Mechanics of caloric stimulation in a case with a plug in the horizontal semicircular canal

Horizontal eye movements were recorded by electronystagmography in a subject with a plug of granulation tissue in the unilateral horizontal semicircular canal by the room temperature air caloric stimulation. Stimuli were sequentially presented to both ears with the subject supine and prone, keeping the horizontal canal vertical. Caloric stimuli presented to the unplugged ear resulted in a vestibular nystagmus whose slow phase eye velocity (SPEV) was always directed toward the stimulated side when the subject was supine and toward the opposite side when he was prone. In contrast, calorics presented to the horizontal canal plugged ear showed that SPEV was always directed toward the stimulated side, irrespective of whether the subject was supine or prone. Results were consistent with consensus that the caloric responses noted in the horizontal canal plugged ear reflected horizontal canal afferent activity which may correspond to direct effect of temperature on end organs or afferents. The findings imply that a caloric mechanism exists which is independent of the conventionally accepted one involving concection currents within the canal.