Three-dimensional aspects of caloric nystagmus in humans: II. Caloric-induced torsional deviation

The oculomotor response to caloric stimulation was recorded using video-oculography (VOG), yielding measures of eye rotation about all three orthogonal axes. During an ongoing caloric nystagmus response, a slow torsional deviation of the eye in the direction of the slow phase of the torsional component of nystagmus was observed. The effect appeared to be systematically influenced by the gravitoinertial force employed during a centrifuge study (see Part I, this issue), and was observed in all tested subjects. Furthermore, in subsequent laboratory tests warm and cold irrigations elicited opposite responses, as did stimulation to right and left ears. Testing in the supine and prone positions, however, did not lead to a reversal of direction, as is the case with the horizontal, vertical and torsional nystagmus components as such. The rate of torsional deviation was of the order of 0.05 degrees/s, well below the physiological transduction range of the cupular organs. Testing with a torsional optokinetic stimulus produced only the expected OK nystagmus without any slow deviation. This would argue against a central vestibular integratory, or oculomotor origin for the phenomenon. It is proposed that this caloric-induced, torsional deviation is of peripheral otolithic origin, most likely elicited by way of direct thermal stimulation to the hair cells of the utricular maculae.