The directions of nystagmus and apparent self-motion evoked by caloric tests and angular accelerations

To further investigate the direction of (I) nystagmus and (II) self-motion perception induced by two stimuli: (a) caloric vestibular stimulations and (b) a sudden halt during vertical axis rotation.

Twelve normal humans received caloric stimulation at 44 degrees C, 30 degrees C, and 20 degrees C while in a supine position with the head inclined 30 degrees upwards. In a second test they were rotated around the vertical axis with the head randomly placed in two positions: tilted 30 degrees forward or tilted 60 degrees backward, at a constant velocity of 90 degrees /sec for 2 minutes and then suddenly stopped. After both tests they were asked to describe their sensations of self-motion. Eye movements were recorded with an infrared video-technique.

Caloric stimulation evoked only horizontal nystagmus in all subjects and induced a non-uniform complex perception of angular in frontal and transverse planes (the former dominated) and linear movements along the antero-posterior axis (sinking dominated) of the subject's coordinates. The self-motion was felt with the whole body or with a part of the body. Generally the perception evoked by cold (30 degrees C) and warm (44 degrees C) calorics was similar, although there were some differences. The stronger stimulus (20 degrees C) evoked not only quantitative but also qualitative differences in perception. The abrupt halt of rotation induced self-motion perception and nystagmus only in the plane of rotation. The self-motion was felt with the whole body.

There was no difference in the nystagmus evoked by caloric stimulation and a sudden halt of vertical axis rotation (in head positions to stimulate the horizontal canals); however, the two stimuli evoked different perceptions of self-motion. Calorics provoked the sensation of self-rotation in the frontal plane and linear motion, which did not correspond to the direction of nystagmus, as well as arcing and a reset phenomenon during angular and linear self-motion, caloric-induced self-motion can be felt predominantly or only with a part of the body, depending on the self-motion intensity. The findings indicate that, unlike the self-motion induced by sudden halt of vertical axis rotation, several mechanisms take part in generating caloric-induced self-motion.