Effects of semicircular canal plugging on caloric nystagmus recorded in three dimensions

In Rhesus monkeys, distinct eye movement response components in the planes of the semicircular canals (canal plane vectors, CPV) were identified by 3-dimensional recordings of caloric nystagmus elicited in different head positions relative to gravity. Surgical plugging of single semicircular canals (SCC) was used to identify the generators of these response components. The lateral CPV is a sinusoidal function of head position, compatible with a thermovective mechanisms in this canal. Surgical plugging of the lateral SCC leaves only a small head position independent, lateral CPV, probably due to direct temperature effects on lateral SCC afferents. The posterior and anterior CPVs are complex responses composed of various components of which the present plugging experiments identified two generators: i) effects of thermovection in the lateral SCC, demonstrated by modifications of posterior and anterior CPVs after occlusion of the lateral SCC; and ii) thermovection in the vertical SCCs, demonstrated by effects of plugging a single vertical SCC exclusively on the corresponding CPV. This is the first definite identification of the vertical SCCs as contributors to the generation of nystagmus induced by caloric irrigation of the outer ear canal. It may provide a basis for the development of a modified caloric test measuring more than only lateral canal function in human patients.