title: Development of eye position dependency of slow phase velocity during caloric stimulation.
abstract: The nystagmus in patients with vestibular disorders often has an eye position dependency, called Alexander's law, where the slow phase velocity is higher with gaze in the fast phase direction compared with gaze in the slow phase direction. Alexander's law has been hypothesized to arise either due to adaptive changes in the velocity-to-position neural integrator, or as a consequence of processing of the vestibular-ocular reflex. We tested whether Alexander's law arises only as a consequence of non-physiologic vestibular stimulation. We measured the time course of the development of Alexander's law in healthy humans with nystagmus caused by three types of caloric vestibular stimulation: cold (unilateral inhibition), warm (unilateral excitation), and simultaneous bilateral bithermal (one side cold, the other warm) stimulation, mimicking the normal push-pull pattern of vestibular stimulation. Alexander's law, measured as a negative slope of the velocity versus position curve, was observed in all conditions. A reversed pattern of eye position dependency (positive slope) was found <10% of the time. The slope often changed with nystagmus velocity (cross-correlation of nystagmus speed and slope was significant in 50% of cases), and the average lag of the slope with the speed was not significantly different from zero. Our results do not support the hypothesis that Alexander's law can only be observed with non-physiologic vestibular stimulation. Further, the rapid development of Alexander's law, while possible for an adaptive mechanism, is nonetheless quite fast compared to most other ocular motor adaptations. These results suggest that Alexander's law may not be a consequence of a true adaptive mechanism.
authors.affiliation: Department of Otorhinolaryngology, Head & Neck Surgery, University Hospital Zürich, Zürich, Switzerland. chris.bockisch@usz.ch
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Bockisch
authors.forename: Elham
authors.initials: E
authors.lastname: Khojasteh
authors.forename: Dominik
authors.initials: D
authors.lastname: Straumann
authors.forename: Stefan C A
authors.initials: SC
authors.lastname: Hegemann
mesh_terms: D003080:Cold Temperature; D005133:Eye Movements; D005260:Female; D006358:Hot Temperature; D006801:Humans; D008297:Male; D009760:Nystagmus, Physiologic