# Development of a New Eye Movement Measurement Device Using Eye-Tracking Analysis Technology
## 4 Discussion  
We developed an all-in-one device based on eye tracking to evaluate the accuracy of eye movements to a moving target and tested its feasibility by measuring eye movements at various visual target speeds, including those faster than human pursuit eye movements. Because the device consists of eye-tracking systems to record data of both eyes and an internal monitor to display the visual target, it allows us to easily record eye movement responses of both eyes simultaneously. We examined whether the data measured using this device reflect eye movement characteristics. When the lag was zero, the rho max between both eye positions was maximum (0.995 \(\pm\) 0.008 and 0.967 \(\pm\) 0.062 for horizontal and vertical eye movements, respectively; Figure 4). From these results, it was clarified that this device can accurately record fronto-parallel eye movements and that simultaneous measurement of both eyes is possible. Rho max significantly decreased for target movements faster than 40 deg/s (Figure 5). Since pursuit was possible with a gain close to 1.00 at target speeds up to 30 deg/s [19], our data may reflect the characteristics of smooth pursuit and saccades in humans. In addition, the frequency of saccades increased as the visual target speed increased, suggesting increased compensation by saccades in addition to pursuit as target speed increased. At all visual target speeds, saccades occurred most frequently at 5.5 to 9.5 frames (0.17 to 0.29 s) after the target moves (Figure 7). This result was considered reasonable, as the saccade latency was approximately 0.20 s [20]. The correlation between the visual target and gaze was significantly lower for vertical target movements than for horizontal target movements. Vertical eye movements have been reported to have a lower gain and higher acceleration compared to horizontal eye movements [21]. In our study, even at a target speed of 100 deg/s, the rho max of the target and eye position was as high as 0.85 for both horizontal and vertical gaze movements. [PERSON] [22] reported that the maximum velocity gain of smooth pursuit was, on average, 0.98-0.75, gradually diminishing with increasing target velocities of 10-60 deg/s. [PERSON] et al. [18] reported that the gain for targets with predictable movements was as high as approximately 90%, even at 100 deg/s for 4/5 participants. The results of this study are in good agreement with those obtained using conventional methods. In our study, when examining the correlation between the frequency of saccades and rho max, we identified a negative correlation when the fixation target speed was 20 deg/s and a positive correlation when the target speed was 100 deg/s (Figure 8a,b). This indicated  
Figure 8: Characterization of participants by saccade frequency. **(a,b)** The average number of saccades per cycle was calculated from the data shown in Figure 7. Rho max against the frequency of saccades at **(a)** the lowest visual target speed (20 deg/s) and **(b)** highest visual target speed (100 deg/s). **(c)** Correlation between frequency of saccades at the lowest and highest visual target speeds. In **(a–c)**, each data point represents a participant. that participants who could follow the target with pursuit when the target speed was slow had a high rho max, whereas participants who could use saccades frequently when the target speed was fast had a high rho max. Notably, in participants who used fewer saccades at low speeds, a negative correlation was observed with more saccades at high speeds (Figure 8c). These findings demonstrated the applicability of the EyeScore for characterizing eye movements. In this regard, the two following types of visual behavior may exist: (1) saccades for a slowly moving target (lower-right corner in Figure 8c) and (2) saccades for faster-moving targets (upper-left corner in Figure 8c). Of note, eye movement accuracies were higher for participants who made more saccades for faster-moving targets.