# Development of a New Eye Movement Measurement Device Using Eye-Tracking Analysis Technology
## 3 Results
### Horizontal Eye Movement Velocity  
As tracking by saccades is critical for fast target speeds, we investigated the timing and frequency of saccades relative to the to-and-fr motion of a visual target. Examples of eye position and eye velocity obtained at each visual target speed are superimposed on one cycle, as shown in the upper part of Figure 6. The velocity waveforms corresponding to the positions of the cycles in the upper part of Figure 6 are shown in the lower part of Figure 6. For low visual target speeds, eye movement velocity was generally less than 40 deg/s, which was within the pursuit range. However, as the visual target speed increased, spike-like waveforms with higher eye velocities were observed more frequently. In this study, we defined saccades as eye movements of 100 deg/s or more [17; 18], and the frequency with which saccades appeared was examined. The results showed that saccades tended to appear most frequently about 10-14 frames (0.30-0.42 s) after the onset of target motion at any visual target speed (Figure 7). The real timing of saccades was 5.5-9.5 frames (0.17-0.29 s) after the onset of target motion due to the sampling delay of eye position measurement (3-5 frames, mean of 4.5) in this device.  
Figure 4: Rho max between both eye positions at each target speed. The cross-correlation coefficient reached a maximum (rho max) when the time lag was zero for both (green) horizontal and (orange) vertical eye movements. The average rho max values across all velocities were 0.995 \(\pm\) 0.008 and 0.967 \(\pm\) 0.062 in the horizontal and vertical directions, respectively (mean \(\pm\) standard deviation).  
The correlation between the frequency of saccades and rho max at each target speed was analyzed. The correlation coefficients for visual target speeds of 20, 40, 60, 80, and 100 deg/s were \(-\)0.75 (\(p\) < 0.001), \(-\)0.42 (\(p\) = 0.102), \(-\)0.24 (\(p\) = 0.367), 0.09 (\(p\) = 0.745), and 0.66 (\(p\) = 0.005), respectively. At the lowest visual target speed (20 deg/s, Figure 8a), rho max was negatively correlated with the frequency of saccades. Conversely, positive correlations were observed at the highest visual target speeds (100 deg/s, Figure 8b). A negative correlation was observed between the frequency of saccades at the lowest and highest visual target speeds (Figure 8c).  
Figure 5: Rho max decreased for faster visual target motions. The upper and lower figures show the horizontal and vertical gazes, respectively. The left and right figures show the results for the left and right eyes, respectively. Error bars represent standard errors.  
Figure 6: Example of eye position and eye velocity. (**Upper**) Horizontal eye movement waveforms measured continuously for 20 s are superimposed on one to-and-fro cycle of the target motion. (**Lower**) Eye velocity at the position corresponding to the upper panel. The black line indicates the (**upper**) position or (**lower**) velocity of the visual target, and the red and blue lines indicate those of the right and left eyes, respectively.  
Figure 7: Frequency of saccades at each visual target speed. The number of saccades within a frame during horizontal eye movements at each target speed. The data are summed across all the participants. The first and second halves of the histograms depict the results when the target moved to the right (green) and left (orange), respectively. The horizontal axis represents the frames of the cycle. Note: The center of the ordinate represents zero (no saccades). The upper and lower scales are for rightward motion (green) and leftward motion (orange), respectively.