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3From the Department of Neurology, Department of Veterans Affairs Medical Center and University Hospitals, Case Western Reserve University, Cleveland, Ohio; the 1Department of Ophthalmology, Great Ormond Street Hospital for Children National Health Service Trust, and 2Department of Visual Science, Institute of Child Health, London, United Kingdom; and the 4Institute of Neuroscience, Faculty of Neuroscience, Plymouth University, Plymouth, United Kingdom.
PURPOSE. Optokinetic stimulation induces nystagmus that can be used to test the saccadic and visual-tracking systems in some patients with voluntary gaze palsies. The purpose of this study was to characterize vertical optokinetic nystagmus (OKN) in normal human subjects, comparing the dynamic properties of the quick phases with voluntary saccades of similar size and measuring the slow-phase responses to visual stimuli with a range of spatial and temporal frequencies.
METHODS. Vertical OKN and saccades were recorded in 10 healthy adult subjects (age range, 24–54 years) using the magnetic search coil technique. The optokinetic (OK) stimulus subtended 72° horizontally and 60° vertically, consisted of black-and-white stripes with a spatial frequency of 0.04, 0.08, or 0.16 cyc/deg, and moved vertically at 10 to 50 deg/s. Vertical and horizontal saccades to visual targets separated by 1° to 10° were also elicited.
RESULTS. Over 95% of quick phases were less than 10° in amplitude; voluntary saccades of this amplitude range were slightly faster than quick phases of similar size. The amplitude–peak velocity relationships and amplitude–duration relationships of upward and downward fast movements (saccades or quick phases) were similar. Most vertical slow-phase OK responses showed greater gain for upward stimulus motion. OK gain decreased with increasing stimulus speed and increased spatial frequency, so that there was a general decrease in slow-phase velocity gain with increasing temporal frequency.
CONCLUSIONS. In this study, the best OK responses were obtained using stripes with lower spatial frequencies and lower stripe speeds (0.4 cyc/deg at 10 deg/s). The dynamic properties of vertical quick phases of nystagmus are similar enough to those of voluntary saccades for OK stimulation to be used as a clinical test of the vertical saccadic system in individuals with voluntary gaze palsy.
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