## 1 Introduction  
Athletes, especially those engaged in ball games, must focus on fast-moving visual targets. Eye movements include involuntary actions like the optokinetic nystagmus, vestibular-ocular reflex, and smooth pursuit eye movements, as well as deliberate movements like vergence eye movements and saccadic eye movements (saccades) [1]. Among these, saccades are pivotal in quickly fixing the gaze on moving visual targets.  
Research indicates differences in gaze utilization between non-athletes and athletes during sports play [2, 3]. [PERSON] et al. suggested that skilled athletes, owing to their anticipatory abilities, could execute saccadic eye movements toward the anticipated position of the ball. However, this possibility remains unclear because reproducibility is limited in real gaming conditions. To address this, repeated measurements of eye positions under controlled laboratory conditions may be necessary. Conducting research in the laboratory offers advantages, allowing for the assessment of visual function in athletes through various ophthalmicamologic examinations and ensuring reproducibility--a key factor in cumulative research efforts.  
One method for comparing eye movements between athletes and non-athletes is the dynamic visual acuity (DVA) test [4]. This test measures the ability to discern a horizontally moving visual target, typically represented by a Landolt \"C\" ring. [PERSON] et al. [5] conducted a study comparing the DVA of baseball players and non-athletes. They found no difference in the results when the subjects fixated on the front, but when the subjects tracked the target horizontally, the baseball players performed better than non-athletes. However, DVA measurements involve the detection of the orientation of the Landolt ring, making it challenging to isolate eye movement ability.  
Clinical ophthalmology offers various eye movement measurement methods, including the invasive search coil method [6] and electro-oculography (EOG) [7]. The search coil method, which uses contact lens electrodes, is highly invasive, while EOG may introduce artifacts due to body movements and electrode misalignment, limiting its use in measuring athletes' eye movements [8; 9]. Video-based eye trackers have recently advanced, using a camera with infrared diodes to map reflection patterns on the subjects' corneas, enabling eye movement measurement [10]. Although eye tracking is commonly employed to assess eye movement during sports play by synchronizing with external camera images [3], its predominant use has been during actual games. However, evaluating eye movements in a laboratory setting using an eye-tracking device requires adequate visual stimuli and a monitor for display before measurements. Due to the large size of these equipment, measurements are limited to specific laboratories. Moreover, each facility using its own stimuli makes it challenging to compare findings. An additional drawback is the time-consuming need for thorough calibration before measurements.  
We engineered a comprehensive eye movement measurement device capable of presenting visual stimuli internally while concurrently capturing bilateral eye positions. This was achieved through modifications to an eye-tracking-based ophthalmic examination device (ORTe EYENAC, NAC Image Technology Inc., Tokyo, Japan) [11]. Measurements with this device revealed that some healthy people are good at eye movements while others are not [12].  
In this study, we hypothesized that athletes can perform more accurate saccades compared to non-athletes. To examine this hypothesis, the eye movements of professional basketball players were measured and compared with those of healthy subjects as previously reported [12].