Caloric testing with small temperature gradients. Caloric zero.

Caloric nystagmus was investigated in 50 normal subjects with stimulation of the temperatures 30, 33, 35, 39, 41, and 44 degrees C. Mean values of the duration and the eye speed of the slow phase of adjacent temperatures were significantly different. No difference could be demonstrated between the equidistant temperatures 30/44 and 35/41 degrees C, but the response was significantly stronger with 35 than with 39 degrees C. This indicates that the caloric zero, i.e., the theoretical neutral temperature, is higher than 37 degrees C. The caloric zero was determined by extrapolation of the regression lines of the mean values for the six temperatures investigated. Determination by means of maximum eye speed of the slow phase values showed to be more accurate than with the duration, probably due to smaller interindividual variations of the responses.