Evidence of Direct Thermal Action upon the Vestibular Receptors in the Caloric Test: A Re-interpretation of the Data of Coats and Smith

Abstract

Although Bárány's convection theory has gained wide acceptance, there is strong evidence that the caloric response may include a component resulting from direct thermal stimulation of the vestibular end organs. It is argued on theoretical grounds that important new information could be obtained from a precise determination of the neutral head positions at which caloric irrigation fails to elicit a response. Although experimental studies using a bracketing procedure proved unsuccessful for this purpose, a re-examination of the extensive studies of Coats and Smith revealed that this information was already available. Reinterpreted in this way, their data clearly indicate that the hot and cold components resulting from direct thermal action equate to 42% and 34% respectively of the responses obtained in the planes of maximum reactivity. This is of sufficient magnitude to account for the anomalous findings in Spacelab I. Because of non-linearity in the relationship between neural discharge evoked by caloric irrigation and nystagmus magnitude, this component may be significantly less in the conventional caloric position. For this and other reasons there would seem to be no good reason to re-evaluate the clinical reliability of the caloric test.

Key Words:

• caloric response
• neutral angle
• sensory epithelium
• Spacelab I
• theoretical considerations
• thermal reaction