# Development of a New Eye Movement Measurement Device Using Eye-Tracking Analysis Technology
## 1 Introduction  
Stable fixation on a visual target and stabilization of retinal images are crucial for achieving precise binocular vision. The stabilization of retinal images is supported by several types of eye movements, including reflex eye movements, such as vestibular-ocular reflex and optokinetic nystagmus, smooth pursuit eye movements (pursuit), and voluntary eye movements, such as saccadic eye movements (saccades) andvergence eye movements [1]. Each eye movement is controlled by several brain regions [2]. Eye movement measurements are used to assess brain and extraocular muscle damage. Moreover, saccades and pursuits in patients with ambylopia [3, 4] and low vision [5] differ from those in healthy individuals. A recent study reported that patients with macular degeneration exhibited misdirected saccades and misalignment with the target trajectory during pursuit [6]. Eye movements are typically only measured in patients with brain injury or paralysis of the extraocular muscles in routine ophthalmological practice. Several methods for measuring eye movements exist, such as the search coil [7] and electro-oculography (EOG) [8]. The search coil method is accurate but is highly invasive, as it uses contact-lens electrodes. EOG is performed by attaching electrodes around the eyes. However, body movements, fluctuating test room illumination, and electrode misalignment may cause artifacts that lead to inaccurate measurement results [9; 10]. Common disadvantages of both methods include the need for a polygraph system, electrodes, amplifier, and personal computer for data analysis [11]. Given the lack of all-in-one equipment, these methods require complex and large-scale measurement equipment. Therefore, eye movements are not routinely measured in ophthalmology. Recently, video-based eye trackers have emerged, which enable the determination of gaze direction with a high degree of accuracy. This approach measures the position of the corneal reflex of an infrared light relative to the pupil, which can be observed in both table and head-mounted configurations and allows eye-tracking in real time, thus enabling a much wider range of experimentation than was previously possible [12]. Reports suggest that measurement accuracy is similar between mobile and stationary video-based eye trackers, and eye trackers have the advantage of high measurement accuracy regardless of the model [13]. By measuring eye movements using an eye-tracking system, one study demonstrated the feasibility of measuring saccade reaction times and saccades in 115 children between 5 and 42 months of age [14], which was previously intractable using conventional methods. These developments indicate that eye movement measurements using eye trackers are accurate and simple. However, in current measurements using eye trackers, the stimulus-presenting monitor and recording device are separate devices, and the measurement conditions are not standardized. Therefore, it is difficult to measure eye movements using eye-tracking in ophthalmology clinics, which are limited to laboratory measurements. We have developed a new all-in-one eye movement measuring device by modifying an eye-tracking-based ophthalmic examination device (ORTe EYENAC; NAC Image Technology Inc., Tokyo, Japan) [15]. This device simultaneously presents visual stimuli internally and records bilateral eye positions. Our device (referred to as \"EyeScore\") (Figure 1) comprises a visual simulation generator, stimulus-presenting monitor, and two eye-tracking systems (for both eyes) within a single body. In addition, the device is portable and standalone, similar to many ophthalmic examination devices, which enables the same experiment to be performed in different facilities without any additional apparatus (e.g., personal computers). Experiments using original stimuli tend to be conducted at a single facility and are prone to bias [16]; however, this device facilitates the reproducibility of experiments.