Abstract
Abstract: :
Background: There have been very few studies investigating the normal response characteristics of torsional optokinetic nystagmus (tOKN). The magnitude of this response has been found to be considerably reduced compared to horizontal and vertical optokinetic nystagmus. There is also limited literature relating to the effects of central and peripheral stimuli on the response. Purpose: To investigate the effect of stimulus velocity, cycle size and central or peripheral stimulation upon the slow phase velocity (SPV) of the tOKN response. Methods: Torsional OKN was induced by a rotating sinusoidal grating pattern that subtended 50° in diameter. The stimulus was rotated both clockwise and anticlockwise directions in 16 normal subjects. Stimulus velocities ranged from 20°/s to 1000°/s subtending two cycle sizes of either 30 or 90 degrees. An artificial central scotoma of increasing sizes was used to occlude the central portion of the stimulus. Torsional eye movements were recorded using a pupil and iris tracking video oculography technique. Results: The response in either eye was conjugate and symmetrical for both directions. For the smaller cycle size of 30°, the mean torsional SPV increased from 1.9°/s to 3.7°/s for stimulus velocities of 20°/s to 100°/s respectively. For the larger cycle size of 90°, the mean torsional SPV increased from 1.3°/s to 3.2°/s up to stimulus velocities of 200°/s. The gain (eye velocity/stimulus velocity) in both groups decreased as stimulus velocity increased. This decrease was exponential for stimulus velocities ranging from 20°/s to 1000°/s. An increased mean SPV was demonstrated to stimuli subtending a cycle size of 30° compared to those subtending 90°. When an artificial central scotoma of increasing size was applied to the central portion of the stimulus, the subjects actively 'filled-in' the occluded portion of the stimulus and still showed a tOKN response. The filling- in response was much stronger with the smallest size of central occluder. In some cases tOKN was demonstrated even when 80% of the central stimulus was occluded. Conclusion: Torsional OKN demonstrates precise response characteristics to variations in stimulus velocity and cycle size. The tOKN response was also elicited without central stimulation indicating that either the peripheral stimuli or more likely that filling-in has the capacity to generate active eye movements. This may suggest that filling-in is an active process.
Keywords: 407 eye movements: conjugate • 495 ocular motor control