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Jeremy Wilmer, Benjamin Backus; Behavioral genetic evidence for plasticity in the oculomotor system. Journal of Vision 2008;8(6):633. doi: 10.1167/8.6.633.
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By establishing the relative influence of genes versus environment, a classic twin study can point to visual functions that exhibit more or less plasticity due to experience. Such knowledge informs theories of development and helps to identify those visual functions most likely to respond to therapeutic intervention. Here, we use a twin study to demonstrate substantial plasticity in one important aspect of oculomotor function. Specifically, we find evidence that an individual's vergence position with inputs to the eyes dissociated, or phoria, is due mainly to environmental influences rather than genes.
We measured phoria at nearpoint (40cm) in 310 pairs of identical (n=258) and fraternal (n=52) twins using a Maddox Rod with a target for accommodation. Twins had highly similar phorias (r=0.60), which in isolation could result from sharing either genes or environment. However, despite sharing twice as many genes (100% vs. 50%), identical twins' phorias were no more similar (r=0.58) than fraternal twins' (r=0.67), suggesting that phoria results from environmental rather than genetic influences.
Phoria has traditionally been thought of as one's latent tendency toward manifest oculomotor deviation, or strabismus. Our lack of evidence for a genetic contribution to phoria must therefore be reconciled with previous evidence that strabismus is substantially genetic. We propose that phoria and strabismus may not be simply graded expressions of a common tendency. Instead, distinct mechanisms with different developmental origins may contribute to phoria and strabismus. Our study is the first we are aware of to demonstrate substantial oculomotor plasticity using the twin method.
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