In part 1 we introduced you to book by Goldberg et al summarizing why the vestibular system can truly be considered a sixth sense. It is not surprising, given the extent to which vision is linked to cognitive, sensory, and motor processes, that we should be interested in the function of the vestibular system in vision therapy. Solan, Shelley-Tremblay, and Larson set the tone for the role of the vestibular system, sensory integration and balance anomalies in vision in this paper published in Optometry and Vision Development in 2007. Yet one need not be engaged in vision therapy to appreciate the role of the vestibular system in vision. Every optometrist who prescribes eyeglasses has advised patients at one time or another that a period of adaptation may be required in order to get adjusted to a new prescription. Goldberg et al cite this up front in their book as a model of adaptive plasticity.
The issue of adaptive plasticity in response to the introduction of a lens or prism is directly related to the time required to recalibrate the gain in the VOR (Vestibulo-Ocular Reflex). The gain of the VOR is defined as the ratio of eye velocity to head velocity. With an increase in magnification, a movement of the head will result in a larger, faster movement of the image across the retina and will require an increase in the gain of the VOR to maintain retinal stability. A decrease in magnification, or relative magnification, will require a decrease in VOR gain. As a brief review from part 1, here is a simplified image from Wiki Commons that illustrates the inhibition/excitation balance in the VOR between the semicircular canals and EOMs.
Adapting to a new lens Rx can be an adventure, even under optimal conditions. One of the greater challenges as we know is the initial adaptation to Progressive Addition Lenses (PALs). Want to drive an autolensometer crazy? Take a PAL and purposely move it just a little bit off center, and see what the computer thinks of the image. You’ll get a series of wild cylinder readings in the region of 10D, and a bunch of #@?!#@?#@! Hence the common piece of advice given to patients initially is to point your nose in the direction of the object rather than scanning with your eyes.
Your brain accomplishes two things this way. One, of course, is that you don’t have to deal with the distorted image. Much like the lensometer, your brain is thinking: “That is some kind of cr*ppy image out there!”. The second is that it’s less work to recalibrate the gain in the VOR as you look around if you minimize the difference between head movements and eye movements. In due time, your brain taps into adaptive plasticity, and it can start to scan just a wee bit more into the periphery. Recalibration of the VOR is what Goldberg et al refer to as context specific adaptation. That is why you sometimes get into trouble when you switch someone from a lens they perform well in to a “new and improved” version. Patients with tightly calibrated VORs may lack sufficient adaptive plasticity to readily change base curves, lens thickness, frame/lens shape, etc. It should be part of our consideration in profiling a patient as a “central processor” vs. a “peripheral processor”.
All this talk about the vestibular system and eye movements makes me hungry to get back to one of the most prized possessions in my library, “Leigh and Zee”. After I did my first pass through the 4th edition in 2006 I found at least one gem on every page. Ah … so many colored tabs; so little time … (no longer use the colored tabs in 2014, but not yet converted from the feel of a book to eReaders).