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 $100\text{deg}\mathrm{/}{\text{sec}}^2$ . 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; $403.4\pm 29$ degree (1st trial), $346.2\pm 37$ degree (2nd trial), $278.3\pm 33$ degree (3rd trial) and $256.6\pm 36$ degree (4th trial). The time constant of the nystagmus was also reduced as follows; $12.9\pm 0.78$ second (1st trial), $12\pm 0.63$ second (2nd trial), $9.7\pm 0.78$ second (3rd trial) and $9.9\pm 0.54$ second (4th trial). In group II (3 subjects, mean age 28.3 year), the mean values of SCEP gradually increased; $774.3\pm 135$ degree (1st trial), $1127\pm 178$ degree (2nd trial), $1096.3\pm 123.4$ degree (3rd trial) and $1225.7\pm 199.7$ degree (4th trial). The time constant of the nystagmus increased; $15.7\pm 2.7$ second (1st trial), $22\pm 4.5$ second (2nd trial), $22.3\pm 3.3$ second (3rd trial) and $23.3\pm 5.7$ second (4th trial). The gain of vestibulo-ocular reflex (VOR) induced by sinusoidal rotations at a frequency 0.16 Hz increased in both groups: $0.59\pm 0.03$ to $0.78\pm 0.06$ (group I) and $0.65\pm 0.07$ to $1.15\pm 0.06$ (group II). Phase changes were also observed. In group I, the phase was shifted from $0.6\pm 0.6$ degree to $-0.40\pm 0.6$ degree. In group II, the phase was shifted from $3.67\pm 1.86$ degree to $-0.33\pm 0.33$ 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.