Hering's law of equal innervation

Hering's law of equal innervation is used to explain the conjugacy of saccadic eye movement in stereoptic animals. The law proposes that conjugacy of saccades is due to innate connections in which the eye muscles responsible for each eye's movements are innervated equally. The law also states that apparent monocular eye movements are actually the mathematical summation of conjugate version and vergence eye movements. The law was put forward by Ewald Hering in the 19th century, though the underlying principles of the law date back considerably. Aristotle had commented upon this phenomenon and Ptolemy put forward a theory of why such a physiological law might be useful. It was clearly stated for the first time by Alhacen in his Book of Optics (1021).

This theory is in contrast to the theory proposed by Von Helmholtz (1911) which states that conjugacy is a learned, coordinated response and that the movements of the eyes are individually controlled. Although for most of the 20th century, it was believed that Hering was right, recent evidence has suggested that the eye movements may be separately encoded.