New ideas about binocular coordination of eye movements: Is there a chameleon in the primate family tree?

Abstract

Many animals with laterally placed eyes, such as chameleons, move their eyes independently of one another. In contrast, primates with frontally placed eyes and binocular vision must move them together so that both eyes are aimed at the same point in visual space. Is binocular coordination an innate feature of how our brains are wired, or have we simply learned to move our eyes together? This question sparked a controversy in the 19(th) century between two eminent German scientists, Ewald Hering and Hermann von Helmholtz. Hering took the position that binocular coordination was innate and vigorously challenged von Helmholtz's view that it was learned. Hering won the argument and his hypothesis, known as Hering's Law of Equal Innervation, became generally accepted. New evidence suggests, however, that similar to chameleons, primates may program movements of each eye independently. Binocular coordination is achieved by a neural network at the motor periphery comprised of motoneurons and specialized interneurons located near or in the cranial nerve nuclei that innervate the extraocular muscles. It is assumed that this network must be trained and calibrated during infancy and probably throughout life in order to maintain the precise binocular coordination characteristic of primate eye movements despite growth, aging effects, and injuries to the eye movement neuromuscular system. Malfunction of this network or its ability to adaptively learn may be a contributing cause of strabismus.