CONTRIBUTIONS OF SINGLE SEMICIRCULAR CANALS TO CALORIC NYSTAGMUS AS REVEALED BY CANAL PLUGGING IN RHESUS-MONKEYS

We demonstrated specific responses from the anterior and the posterior semicircular canal to irrigation of the outer ear canal with cold wat er in the Rhesus monkey. This required i) three-dimensional analysis o f the evoked eye movements in the planes of the semicircular canals (c anal plane vectors, CPV); ii) assessing these CPV responses in eight d ifferent head positions relative to gravity; iii) comparing the respon ses in 6 normal animals (12 ears) with responses after selective plugg ing of pairs of semicircular canals (all, both lateral, and right ante rior+left posterior). The results showed: i) Irrigation of the outer e ar canal with cold water induces thermoconvection also in the posterio r and anterior semicircular canals. This can be inferred from the sinu soidal modulation of eye movement components with changes in position of the corresponding semicircular canal plane relative to gravity; ii) Specific vertical canal responses occur exclusively in the direction of the corresponding semicircular canal, though they are superimposed with response components of other origin, one probably related to endo lymph shift in the lateral semicircular canal; iii) before possible cl inical application, these different response components of vertical ca nals will need to be determined in humans.