Hering’s law of equal innervation

Hering’s law of equal innervation states that whenever an impulse for an eye movement is sent out, corresponding muscles of each eye receive equal innervation to contract or relax.¹ The law is also called the law of motor correspondence of the eyes.¹ The paired muscles are yoke muscles, one in each eye, and the law relates the innervation of an agonist in one eye to its contralateral synergist in the other eye.^1,2

Hering first stated the law in 1868 for voluntary eye movements, but the law applies to all normal eye movements, including vergences.¹ One eye need not make an observable movement, and an eye that seems to stay still does not invalidate the law.¹

Hering’s law explains primary and secondary deviations in paralytic strabismus.¹ If a patient with a paretic left lateral rectus fixates with the paretic eye, the left lateral rectus needs much more innervation to hold the eye straight.² The non-paretic right eye receives the same increase to the right medial rectus, causing a very large secondary deviation.²