SUMMARY A midline experimental lesion separating the medial longitudinal fasciculi at and below the level of the abducens nuclei without damaging either fasciculus at the level of the nuclei has produced defects of ocular motility resembling those of clinical internuclear ophthalmoplegia. Electromyographic recordings during lateral gaze demonstrate: (1) lack of inhibition of the lateral rectus muscle in the adducting eye, (2) delayed inhibition of the medial rectus muscle in the abducting eye, and (3) occasional evidence of excitation of the medial rectus muscle of the abducting eye probably associated with pupillary constriction. The presumed physiologic mechanisms involved in conjugate gaze movements are discussed in the light of the experimental findings.Internuclear ophthalmoplegia (INO) is the dissociation of ocular movements during horizontal versions, consisting of an underacting medial rectus muscle (in the adducting eye), often accompanied by abducting nystagmus of the fellow eye. The presence of this movement disorder is considered to be pathognomonic of a lesion in the medial longitudinal fasciculus (MLF) in the brainstem. The pathophysiologic mechanism by which the dissociated nystagmus of the abducting eye is produced has remained elusive. Stroud et al. (1973Stroud et al. ( , 1974 proposed that asymmetric convergence is used to drive the weak medial rectus muscle, which secondarily produces the abducting nystagmus on the uninvolved side. More recently, Pola and Robinson (1976) have postulated that a lesion of one MLF would interrupt both ipsilateral excitatory and contralateral inhibitory fibres to the respective medial rectus subnuclei of the third cranial nerve. This postulate would fit well with clinical findings previously reported by Loeffler et al. (1966). In contrast to Stroud et al. these authors believe that the nystagmus of the abducting eye after a contralateral MLF lesion is due to loss of inhibition of the medial rectus activity of that eye. A third hypothesis attributes the nystagmus to interruption of nerve fibre pathways passing to The purpose of this study was to produce an experimental model of inter-nuclear ophthalmoplegia in the primate without damaging the parapontine gaze centres or their subservient ocular motor nuclei. The animals were studied by electromyography in order to gain an understanding of the underlying pathophysiology of INO.
MethodsFour male and one female rhesus monkeys of weights ranging from 9 5 to 12-5 lb (4 3 to 5 7 kg) underwent suboccipital craniectomy under barbiturate anaesthesia. In each case the cerebellar vermis overlying the caudal portion of the fourth ventricle was split in the midline to a depth of approximately 1'5 cm. Under magnification the two cerebellar hemispheres were held apart in this region, and a midline cut was made in the floor of the fourth ventricle with a fine knife.A week or more after surgery the animals were studied in a primate chair with and without head restraint. Motion pictures, 8 and 16 mm, were made of the extraocular mo...