# Parameters of Optokinetic Nystagmus Are Influenced by the Nature of a Visual Stimulus
###### Abstract  
Studies on contrast sensitivity (CS) testing using optokinetic nystagmus (OKN) proposed adjusting the stimulus presentation duration based on its contrast, to increase the time efficiency of such measurement. Furthermore, stimulus-specific limits of the least OKN gain might reduce false negatives in OKN detection procedures. Therefore, we aimed to test the effects of various stimulus characteristics on OKN and to propose the stimulus-specific limits for the OKN gain and stimulus presentation duration. We tested the effect of contrast (C), spatial frequency (_SF_), and color on selected parameters of robust OKN response, namely its onset and offset time, amplitude, and gain. The right eyes of fifteen emmetropes were tracked with an infrared eye tracker during monocular observations of sinusoidal gratings moving over the horizontal plane with a velocity of (21 \({}^{\circ}\)/s). The available contrast levels were \(C\): 0.5%, 2.0%, 8.2%, 16.5%, 33.0%, and 55.5% presented in a random order for ten times in all measurements of _SF_: 0.12, 0.25, 0.5, and 1.00 cycles per degree and grating type: luminance, red-green, and blue-yellow. This study showed a significant effect of the stimulus characteristics on the OKN onset, offset and gain. The effect of _SF_ was insignificant in OKN amplitude; however, it indicated significance for the C and grating type. Furthermore, the OKN gain and offset limits were proposed as functions of contrast for the luminance and chromatic gratings. This study concludes the characteristics of a visual stimulus have an effect on the OKN gain and onset and offset time, yet do not affect the eye-movement amplitude considerably. Moreover, the proposed limits are expected to improve the time efficiency and eye-movement detection in OKN-based contrast sensitivity measurements. eye tracking; detection; optokinetic nystagmus; contrast sensitivity; color; adjustments +
Footnote †: journal: Applied Sciences  
saccade-like fashion in the direction against the stimulus. Although the previous research showed the OKN-QP to be in a similar velocity range compared to normal saccades [16], no attentional input was found to be a trigger of the OKN-QP and therefore the statement of OKN-QP to be same as normal saccades was not supported [17]. Nonetheless, the velocity-based algorithms for saccade detection [18] were successfully implemented [14; 19]. Since the OKN occurrence is dependent on contrast and spatial frequency of a target, previous studies proposed this type of eye movement as a reliable tool for CS appraisal [13; 14; 19], although the contrast sensitivity functions (CSFs) were shown to be shifted over the x-axis to the left due to the velocity of a moving target [20]. In OKN-based CS measurements, the CSFs were shown to correlate with a subjective judgment of the direction the visual stimulus moved in [13] or were shown to be tending toward lower values in conditions of defocused vision, especially in measurements of higher spatial frequencies [14]. This effect was considered a successful replication of the clinical measurements of CS under defocus, as shown in some previous works [21; 22; 23]. Moreover, [PERSON] et al. performed a CS appraisal using a VR headset and simulated low-vision conditions [19]. On top of that, the previous research showed a possible implementation of adaptive psychometric procedures in methods of searching for the contrast thresholds for future creation of OKN-based CSFs, making the testing adaptive and time-efficient, but requiring advanced skills in programming [14]. In this context, another way to possibly increase the time efficiency of the OKN-based CS testing might be useful, while avoiding the already established live-detection method. To reach this aim, we targeted the optimization of the stimulus presentation duration based on its parameters: \(SF\), \(C\), and type (luminance/chromatic).