Unilateral muscimol inactivations of the interstitial nucleus of Cajal in the alert rhesus monkey do not elicit seesaw nystagmus.

Seesaw-nystagmus (SSN) is a unique form of nystagmus with disconjugate vertical and conjugate torsional eye movements. Although rare, this disorder serves as a model for neuronal binocular control of the alignment of vertical-torsional eye movements of both eyes. The pathomechanism of SSN, however, is unclear. Studies in patients have suggested that the jerk SSN is associated with a midbrain lesion, i.e. a lesion of the interstitial nucleus of Cajal (iC), a center of integration of vertical and torsional eye movements. To test this hypothesis, we examined three dimensional binocular eye movements after reversible local inactivations of the iC and its immediate vicinity in the midbrain of the alert monkey. Inactivations were induced by muscimol microinjections. Eye movements were recorded with binocular scleral search coils. Isolated inactivations of neither the iC nor its immediate vicinity in the midbrain (including the adjacent rostral interstitial nucleus of the medial longitudinal fascicle, riMLF) elicited a disconjugate vertical/torsional nystagmus (SSN). However, there was a direction-specific right/left asymmetry in which a larger vertical amplitude was associated with the contralesional eye and a larger torsional amplitude with the ipsilesional eye, indicating a vestibular imbalance. We conclude that, first, iC lesions do not elicit SSN and, second, that apart from the gaze holding deficit a vestibular imbalance contributes to the vertical/torsional nystagmus after iC lesions.