Spontaneous nystagmus modifies the degree of rotatory pendular and caloric vestibular nystagmic responses, provoking directional preponderance towards the same side. Until now, no true correlation has been able to be established between directional preponderance intensity and that of spontaneous nystagmus, correlation between these two parameters depending, in fact, on a third factor; vestibular reflectivity. Spontaneous nystagmus can apparently be defined as a directional preponderance inversely proportional in degree to vestibular reflectivity (the reflectivity of a vestibule being the sum of the hot and cold stimulation response indices). This phenomenon produces a thermal preponderance during the caloric test, and "vestibular recrutment" during the rotatory pendulum test. These findings are the result of 56 measurements conducted in 34 patients with vestibular neuritis. There was also a significant increase in vestibular reflectivity on the healthy side in these patients, suggesting both central and peripheral lesions. Because of the difficulty sometimes encountered when attempting to relaste the different concepts of overall vestibular responses, a new clinical unit of measure is proposed: the number of nystagmic jerks in 30 s. Hinchcliffe has previously employed this unit for the vestibular response, but it is now proposed for more general application to measurement of reflectivity, mean directional preponderance, unilateral directional preponderance, and mean thermal preponderance.