Rotations. I’ve got a confession to make here. When I first embarked on a career in vision therapy I thought to myself: “What a silly procedure. Thumb rotations. How can that be relevant? It looks too simplistic.” Boy was I wrong!
Here’s the key: Optic Flow Disturbance! When first beginning with thumb rotations, many of the patients with whom we enter into a therapy relationship have trouble managing optic flow, so we need to select the background carefully to minimize visual confusion. The best way to do this is to have a solid background. The patient moves her thumb slowly, to the left and right of midline. maintaining fixation on her thumb. Next the patient rotates her thumb clockwise for several cycles, followed by counterclockwise. Then alternating between clockwise and counterclockwise. Now if you try this, and it doesn’t seem challenging, proceed to having objects such as pictures on the wall, or a TV in the foreground while doing thumb rotations. Using a patterned carpet as a background to the thumb can be dizzying. So you have to set up a protocol in moving from a less complex environment to a progressively more complex environment. A patient who is particularly poor at managing optic flow, might have to begin doing thumb rotations while seated, then progress to standing up, then on a balance board, and then while walking around the room. Ultimately optic flow, or movement of the surround, will be difficult to ignore but eventually the patient will learn to manage this effectively. Think of this a type of central/peripheral balance that is unique to the dynamics of moving in the environment.
Now here’s the kicker: How do we know that the patient’s ability to manage optic flow is the same with right eye vs. left eye? The answer is we don’t know until we probe it. So when the patient has strabismus, amblyopia, or central suppression of one eye relative to the other, presume they have a difference in how they manage optic flow with one eye as compared to the other eye, and work toward symmetry. This opens the Pandora’s Box to ecological optics as related to VT procedures. Obviously we’ll need a part 6 to continue!