Am Orthopt J  49: 156-160  (1999).

Vision-Dependent Tonus Mechanisms of Torticollis: An Evolutionary Perspective

Michael C. Brodsky, M.D.

Duke-Elder states that "The control of the movements of living organisms, both plants and animals, by light is a fundamental function of great phylogenetic age, preceding the acquirement of vision and, indeed, leading directly to its development. The association of the functions of equilibration and orientation with the visual system of higher animals is in every sense basic. This primitive control of movement by light is undoubtedly an adaptive process, directing the organism to regions in the environment which are favorable to it."

Postural abnormalities in lower animals reflect asymmetries in neuronal tonus pools which can be driven by unequal input from the eyes, the labyrinths, or by other afferent pathways involved in proprioception. Tonus asymmetries between antagonist pools produce turning movements in lower animals and may manifest as torticollis in humans. These postural abnormalities appear to be atavistic in nature, since they emerge in humans when normal binocular vision fails to develop. The role of pretectal visual input in calibrating and modulating tonus levels in humans and other frontal eyed animals remains to be determined. The emergence of these primitive reflexes in humans with abnormal visual input early in life may offer insight into the evolutionary role of visual input in the tonic modulation of extraocular and postural muscle tone.

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