Caloric stimulation of the vestibular system of the pigeon under minimal influence of gravity.

Vestibular microphonics in response to acoustical stimulation were recorded in cochlea-deprived pigeons. Endolymph flow in the semicircular canals alters this response. A caloric stimulus was applied to the horizontal canal or the posterior vertical canal close to the ampulla. To minimize the influence of gravity, the canal being studied was positioned in the horizontal plane. Local temperature changes were generated with an electrically heated miniature copper probe, producing an approximately trapezoidal temperature profile. Because of the opposite hair cell polarization in the horizontal and the vertical ampulla, we could prove that the observed effects were of (hydro-)mechanical origin and were not caused by a direct effect of sensory epithelium temperature change. The first effect of temperature increase was an utriculopetal endolymph flow, as predicted by the 'expansion theory'.