Figure 8.

How eyelid obstruction of the pupil during the head impulse generates artifactual eye movement records. (A,B) show the results of an eyelid flick touching the top of the pupil: biphasic for the anterior response (A) and uniphasic for the posterior response (B). The lower panels show stills from a grossly exaggerated version of how this occurs during the anterior impulse: the pupil starts in the centre of the vertical range (C), then is moving upwards as the eyelid “flicks” down to touch the top of the pupil (D,E), producing an apparent deceleration of the motion. As the eyelid then moves back up (F), the center of mass appears to accelerate upward. (G–I) A set of posterior canal impulses (G) in the situation where the pupil slides behind a stationary lower eyelid as it moves downward during the stimulus (H,I).