Video-based head movement compensation for novel haploscopic eye-tracking apparatus.

PURPOSE To describe a video-based head-tracking technique to compensate for torsional, horizontal, and vertical in-plane head movements during pupil/iris-tracking video-oculography with a tilting haploscope. METHODS Custom software was developed for image acquisition and off-line analysis for a novel haploscopic viewing device. Head movements were constrained to the frontal plane with the use of a bite plate and a forehead rest. Head movements were monitored by tracking black adhesive dots with a white border placed near the subject's inner canthi. Dot and pupil positions were computed using feature detection based on the Hough transform modified for ellipses. With the inter-dot distance and the relative vertical shifts of the dots during head motion, each video frame was rotated and translated to remove the effects of head movement from eye movement data. This technique was verified with a model head and with healthy subjects, who were asked to strain their heads purposefully against the bite plate during the recording. Head-tracker performance during 45 degrees head tilting was also assessed. RESULTS Validation experiments with the model head indicated a linear relationship, between true and measured positions, with a Pearson's correlation coefficient of R = 1.00. For human subjects, binocular video-oculographic recordings showed essential elimination of head movement artifacts from the recorded eye movements. CONCLUSIONS Tracking black dots placed near the inner canthi is an effective method of compensation for horizontal, vertical, and torsional in-plane head movements during pupil and iris crypt-based video-oculography.