Contribution of the Vertical Semicircular Canals to the Caloric Nystagmus

Abstract

Modulation of the caloric nystagmus in response to repositioning the plane of one vertical semicircular canal from gravitational horizontal to vertical during continuous caloric stimulation was used to measure the vertical canal's contribution to the nystagmus. The rationale was to examine the thermovective response from one vertical canal at a time, after a temperature gradient had been established across its two limbs. The nystagmus was measured and analysed in three dimensions using orthogonal head-referenced coordinates. The magnitude of each semicircular canal's contribution to the overall caloric response, the canal vector, was determined in non-orthogonal, contravariant semicircular canal plane coordinates. By using the canal plane reorientation technique and contravariant canal plane coordinates, we were able to measure the proportional thermovective response magnitude generated by each vertical canal during caloric stimulation. We found that the anterior canal contributed about one-third and the posterior canal about one-tenth as much as the lateral canal did to the overall caloric response magnitude when it was reoriented from horizontal to vertical. Comparison of the eye rotation axis