Departments of Neurology and Anatomy, University of Mississippi, USA (e-mail:email@example.com)
More than a century ago, Helmholtz and Hering proposed conflicting hypotheses about how binocular co-ordination of eye movements was achieved. Helmholtz proposed that each eye is controlled independently and that binocular co-ordination is a learned behaviour. Hering proposed that both eyes are innervated by common conjugate and vergence commands that yoke their movements (Hering's Law of Equal Innervation). Neurophysiological evidence has, until recently, provided apparent support for Hering's hypothesis. Recent studies suggest, however, that Helmholtz' hypothesis should be reconsidered. For example, we have reported that premotor medium lead burst neurones in the paramedian pontine reticular formation (PPRF) encode monocular commands for right or left eye movements, not conjugate eye movement commands as was previously assumed. We have also shown that many abducens motoneurones are not monocular, as previously assumed, but binocular, and exhibit discharge activity related to movements of either eye. The premotor data suggest that Helmholtz may have been correct in assuming a degree of independent neural control of each eye. The motoneurone data, however, suggest a novel reinterpretation of Hering's Law that may provide a way to reconcile their conflicting hypotheses.