1991 年 50 巻 2 号 p. 221-226
The possibility of detecting the unilateral recruitment phenomenon in the vestibular system by the rotation test was examined in an analogy model of cochlear recruitment, in which a response from the affected side is characterized by a raised threshold and the intensity of the response above the thresdold asymptotic to that from the normal side.
According to the analogy model, the difference of intensity between leftward and rightward rotational nystagmus reaches its maximum at the threshold stimulus intensity of the affected side and keeps that maximum or decreases above the threshold, so that only in patients with mild hypofunction in comparison with the stimulus intensity range can the difference decrease monotonously (“converge”) with the stimulus intensity, as previouly reported. On the other hand, directional preponderence keeps constant below the threshold and decreases above the threshold.
Moreover, assuming that the ratio of response to ampullopetal to amullofugal stimulation is 2 : 1, the predicted maximum difference of the nystagmus intensity remains within the normal range of variation when the contribution from spontaneous nystagmus is removed. These results indicate that the unilateral vestibular recruitment phenomenon, when it is similar to cochlear recruitment, is difficult to detect by the rotation test. Directional preponderence usaally comes from unilateral hyperfunction in the acute stage or the compensation process of unilateral hypofunction. These factors are not directly related to vestibular recruitment.