Put aside this article to read and came across it again this morning: The Necessity of Amblyopia by Steven M. Archer, M.D., the 46th Richard G. Scobee Memorial Lecture presented as part of a Symposium of the Joint Meeting of the American Orthoptic Council, the American Association of Certified Orthoptists, and the American Academy of Ophthalmology, Las Vegas, Nevada, November 16, 2015, and published in the American Orthoptic Journal last year.
From the abstract:
“A general property of nervous system development is that correlated activity is used to organize topographic projections. This correlated activity is typically produced by electrical coupling of adjacent neurons; however, electrical coupling is not possible for retinal ganglion cells in separate eyes that need to be precisely mapped to the same place in the brain. This forces the visual system to rely on environmental stimuli to produce the correlated activity that drives the development of binocularity, with amblyopia as necessary consequence when visual experience is abnormal. The characteristic visual deficits in both the amblyopic and the sound eyes can be understood in the context of these normal developmental processes. The auditory system provides another example where precise connections between paired sense organs must rely on environmental stimuli for normal development in which the analogous condition of amblyaudia occurs.”
Archer explains his premise in the introduction as follows:
“My thesis for this lecture is that amblyopia is not a result of visual system adaptation gone wrong, but rather a necessary consequence of a general strategy of nervous system development, well recognized in the field of neurobiology, that is left vulnerable to environmental disruption, in this special instance, by the physical separation of the two eyes.”
If the visual system relies on correlated activity from the environment, how might this this neural activity between the two eyes work? Archer suggests the following construct:
“Imagine an apartment building with a phone in each unit and walls that are not very soundproof. When you listen to the conversation on one of the phone lines coming out of the building, you hear not only the person in the unit from which that line comes, but also a little of the conversations going on in adjacent units. By listening to pairs of lines coming out of the building, you could ascertain which units are adjacent to each other and ultimately sort the lines into the same order as the units from which they came. In the retina, correlated activity of adjacent ganglion cells could theoretically provide the information – analogous to the crosstalk between rooms in the apartment building needed to organize the projections to the lateral geniculate nucleus (LGN) and visual cortex.” Watch these videos from the Kellogg Eye Center at the University of Michigan to see how this works.
Why is amblyopia truly a binocular problem? Archer continues:
“Because the eyes are physically separated, there is no mechanism by which corresponding points in the two retinae can be electrically coupled to provide correlated activity. This means that if the same developmental strategy of using correlated activity is used to refine the alignment of the cortical maps, the correlation of corresponding retinal points in each eye will have to be generated by external visual stimulation of aligned eyes. So, while the projections from each eye are already largely retinotopic by the time of birth or eye opening, refining the binocular alignment of those projections must wait for visual stimulation. At that point, each cortical cell is influenced by both eyes as it establishes its spatial identity. It is most likely to pick a spatial identity that provides the best correlation between the inputs it receives from each eye. In this way, the mapping of each eye influences the organization of the other until they match.”
Parenthetically, Archer positions the loss of stereopsis as a barometer of the lack of refinement of binocular correspondence between the maps from each eye – something that persists even after surgical correction of ocular misalignment in the case of strabismus. And given my penchant for the parallels between auditory and visual processing, I was drawn to Archer’s concluding paragraph:
“If organization of projections using information from correlated activity is a general feature of the developing nervous system, then are there other examples in which the function of paired sensory structures is vulnerable to abnormal environmental experience? As it turns out, the auditory system is similar in many regards. In a process analogous to stereopsis, differences in loudness and timing between the two ears are integrated centrally to localize sound in three dimensions. As with stereopsis, stereophonic sound gives a spatial sense of the environment that is qualitatively different from monaural hearing. Binaural hearing also facilitates the so-called “cocktail party effect” – the ability to focus on understanding one person speaking in a noisy environment – which may be particularly important for children in complex environments such as the classroom. A number of animal models show long-lasting functional deficits from monaural deprivation during a critical period development. Clinical models of asymmetric hearing loss during childhood are more difficult to study, but there is growing recognition of an “aural preference syndrome” or “amblyaudia.”