Plasticity of the human vestibulo-ocular reflex during off-vertical axis rotation

Abstract

We investigated whether adaptive plasticity of the vestibulo-ocular reflex in humans occurs in response to visual-vestibular conflict stimulation during rotation about a 30° incline (off-vertical earth axis rotation, OVAR). Subjects were 26 healthy adults (17 males and 9 females), ranging in age from 22 to 33 years (mean: 24.4) with no history of neurotological symptoms. Each testing session consisted of a pre-test, an adaptation period, and a post-test. The pre-test and the post-test were performed in complete darkness with the subjects' eyes opened. Subjects were rotated sinusoidally at 0.16Hz under OVAR, with a maximum angular velocity of 60°/s for 30s. Subjects were divided into two groups depending on the kind of visual stimulation. One group of subjects was rotated sinusoidally at 0.16Hz and 60°/s peak velocity under OVAR for 20min while viewing optokinetic stripes, which moved at the same frequency and peak velocity as the rotational chair but in the opposite direction (X2 adaptation paradigm). The other group of subjects was rotated sinusoidally at 0.16Hz and 60°/s peak velocity under OVAR for 20min while viewing optokinetic stripes, which moved at the same frequency and peak velocity as the rotatory chair but in the same direction (X0 adaptation paradigm). There was no significant difference in gain before or after adaptation using the X2 adaptation paradigm. VOR gain decreased significantly after adaptation using the X0 adaptation paradigm. We hypothesize that attenuation of VOR gain increase after the X2 adaptation paradigm is caused by tilt suppression. In the X0 adaptation paradigm, the decrease in VOR gain was facilitated by tilt suppression in addition to the plastic change of the VOR gain caused by visual-vestibular conflict stimulation. Consequently, the VOR gain change ratio in the X0 adaptation paradigm increased significantly compared to that in the X2 adaptation paradigm.