Persistence of Covert but Not Overt Corrective Saccades in Darkness: Insights From Unilateral and Bilateral Vestibular Loss

Corrective saccades identified during the video head impulse test (vHIT) are commonly classified as overt or covert according to their timing relative to head movement. Although overt saccades are generally considered visually driven, the mechanisms underlying covert saccades remain incompletely understood. We report two illustrative cases of vestibular hypofunction evaluated with vHIT under standard illuminated conditions and in complete darkness. The first patient had unilateral vestibular loss, whereas the second had bilateral vestibular loss. In both cases, overt corrective saccades observed during testing in light conditions disappeared in darkness, while covert saccades remained detectable. These findings are consistent with the hypothesis that overt and covert corrective saccades may rely, at least in part, on different physiological mechanisms. The abolition of overt saccades following elimination of visual feedback is consistent with a visually mediated origin. In contrast, the persistence of covert saccades suggests participation of non-visual pathways involved in gaze stabilization. The observed latencies were compatible with previously reported latencies of cervical proprioceptive responses, including the cervico-ocular reflex (COR), although a direct causal relationship cannot be established from the present observations. This simple light-versus-darkness vHIT protocol may provide a clinically accessible approach for exploring compensatory gaze stabilization mechanisms in patients with vestibular disorders.