# Parameters of Optokinetic Nystagmus Are Influenced by the Nature of a Visual Stimulus
## 4 Discussion  
The current study aimed to test the effects of contrast, spatial frequency, and grating type on four selected OKN parameters, namely the onset and offset time, amplitude, and gain. Furthermore, the current study aimed to propose limits for optimized contrast sensitivity testing using the OKN responses. In this context, the OKN offset time limit was ratioinated to be used as a maximum grating presentation time in OKN-based contrast sensitivity testing to increase the time efficiency of such measurement [24], while avoiding any live-detection procedure [14]. Second, we aimed to propose limits of the OKN gain based on parameters of the visual stimulus targeting prevention from false negative eye movement detection [14]. In a previous work investigating the relationship of the OKN onset time and contrast [24], the resulting function showed shorter latency with increasing contrast level as well. Here a latency range from approximately 361 to 525 ms for contrast levels ranging from 58% to 0.1% in low-speed stimuli (2.5 deg/s) was found. In the current study, we confirmed this trend for luminance as well as the R-G and B-Y gratings drifting with a higher velocity. In contrast to a previous study [24], we found a statistically significant effect of the spatial frequency. This could be first due to a different velocity and range of spatial frequencies of the grating used in the current study. More importantly, however, we used linear mixed-effects models for the statistical evaluation considering the nature of our data. As the models have been violated by the heteroscedasticity of the variance of residuals, we consider a potential effect on the statistical results, although a previous study allows such statistical testing even with the residuals being heteroscedastic [36]. Similarly to the dependence of the OKN onset time on contrast, this effect can also be observed for the onset (latency) of other types of eye movements as microsaccades [7] or reflexive (reactive) saccades [37,38]. Furthermore, the relative eye velocity to the target velocity in smooth pursuit eye movements was found to be higher in contrast-rich stimuli [39]. The described contrast dependence can be attributed to the higher recognizability of the stimulus due \begin{table}
\begin{tabular}{c c c c c} \hline \hline – & **SF = 0.12 cpd** & **SF = 0.25 cpd** & **SF = 0.5 cpd** & **SF = 1 cpd** \\ \hline C1 ( luminance) & 1077 (1114 \(\pm\) 250) & 1022 (1070 \(\pm\) 267) & 999 (1031 \(\pm\) 234) & 1036 (1066 \(\pm\) 231) \\ C2 (R-G) & 963 (987 \(\pm\) 204) & 892 (926 \(\pm\) 172) & 868 (937 \(\pm\) 200) & 949 (1041 \(\pm\) 265) \\ C3 (B-Y) & 952 (973 \(\pm\) 180) & 894 (925 \(\pm\) 152) & 908 (972 \(\pm\) 200) & 1010 (1080 \(\pm\) 262) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Median (mean \(\pm\) SD) of the OKN offset time values were calculated over all subjects and contrast levels, individually for the three gratings and the four spatial frequencies. All values in the table are provided in ms.  
Figure 8: Proposed limits as a function of contrast for the luminance and chromatic gratings. to the increase in contrast [38]. Moreover, comparing the onset time of OKN stimulated by the two types of our grating, the two chromatic gratings yield a faster OKN response faster compared to the luminance visual stimulus. As the relative-to-fixation sensitivity for chromatic gratings in smooth-pursuit eye movements was previously shown to be increased compared to a luminance grating, ref. [30], the authors assume faster OKN onset due to the enhanced sensitivity in the initial tracking phase (before the first OKN-QP). Similarly, the offset time of OKN, defined as the end time of the third OKN-QP (end of the robust OKN response [14; 25]) was found to be influenced by the contrast of our visual stimuli as well, following a comparable trend function to the OKN onset time.