# Parameters of Optokinetic Nystagmus Are Influenced by the Nature of a Visual Stimulus
## 2 Materials and Methods
### Visual Stimulus and Eye Tracking  
For the OKN stimulation, we used a vertically oriented sinusoidal grating drifting over the horizontal plane with a constant velocity of \(v=21\,^{\circ}/\mathrm{s}\), which was comparable to a previous work focused on contrast sensitivity using OKN [30]. Because no clear effect of the gain of OKN has been found between the two horizontal directions [31], the grating drifted either nasally or temporally in a random order, in an equal number of trials. The stimuli were created in MATLAB (MATLAB2018b; MathWorks, Natick, MA, USA) using the Psychtoolbox-3 extension [32,33] and covered the entire Viewpix screen (VIEWxx; VPixx Technologies Inc., Saint Bruno, Quebec, Canada), refreshing at a rate of 120 Hz. Because the screen provided a resolution of 1920 \(\times\) 1200 pixels with a pixel pitch of 0.252 mm, the covered visual field from the viewing distance of 75 cm was \(\approx 36^{\circ}\) and \(\approx 23^{\circ}\) in the horizontal and vertical planes, respectively. Furthermore, the screen provided a bit depth of 12 bits, and the luminance nonlinearity was corrected via gamma correction. The spatial frequencies (SFs) calculated for the observing distance were \(SF=0.12,0.25,0.5,1.0\) cycles per degree (cpd). These were selected for their relevance, considering the velocity of the stimulus [20]. The contrast of the stimulus for each trial was randomly selected from the 6 available levels (\(0.5\%,2.0\%,8.2\%,16.5\%,33.0\%,55.5\%\)), while every contrast level was always presented ten times in every measurement defined by the \(SF\) and the grating type (luminance, R-G, B-Y). These contrast levels were selected upon consideration that this range was relevant in contrast sensitivity testing. The motion of the contrast stimulus was aborted at time \(t=2\,\mathrm{s}\) after stimulus onset. After every presentation of the stimulus, a gray cross of \(1.25^{\circ}\) in size appeared in the center of the screen for \(t=4\,\mathrm{s}\), while during its presentation the participants were asked to blink and rest their vision. During the presentation of the stimulus, the participants were instructed to fixate on the center of the stimulus to stimulate the stare OKN, as performed in previous studies examining CS with OKN [13,14]. The workflow of the stimulus with all its phases is shown in the Figure 1.  
Because the current study targeted testing of the effects in normal and chromatic visual stimuli, three types of sinusoidal gratings were used. The first grating was a commonly used sinusoidal luminance grating for standard contrast sensitivity measurements. Secondly, we used two chromatic gratings (R-G and B-Y), following previous studies [5; 27; 28; 30; 34], as shown on the Figure 2. Here, the chromatic gratings of the R-G or B-Y modulation were created as the sum of red and green or blue and yellow luminance-modulated monochromatic gratings with a phase difference of 180\({}^{\circ}\) between them. The nature of the three gratings was such that the luminance grating converged towards a gray value of the background (middle gray value), whereas the chromatic gratings converged towards black, with decreasing contrast as depicted in the Figure 2. We used the convergence to the middle grey value in the luminance target as we aimed to use an iso-luminant stimulus. In the chromatic stimuli, this approach was not applicable, as even the low-contrast stimuli would have elicited the eye movement by the respective color difference. For this reason, we let the gratings converge towards black. The color gratings were created with the use of a predefined color lookup table computed for the gamma correction, similarly as done by [PERSON] et al. [35] for cone-specific stimuli. Furthermore, the standard luminosity function \(v(\lambda)\) functioned as the baseline for the monitor calibration. The eye-tracking was performed in a head-fixed condition, using the EyeLink 1000 Plus eye tracker (SR Research, Kanata, ON, Canada) with a fixed sampling rate of 1000 Hz.