Blink parameters are confounded by vertical eye orientation in video-based eye tracking: Comparing pupil- and eyelid-based methods.

Blink characteristics such as duration, amplitude, and eyelid velocity are widely used indicators of cognitive and physiological states. While early magnetic search coil studies suggested that vertical eye orientation relative to the head influences blink measurements, subsequent research has largely ignored this factor. No studies have investigated whether vertical eye orientation effects replicate in modern video-based methods or whether different video-based blink-detection approaches show similar sensitivities to changes in vertical eye orientation. In this study, we investigated how vertical eye orientation affects blink parameters estimated using both pupil-based and eyelid-based detection. We recorded pupil diameter and estimated eye openness from video data as seventeen participants performed voluntary blinks from three vertical eye orientations while keeping their heads stationary. Vertical eye orientation systematically influenced all measured blink parameters. Eye openness at blink onset and closing amplitude decreased with downward eye orientation. With more downward eye orientation, closing velocity increased, whereas opening velocity decreased. Crucially, pupil-based measurements of blink duration showed much larger vertical eye orientation effects than measurements of eye openness (32% vs. 8% increase of blink duration from upward to downward eye orientation), though eyelid-based estimates are sensitive to how blink onset and offset are derived. These results show that the vertical eye orientation is a systematic confounding factor in video-based blink measurement, with the measurement method influencing the magnitude of observed effects. The findings have important implications for studies investigating blink characteristics where vertical eye orientation varies, and we conclude with practical recommendations for study design and reporting.