# Parameters of Optokinetic Nystagmus Are Influenced by the Nature of a Visual Stimulus
## 4 Discussion
Here the spatial frequency and color showed a statistical significance and, as with the OKN onset time, the linear model for statistical analysis contained heteroscedasticity of variance of the residuals. Interestingly, although the trend of the offset time over the tested contrast levels has been found to be similar to the onset time, the visually inspected standard deviation was found to be bigger in the offset time. This effect could be due to the unequal amplitude in every OKN pattern. Furthermore, similarly to the onset time, the offset time of the OKN stimulated by the two chromatic gratings was found to be shorter compared to the luminance visual stimulus. We consider the reason for this effect to be enhanced sensitivity during the tracking phases in stimulation with chromatic stimuli, resulting in higher gain [30]. Furthermore, we found the amplitude of the OKN-SPs and the OKN gain to be influenced by the grating type (chromatic or luminance), both having generally higher values in simulations with the chromatic visual stimuli. In terms of the OKN amplitude, [PERSON] et al. found a trend of saturation in the OKN amplitude for grating beyond a luminance of 2 \(\times\) 10\({}^{-5}\)cd/m\({}^{2}\) in one defined spatial frequency (0.1 cpd) [40] in scotopically simulated OKN. In comparison to this study, we found a similar trend in stimulation with the luminance as well as with the two chromatic gratings, giving the first evidence the eye movement amplitude is influenced by contrast only in the low-contrast range. The gain of the OKN showed an expected trend of increasing its value with increasing contrast of a visual stimulus. However, this effect was already shown in the past, for an animal (zebra-fish) model [26]. In addition, the current study supports the result of [PERSON] et al. [26] of spatial frequency having an impact on the OKN gain, although both studies provide only an initial evidence. In this context, the authors suggest further investigations, as all the selected spatial frequencies in the current study were around the peak of the expected CS function for the selected stimulus velocity [20]. Furthermore, the OKN gain obtained for the stimulation with chromatic gratings showed saturation for contrast levels higher than 2%, while generally in visual inspection the gain was found to be higher for the two chromatic stimuli compared to the luminance grating. We expect this effect to originate in the enhanced sensitivity for chromatic targets, as already addressed [30]. In further connection to the previous research, the OKN gain has not only been found to be a function of contrast or spatial frequency but was also found to be dependent on the size of the simulating area on the retina, indicating a possibility to use the OKN gain in investigations of visual field loss [41]. Equally important to the statistical testing of the effect of various stimulus parameters on selected parameters of the OKN eye movement, this study proposes limits, allowing optimized OKN-based contrast sensitivity testing. As an example of the potential effect of the stimulus duration limitation, one measurement of one \(SF\) consisting of ten presentations of the six contrast levels from this study, each lasting for 2 s, would result in a 120 s long measurement, whereas in utilization of the proposed limits the duration of the test will be reduced by approximately 33%, considering equal testing of high and low contrast levels. However, since the equal amount of presentations of low and high contrast levels is not expected in a clinical testing of a healthy patient, conducting the test rather in a low-contrast range, the method of the adjusted presentation duration upon the stimulus contrast might be even more relevant. The respective limits for the OKN offset time and gain have been derived as percentiles (95 th and 5 th, respectively), as the data did not follow a normal distribution.