A Model of Alexander's Law of Vestibular Nystagmus

The observation that the amplitude of ves­ tibular nystagmus grows as gaze is increased in the direction of the nystagmus fast phase and diminished with gaze in the opposite direction is known as "Alexander's law". We have developed an analog computer model to simulate Alexander's law in nys­ tagmus secondary to dysfunction of a semicircular canal. The model utilizes relevant brainstem anatomy and physiology and includes gaze modulation of ves­ tibular signals and push-pull integration to create eye positition commands. When simulating normally func­ tioning semicircular canals, the model produced no nystagmus. When simulating total impairment of the canal on one side with gaze directed maximally in the opposite direction, the model produced a large ampli­ tude nystagmus with linear slow phases directed to­ ward the affected side. As gaze was changed from far contralateral to ipsilateral, the nystagmus gradually diminished to zero. When simulating partial impair­ ment of one canal, the nystagmus was smaller in amplitude and absent in ipsilateral gaze.