A visit with the kids/grandkids in Cincinnati this holiday weekend prompted a trip to Joseph-Beth Booksellers where this time I unearthed a gem by the internationally renowned psychiatrist Iain McGilchrist titled The Master and His Emissary.
This new expanded edition published ten years after the original was published is a bargain at $20 just for the annotated notes and references, but there is an additional treasure trove of information between the covers. The introduction and bibliography to the original 2009 edition in available the book’s website. You can view a nice synopsis of the previous edition in this fast-paced video on YouTube (amazing how that RSA animation learning style with the doodling white board marker was so popular once upon a time and has essentially disappeared):
On page 47, McGilchrist makes the attention grabbing statement that: “chicks use either eye for different purposes and different views of the world“, as reviewed in Current Biology. This is cited as an example of functional asymmetry between the two hemispheres, normally facilitated by communication across the corpus callosum in humans. McGilchrist is careful to point out that although some have gone to extremes in painting distinct jobs for the right and left hemispheres which no doubt have been over-extrapolated (such as “left brain/ right brain” characteristics), there are some basic differences in function between the two hemispheres. Consider for example that the left hemisphere has input to awareness only on the right sight of the body, but the right hemisphere provides input to both sides. That is why damage localized to the left hemisphere does not result in neglect or inattention, and that neglect typically occurs on the left side only when the right hemisphere is damaged.
Which brings this thought to mind: Absent primary mechanical or structural alterations in the eyes or the rest of the visual pathways due to trauma or disease, are there reasons why binocular imbalance might be indicative of a normal functional asymmetry between the two eyes that has gone awry?
Just like handedness in the motor domain, or aural dominance in the auditory domain, we know there is dominance in the visual domain. Practitioners prescribing monovision or modified monovision contact lenses typically fit the dominant eye for distance. However, the hole in card or framing technique to identify the preferred “sighting” eye is really more of a probe for localization and at some level is indicative of hemispheric preference rather than acuity preference, as reflected in golfers’ putting preferences.
The point here is that when one is looking clinically at binocular vision performance, perfect symmetry is not necessarily desirable. However, when asymmetry goes awry, it can become dysfunctional. Tolerance for this asymmetry before it negatively impacts performance is highly individualized. Take for example (on an admittedly simplistic level) a child with no prior Rx who acquires natural monovision, presenting with OD – 0.75 sph and OS -2.25 sph, with an entering complaint of not being able to see the board well enough in school. He is a high achiever and enjoys reading. If your approach is to prescribe for both eyes to attain 20/20 at distance, you’ll likely have an uncomfortable and unhappy patient. This is where “trialing” comes into play, and you’ll often find that just by Rxing -0.25 or -0.50 OD and leaving OS plano will make the child happy at distance and preserve comfort at near. In this instance we have embraced the asymmetry rather than pushing a lens combination that undoes it.
Take another example, in this case a sensitive teenager with no ametropia who exhibits basic exophoria of 15^ at distance and near. He is aware that the right eye tends to drift more than the left eye. When you do a unilateral cover test it is obvious that he right eye recovery time is slower than the left eye. Based on other testing including associated phoria/fixation disparity you decide that 2^ diopters base-in is a “safe” amount of prism to prescribe, and given the asymmetry of imbalance you anticipate best acceptance with 1.5^ BI OD and 0.5^ OS. But when “trailing” the prism, your patient feels more comfortable with the prism even divided between the two eyes. Essentially you are preserving the asymmetry by prescribing equal prism values.
My main message is that purposeful, functional asymmetry is built into the brain and the visual system. It may be that binocular imbalance, including amblyopia and strabismus, is largely indicative of the innate drive toward functional asymmetry gone awry.
When I originally blogged about McGilchrist and the divided brain in 2011, I referenced the psychiatrist Fredrick Schiffer at Harvard who, in his book “Of Two Minds: The Revolutionary Science of Dual-Brain Psychology”, showed that distinct emotions can be evoked by selectively blocking access of the visual field to one of the brain’s hemispheres. This has morphed into an interesting project called EmotiGlass.
I raise this from the standpoint of manipulating symmetry in the brain, which we do when we introduce sector occlusion such as binasals. Should the the occlusion be perfectly symmetrical or should it be purposely asymmetrical? How often should it be changed? Of necessity there will be some trial and error, as the patient comes to terms with the feelings induced by visual field and hemispheric alterations as much as the position of the eyes. All this is a reminder that the binocular balance we achieve resides in the brain more so than the eyes.