Caloric responses after horizontal canal inactivation.

Horizontal eye movements in squirrel monkeys were recorded in response to ice-water caloric stimuli before and after horizontal canal (HC) inactivation, achieved by a plugging procedure. Calorics were presented with the subjects supine and prone, and the maximum slow phase eye velocity (SPEV) of the induced nystagmus was assessed. SPEV responses from normal ears were always directed toward the stimulated side when the monkeys were supine, and toward the opposite side when they were prone. Supine responses were always greater than prone ones. After HC-plug, which abolishes the canal's mechanical response to rotatory or convection current stimulation, small SPEV responses were routinely observed. However, they were always directed toward the stimulated side, regardless of head orientation. The results are consistent with the notion that the normal caloric response is composed of the sum of a convention current component which is dependent upon head orientation, a smaller position-dependent component of unclear origin, and a direct temperature effect on the canal's sensory apparatus which is independent of head orientation.