In Part 2 we cited Hermann von Helmholtz, a brilliant scientist in the 1800s in Germany who understood that the development of binocular vision was a learned rather than innate process. Helmoltz’s mother was Caroline Penn, a remote descendant of William Penn, founder of Pennsylvania in 1681. Michel Meulders, Emeritus Professor of Neuroscience and former Dean of the Medical School at Catholic University of Louvain, has written a wonderful account of Helmhotz’s influences. I’m going to make a bold set of claims here: The origin of the schism that developed between optometry and ophthalmology involving vision therapy, and the drifting even in optometry away from functional vision can be understood through the substance of Helmholtz’s dispute with Hering, as related by Meulders. It is crucial to understand that Hering represented the nativist approach, or a mechanical link between the two eyes in which structural programs unfolded through genetic guidance to a much greater degree than experience.
In contrast, Meulders asserts, Helmholtz’s empirical approach to binocularity was significantly influenced by Kantian principles of time, space, and causality. When I was growing up in Optometry, I listened to Dr. Al Sutton and others talk about “building a visual space world”, along with notions of space and time, terms sounding like they derived more from Einstein than anything I had heard during school in my physiological optics courses. Yet the origins of these concepts are rooted in Helmholtz’s 1857 Treatise on Physiological Optics, volume 3 on Perception, which you can now download courtesy of the Backus Lab on Binocular Vision and Perceptual Learning at the SUNY College of Optometry.
Here is where Meulders shines in summarizing Helmholtz’s theory:
“Other types of movement – of the body or the hand – collabaorated in the exploratory function of the eyes, and this enabled the subject to have a representation of objects in space and record within his inner concept of time his memory of the progress of external events.” He proceeds to describe Helmholtz’s experiments with prisms, comparing reaching behavior withe eyes open vs. eyes closed, and concludes: “So the eye was able to continuously ‘calibrate’ its relations with visual space by taking into account, at the level of perceptual processes, information about the spatial environment derived from other sense organs.”
There is another good encapsulation of the implications of Helmholtz vs. Hering in the Stanford Encyclopledia of Philosophy:
“Hering argues that the ability to experience objects as a single, spatially ordered image is a disposition inborn in human children, and not acquired. While children may not be born with the ability to resolve two images into one, Hering claims that the ability develops when a child grows to maturity, and is not learned. Hering argues that depth perception and stereoscopic vision are inherent physical abilities, like running or even breathing, that can be honed but are not learned wholesale from experience. In fact, to Hering, the adjustments the brain and eyes make to different inputs are automatic and involuntary, like a heartbeat. Helmholtz contends that depth perception and stereoscopic vision require reciprocal adjustment to objects and thus are skills that must be learned through experience …”
That begs the question posed to me by Amber, Stella’s mom, a superb blogger and author of the Life and Times of Stella: “Do you have thoughts on what causes the typical binocular coordination to fail to develop (or should i say, to be learned, as it is a learned skill) in infancy?”
It’s a great question, and I’ll begin by noting that both Helmholtz and Charles Wheatstone, who is credited with inventing the stereoscope, began their scientific careers with explorations in hearing rather than vision. An appreciation of spatial localization through sound launched them into experiemnation in binocular vision. We can gain a clue about the failure of binocular vision development by differentiating the sensory vs. motor processes in auditory defvelopment vs. visual development. Consider the infant who is born with, almost by definition the ulitmate in motor challenges: the child with cererbral palsy (CP). The prevalence of hearing deficits in this population is only 12% but the prevalence of strabismus is much higher. Moreover, the prevalence of strabisumus is directly related to the degree of motor impairment.
One might take this observation to endorse the position that strabismus is purely a motor weakness, but I believe the argument is stronger to support Helmholtz’s argument that motor learning is rooted in head-to-toe early experiences. That is why auditory development is comparatively normal in CP, as cortical localization and tonotopic maps of sound localization can develop largely unimpaired. As I noted in a monograph the primary difference in humans between audition and vision is motor exploration. The sophistication in binocular visual development is reflected in how well the eyes can move independent of head movement. In the human audiory system the ears are never called upon to move independent of the head. But the gross and fine motor restrictions in CP directly limit the ability of the infant to learn binocular integration. The infant is restrained in her ability to develop cross-patterning, reciprocal interweaving, crossing the midline, hand and body feedback to localize space, and body balance to establish that ocluocentric localiization on the midline remains linked with egocentric localization on the midline – all factors in the learned development of normal binocular vision. In this sense, a child with amblyopia has a form of monocular learning disabilty and child with strabismus has a form of binocular learning disability. While the significance of perceptual learning to counteract amblyopia is now widely recognized, its significance in binocular vision development is only now re-emerging in strabismus.
At some level Helmholtz intuitively understood the significance of intersensory integration and motor learning in spatial localization more than 150 years ago. One might say that he was a bit ahead of his time. I suspect that’s why Dr. Backus tipped his cap to Helmholtz by naming his lab at SUNY the Backus Lab on Binocular Vision and Perceptual Learning, positioning Helmholtz’s Treatise front and center.