Lesions of MLF in monkeys (Evinger, Fuchs and Baker, 1977) severed the internuclear pathway connecting ipsilateral internuclear neurons to controlateral motor neurons of the medial right. The experimental lesion caused bilateral palsy of adduction during conjugated eye movements, but preserved adduction associated with slow vergence movements consistent with midbrain cells close to the reaction that are a source of a vergence signal to medial right motor neurons. In a study in my lab on how viewing distance affects the amplification of the vestibulo-ocular reflex, we were surprised that vestibular neurons (positional vestibular pause, PVP) encode the uniocular position; That is, they encode the orbital position of the left or right eye, but not the position of the conjugated eye (McConville et al., 1994), as previously assumed (Fuchs & Kimm, 1975). PVP cells are second-order excitatory neurons in the vestibular eye reflex and synapses directly on the controlateral abducens of motor neurons (Scudder and Fuchs, 1992). The eye position signal is thought to be generated in a neural network that integrates ocular velocity commands generated, for example, by PPRF burst neurons (Robinson, 1973). The integrating circuit includes neurons in the nucleus of prepositus hypoglossi (NPH) and in the medial vestibular nucleus, where the PVP cells are located (Baker, 1977; Baker and Berthoz, 1975; McFarland and Fuchs, 1992). Therefore, the discovery of different populations of vestibular neurons encoding the uniocular ocular position implies that the oculomotor integrator is also uniocular. This conclusion seems to contradict Hering`s Law, but it is possible that the uniocular responses of PVP neurons represent a central sum of distinct commands of conjugated eye movements and convergence. For example, convergence rate neurons in the midbrain and PPRF bursting cells (which represent the conjugated gaze command) could project onto integrator networks in the left and right eye to generate the uniocular signal.

To test this hypothesis, we recorded burst neurons and NPH neurons during disjunctive saccades from the PPRF. Figure 4A shows that 79% of pontoon bursting neurons encoded a uniocular saccadic control, with about 50% favouring the ipsilateral eye (“ocular selectivity”, Zhou & King, 1998). These data were then confirmed by van Horn et al., who found a similar proportion of monocular burst neurons (2008). Similarly, 78% of our recorded NPH neurons coded monocular position (Figure 4B), a finding later confirmed by Sylvestre et al. (2003). “As far as their movements in the service of the sense of sight are concerned, both eyes are treated as one organ. It is irrelevant to the will to mobilize that this body really consists of two distinct parts, because it is not necessary to move each part individually; On the contrary, a single impulse of will directs both eyes at the same time, just as a pair of horses can direct individual reins. “It can be shown that the regularity of these associations [between the movements of the two eyes] is simply a matter of training.” He postulates that the eyes move conjugated (i.e. simultaneously in the same direction and in about the same amount) because the synergistic muscles of both eyes receive the same signal.

It was formulated with two conclusions: (i) the principle of equal innervation is based on the existence of appropriate anatomical connections and (ii) the corresponding anatomical connections are innate. The controversy between Helmholtz and Hering reflected the empiricist vs. nativist philosophical views of every man and his followers. Helmholtz argued that “the connection between the two eyes is not an obligatory anatomical mechanism, but something that can be altered by the mere influence of our own will” (Helmholtz 1962). He described his own experiences of double vision in the sleep state as examples of voluntary control; “If the movements of the eye were coordinated by an anatomical mechanical invention, one would expect it to function in a state of drowsiness with even less resistance when the energy of the will rests.” According to Helmholtz`s observations, eye movements in monkeys during sleep are not coordinated and uniocular movements in any direction can occur, resulting in large divergent vertical and horizontal eye positions (Zhou and King, 1997). Figure 6A shows the vertical vergence angle relative to the horizontal vergence angle in the waking monkey. Vertical vergence is always close to zero and horizontal vergence is always convergent (positive). When the monkey sleeps, horizontal and vertical vergence angles are not correlated, taking positive and negative values (Figure 6B). The seemingly random slow eye movements characteristic of sleep suggest that the oculomotor integrator does not function during sleep and that binocular coordination in turn depends on the function of the integrator and the exercise of voluntary control (Schreyer, Büttner-Ennever, Tang, Mustari and Horn, 2009). These assumptions are behaviorally difficult to distinguish because the movement of each eye (ER, EL) can always be represented as the linear sum or difference of the conjugate (Econj) and Vergence (Everg) components common to each eye. Hering`s law of equal innervation is best understood with Johannes Peter Müller`s stimulus, in which an observer reflects on a point that has moved only one eye.

The least effort to refoveing is to move only the misaligned eye. Instead, Hering`s Law predicts that since both eyes must move in equal amounts, a combination of connective and disjunctive eye movements is required to reflect the target point. Yarbus[5] showed experimentally that binocular eye movements actually consist mainly of combinations of saccades and vergence. In the meantime, however, it is known that there are also significant deviations from Hering`s law. [6] [7] [8] Contributors: BCK Patel, MD, FRCS; Mr. Raman Malhotra, FRCS This theory contrasts with the theory of Hermann von Helmholtz,[9] who asserts that conjugation is a scholarly and coordinated reaction and that eye movements are individually controlled. Helmholtz`s view is now often caricatured as a chameleon and independent eye control, although Helmholtz never defended this theory. Their disagreements concerned the learned aspects of coordinated binocular eye movements. Helmholtz`s arguments mainly concerned Listing`s law and can be simplified because there are eye positions in which muscles have different effects on the two eyes.

Therefore, Hering`s Law simply cannot be correct in its original formulation, as it would lead to situations where the eyes would move in different quantities, which both agree on never happens. Herring then changed his law to state that the eyes behave as if they were receiving the same innervation. (2009). Hering`s “Law of Equal Innervation.” In: Binder, M.D., Hirokawa, N., Windhorst, U. (eds) Encyclopedia of Neuroscience. Springer, Berlin, Heidelberg. doi.org/10.1007/978-3-540-29678-2_2185 This patient was still not visually disturbed by the right upper eyelid, so she decided not to have surgery on the right side. However, other patients may choose to correct the now apparent sagging of the upper right eyelid. The surgeon should aim to give the patient a symmetrical skin fold and fill the upper hollow (sulcus) to achieve as much symmetry as possible. This operation can be requested by the patient at any time. Neurophysiological evidence for Hering`s Law and possible explanations for its failure were obtained only with the technical development of individual extracellular recording in alarmed animals (Evarts, 1968; Fuchs and Luschei, 1970a). With the advent of extracellular recording, the neural information encoded by the advanced traits of extraocular motor neurons and premotor eye movement neurons could be described and quantitatively analyzed in terms of eye movements.