Modification of parameters in vertical optokinetic nystagmus after repeated vertical optokinetic stimulation in patients with vestibular lesions.

Eye movements were recorded in patients with unilateral and bilateral vestibular lesions after upward and downward optokinetic (OK) stimulation before and following 6 weeks' repeated exposure to OK stimulation. In control subjects there was no asymmetry between upward and downward slow-phase velocity (SPV). Before training, less subjects showed that upward and downward SPV was significantly lower than that of controls. There was no asymmetry between upward and downward SPV. After training, in unilateral cases, the values of both upward and downward SPV recovered to the control range. In bilateral cases, the downward SPV values returned to the control range, whereas the values of upward SPV exceeded the control range. The frequencies of both upward and downward OKN in controls were about 3.0 Hz. In unilateral and bilateral cases, before and after training, the OKN frequencies approximated 3.0 Hz, showing no significant differences. The recovery of the SPV in unilateral and bilateral cases after training suggests that OK stimulation acts to stabilize the body and consequently to provoke pronounced OKN, due to eye-head-body co-ordination. The asymmetry of SPV after training in bilateral cases might be a result of the lack of otolith function.