I left you hanging at the end of Part 2 regarding how we’d apply low tech therapy to fusion maldevelopment nystagmus (FMNS), and here is the paper by Tyschen and colleagues that sets the stage. Our colleague Curt Baxtrom convinced me years ago to pay close attention to what Tyschen had to say about asymmetries in motor patterns, and here in the abstract of their 2010 paper Tyschen and colleagues summarize nicely: These experiments have revealed that loss of binocular connections within striate cortex (area V1) in the first months of life is the necessary and sufficient cause of LN. The severity of LN increases systematically with longer durations of binocular and greater losses of V1 connections. Decorrelation durations that exceed the equivalent of 2–3 months in human development result in an LN prevalence of 100%. No manipulation of brain stem motor pathways is required. The binocular maldevelopment originating in area V1 is passed on to downstream extrastriate regions of cerebral cortex that drive conjugate gaze, notably MSTd. Conjugate gaze is stable when MSTd neurons of the right and left cerebral hemispheres have balanced binocular activity. Fusion maldevelopment in infancy causes unbalanced monocular activity. If input from one eye dominates and the other is suppressed, MSTd in one hemisphere becomes more active. Acting through downstream projections to the ipsilateral nucleus of the optic tract, the eyes are driven conjugately to that side. The unbalanced MSTd drive is evident as the nasalward gazeholding bias of LN when viewing with either eye.
How best to combat this nasalward bias? You guessed it! Binasal occlusion.
Reading the paper by Wong’s lab mentioned in Part 1, it occurred to me that arriving at the best plus lens Rx in hyperopic aniso to balance the two hemispheres and using binasal occlusion is a passing starting point for active learning such as their use in conjunction with the Haidinger Brush/M.I.T.
In essence the MIT or Macula Integrity Trainer is a biofeedback tool for time-averaging where the fovea is located relative to a point of fixation. As such it’s the poor man’s MP-1, and lacks the addition of auditory biofeedback, but it does have the added advantage of being able to utilize touch as an aid. So unlike PRL training in AMD which is trying to encourage eccentric viewing, the goal in FMNS is to shrink the cloud of fixation instability so that the brush hangs around the fovea as much as possible. The key element here is that we want the appearance of the target to be as symmetrical as possible when fixating with right eye vs. left eye. While wearing the binasals occlude each eye to do activities, then do it under MFBF conditions (less hyperopic eye views through yellow filter or frosted lens).