PREMOTOR COMMANDS ENCODE MONOCULAR EYE-MOVEMENTS

Binocular coordination of eye movements is essential for stereopsis (d epth perception) and to prevent double vision. More than a century ago , Hering and Helmholtz debated the neural basis of binocular coordinat ion. Helmholtz(1) believed that each eye is controlled independently a nd that binocular coordination is learned, Hering(2) believed that bot h eyes are innervated by common command signals that yoke the eye move ments (Hering's law of equal innervation). Here we provide evidence th at Hering's law is unlikely to be correct. We show that premotor neuro ns in the paramedian pontine reticular formation that were thought to encode conjugate(3-6) velocity commands for saccades (rapid eye moveme nts) actually encode monocular commands for either right or left eye s accades, However, 66% of the abducens motor neurons, which innervate t he ipsilateral lateral rectus muscle, fire as a result of movements of either eye. The distribution of sensitivity to ipsilateral and contra lateral eye movements across the abducens motor neuron pool may provid e a basis for learning binocular coordination in infancy and adapting it throughout life.