Three-dimensional spatial characteristics of caloric nystagmus

We investigated the three-dimensional spatial characteristics of caloric ny stagmus during excitation and inhibition of the lateral semicircular canal in five normal human subjects. Each subject was repositioned in 45 degrees steps at 1-min intervals such that the right lateral semicircular canal pla ne was reoriented in pitch, from 135 degrees backwards from the upright pos ition to 135 degrees forwards, while the right ear was continuously stimula ted with air at 44 degrees C. In orientations in which caloric stimulus res ulted in excitation of the right lateral semicircular canal, the eye veloci ty axis was orthogonal to the average orientation of the right lateral semi circular canal plane. However, in orientations in which caloric stimulus re sulted in inhibition of the right lateral semicircular canal, the eye veloc ity axis was orthogonal to the average orientation of the left and not the right lateral semicircular canal plane. These findings suggest that velocit y and direction of caloric nystagmus depend not only on the absolute magnit ude of vestibular activity on the stimulated side but also on the differenc es in activity between the left and right vestibular nuclei, most probably mediated centrally via brainstem commissural pathways.