Abstract
This study aimed to determine whether speed perception in peripheral vision is modulated by subjective confidence and whether pupil size can serve as an objective indicator of confidence. Previous studies have reported that perceived speed decreases with increasing eccentricity. Based on the principles of a Bayesian model, we employed a two-alternative forced-choice task. Observers judged whether a test stimulus was faster or slower than an 8 deg/s reference stimulus at 5 deg eccentricity, then rated their confidence on a 1-4 scale. The test stimulus appeared at either 5 or 15 deg eccentricity, moving at speeds ranging from 5 to 11 deg/s. As a result, mean confidence was significantly lower at 15 deg eccentricity than at 5 deg, suggesting that blurrier visual input at greater eccentricities reduces perceptual certainty. Furthermore, hierarchical Bayesian modeling showed that lower confidence was associated with slower perceived motion and greater perceptual uncertainty. Higher confidence was also accompanied by greater phasic pupil dilation, indicating that pupil responses reflect subjective confidence. These findings highlight the interactive contribution of sensory and cognitive factors to visual motion perception and provide insight into neural mechanisms linking confidence, visual eccentricity, and speed estimation.