Vestibular responses to different caloric stimulus intensities.

Data are presented concerning stimulus response relationships to weak (44 degrees C) and strong (0 degrees C) caloric stimuli. In clinically hypoactive ears the response to the strong stimulus is not equivalent to the response to the weak stimulus for evaluating hypoactivity when using the usual percentage formula. The ratio of responses to strong and weak stimuli in the hypoactive ear is greater than in the better ear. The absolute differences in mean eye speed response to strong and weak stimuli are the same for hypoactive and better ears as for hypoactive and probably normal groups. A hypothetical schema is described to explain these relationships involving alteration of tonic balance between the two ears in the presence of central compensation. The effect of directional preponderance on stimulus response relationships is described. It is suggested that differences in absolute mean eye speed values in group analysis are the true measure of gain due to the stronger stimulus and are more valid for determining stimulus response relationships between stimuli of differing intensities than the ratio of these means. Using this criterion, no evidence of recruitment was found in the ears described. The concept of vestibular recruitment and decruitment in terms of caloric test characteristics is discussed and the appropriateness of these terms is questioned.