Independent bilateral-eye stimulation for gaze pattern recognition based on steady-state pupil light reflex

Abstract— Objective: Recently, pupil oscillation synchronized with a steady visual stimulus was employed for an input of an interface. The system is inspired by steady-state visual evoked potential (SSVEP) BCIs, but it eliminates the need for contact with the participant because it does not need electrodes to measure electroencephalography. However, the stimulation frequency is restricted to being below 2.5 Hz because of the mechanics of pupillary vibration and information transfer rate (ITR) is lower than SSVEP BCIs. Here, we propose a signal mixture method for stimulation with different frequency patterns to increase ITR of the interface based on pupil oscillation. Methods: Pupil responses were compered between (1) the left and right eyes were stimulated with different frequencies and mixed in the participant's nerve, and (2) signals with different frequencies were mixed in advance and given to both eyes as a common stimulus. Conclusion: In the case of stimulation (2), the beat frequency, which is the difference between input frequencies, contaminated the pupil response signal. In the case of (1) independent stimulation, the power of the beat frequency was lower and thus did not affect the response signal as much. From these results, we develop a target recognition system using 15 targets consists by 3 frequency components by using independent stimulation method. Mean of classification accuracy was 78.3% and ITR was 14.0 bits/min. Significance: This method is functionally compatible with BCI using SSVEP and requires no physical contact or calibration paradigm. Therefore, this study contributes to providing a useful computer interface for patients with motor impairments.