The OK stimulus was rear projected onto a semitranslucent tangent screen at a viewing distance of 1 m in an otherwise dark room. The stimulus subtended 72° horizontally and 60° vertically. Although this stimulus did not fill the visual field and did not produce circular vection (the illusion of self-rotation), it induced robust OKN in all our subjects. The OK stimuli were generated by a visual stimulus generator (VSG2/5; Cambridge Research Systems, Cambridge, UK) and projected by a video projector (Powerlite 9100i; Epson Seiko Corp., Nagano, Japan). The stimulus consisted of alternating black-and-white stripes, with luminance of 0.7 and 13.7 cd/m2, respectively, that moved upward or downward at one of three spatial frequencies: 0.04, 0.08, or 0.16 cyc/deg. The direction and spatial frequency of the stimulus were randomized. Each sequence was viewed with both eyes open and the stimulus moved at 10, 20, 30, 40, and 50 deg/s for periods of 20 seconds each. The screen was blank for 10 seconds between each sequence. Subjects were instructed to keep gazing into the center of the pattern, to try to maintain optimal clarity of the stripes, and not to follow any one stripe deliberately.
The stimulus for eliciting vertical saccades consisted of 13 black dots displayed on the screen. One dot was displayed in the center, one 1° above and below the center and thereafter one every 2° above and below, up to 10° eccentricity. To elicit voluntary vertical saccades, subjects were instructed to perform two patterns of refixations. First, they were asked to move their point of visual fixation from the center dot to the dot located at 1° in the upper field, back to the center dot, to the 2° dot, back to the center dot, and to successive eccentric dots until the 10° dot had been reached. In this pattern, up and down saccades were made entirely within the upper visual field. Second, refixations were made between the center dot and the dots in the lower field. Horizontal saccades were similarly elicited using the same stimulus as for vertical saccades, rotated through 90°.