Parkinson's disease (PD) is associated with a number of oculomotor deficits; however, little is known about changes in vertical optokinetic nystagmus (OKN) associated with PD. We recorded eye movements in 14 PD patients and 14 age-matched controls in response to large field OKN stimulation using stimulus velocities of 20 degrees /second and 40 degrees /second. We compared asymmetry of horizontal and vertical responses in the two groups. We found vertical OKN to be strongly asymmetric in PD with reduced gains for downward-moving stimuli. This asymmetry was significantly greater than that recorded in control volunteers. We postulate that this could result from an abnormal pursuit/early OKN system in PD leading to greater influence of the delayed OKN system.
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