Hering's law of equal innervation and vergence eye movement.

Hering's hypothesis concerning transverse eye movement states that each eye's movement is the sum of two types of component movement. In a purely formal way, the hypothesis can describe any binocular eye movement. However, a mathematical separation of version and vergence components says nothing about the validity of Hering's hypothesis in its empirical form. Accordingly, one must make explicit the empirical propositions that can be derived from the hypothesis. This paper discusses the formal and empirical aspects of the hypothesis and briefly reviews the relevant experimental evidence. Also discussed are accommodative vergence and asymmetrical vergence; it is argued that they do not necessarily violate Hering's law.