Is Vision Therapy All Wet?

Of course not, but I did pique your interest, didn’t I? This not-so-hypothetical question is based on the unique underwater visual abilities of the Moken Sea-Gypsy children of Southeast Asia, which I first encountered in a book about human individuality by David J. Linden, a professor of neuroscience at the Johns Hopkins University School of Medicine and the Kavli Neuroscience Discovery Institute. (By the way, there’s nice review of the book here.)

As noted in Wikipedia, the Moken are an Austronesian people of the Mergui Archipelago, a group of approximately 800 islands within Burma and Thailand. Most of the Moken live a semi-nomadic hunter-gatherer lifestyle heavily based on the sea. How is it that children among these so-called sea gypsies have learned to see well underwater? After all, this seems to defy what first year optometry students learn in optics courses — that eyes normally adapted to terrestrial environments work based on the differences in index of refraction of light in the air (close to 1.0), the watery medium of the eye (close to 1.33) and refractive components of the eye (pre-corneal tear film/cornea/pupil/aqueous/lens/vitreous/retina and on to deeper visual pathways in the cortex).

When you open your eyes underwater, the refractive interface between air and eye is neutralized, resulting in considerable hyperopia that creates blur. This is why you normally put on swim goggles or a scuba mask if you want to see underwater, restoring the normal relationship of light coming in through air as it enters your eye. After all, the cornea is a fixed structure and can’t voluntarily change it’s refractive curvature. The crystalline lens can, but can’t accommodate sufficiently to neutralize the induced hyperopia. But … what if instead of your pupils dilating in the comparative darkness of water, they constricted, resulting in considerably better depth of focus? Apparently that is exactly what the pupils of sea-gypsy Moken children do, in conjunction with bursts of accommodation, as demonstrated in a landmark paper (or perhaps we should call it a seamark paper) by Gislén and colleagues in Current Biology (2003).

Gislén and her colleagues were sufficiently intrigued by the adaptation that Moken children had made to pose another question: Could European children be trained to achieve that same level of clarity? Several years after their original paper, they published their results in Vision Research, demonstrating that visual training can improve underwater vision in children. Training of the European children took place during 11 sessions over the course of 4 weeks, followed by 4 months with no underwater activities. The children showed improved underwater vision related to distinct bursts of accommodation and pupillary constriction. When tested 8 months after the last training session in an outdoor pool in bright sunlight, comparable to light environments in Southeast Asia, children still had the same underwater acuity as the sea-gypsy children.

Now you may ask yourself, that is fine for water flowing underground, but would this training effect result in unwanted residual esophoria on dry land? Apparently not. Excessive convergence wasn’t observed in either the trained European children or the Moken children. In order to see well underwater, these children seem to have learned to uncouple accommodation from convergence while diving. Shades of “degrees of freedom” in conventional optometric vision therapy. As these researchers learned, uncoupling convergence from accommodation can be achieved by most subjects after some training—a requirement for viewing certain types of three-dimensional images.

Same as it ever was …

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