Short-term vestibular responses to repeated rotations

Abstract

To investigate the short-term vestibular habituation, we performed the 4 successive velocity step tests on 28 volunteers, the peak velocity of which was 100 deg/sec with acceleration and deceleration of 100 deg/sec2. As the repeated rotations might alter the vestibulo-ocular reflex (VOR), sinusoidal rotations at a frequency of 0.16 Hz were also given before and after the 4 successive velocity step tests to investigate the changes of gain and phase of VOR. The 28 volunteers were divided into two groups, group I and II, according to their responses to repeated rotations. In group I (25 subjects, mean age 23.7 year), the mean values of the slow cumulative eye position (SCEP), the time integral of eye velocity during nystagmus, was reduced after each trial of the 4-successive velocity step tests as follows; 403.4 +/- 29 degree (1st trial), 346.2 +/- 37 degree (2nd trial), 278.3 +/- 33 degree (3rd trial) and 256.6 +/- 36 degree (4th trial). The time constant of the nystagmus was also reduced as follows; 12.9 +/- 0.78 second (1st trial), 12 +/- 0.63 second (2nd trial), 9.7 +/- 0.78 second (3rd trial) and 9.9 +/- 0.54 second (4th trial). In group II (3 subjects, mean age 28.3 year), the mean values of SCEP gradually increased; 774.3 +/- 135 degree (1st trial), 1127 +/- 178 degree (2nd trial), 1096.3 +/- 123.4 degree (3rd trial) and 1225.7 +/- 199.7 degree (4th trial). The time constant of the nystagmus increased; 15.7 +/- 2.7 second (1st trial), 22 +/- 4.5 second (2nd trial), 22.3 +/- 3.3 second (3rd trial) and 23.3 +/- 5.7 second (4th trial). The gain of vestibulo-ocular reflex (VOR) induced by sinusoidal rotations at a frequency 0.16 Hz increased in both groups: 0.59 +/- 0.03 to 0.78 +/- 0.06 (group I) and 0.65 +/- 0.07 to 1.15 +/- 0.06 (group II). Phase changes were also observed. In group I, the phase was shifted from 0.6 +/- 0.6 degree to -0.40 +/- 0.6 degree. In group II, the phase was shifted from 3.67 +/- 1.86 degree to -0.33 +/- 0.33 degree. The repeated rotation did not induce a common nystagmic response in all subjects. Thus, person to person variations should be considered in short term vestibular habituation.