Vergence increases the gain of the human angular vestibulo-ocular reflex during peripheral hyposensitivity elicited by cold thermal irrigation

BACKGROUND: When viewing a far target, the gain of the horizontal vestibulo-ocular reflex (VOR) is around 1.0, but when viewing a near target there is an increased response. It has been shown that while this convergence-mediated modulation is unaffected by canal plugging and clinically practical transmastoid galvanic stimulation, it is eliminated by a partial peripheral gentamicin lesion.

OBJECTIVE: The aim of this study was to determine if convergence increases the gain during peripheral hyposensitivity elicited by cold thermal irrigation.

METHODS: The high frequency VOR gain was measured using video head impulse testing immediately after the cold caloric stimulus in 9 healthy human subjects with the lateral semicircular canals oriented approximately earth-vertical.

RESULTS: Before caloric irrigation, near viewing (15 cm) increased the average VOR gain by 28% (from 1 to 1.28). Cold (24°C) water irrigation of the right ear decreased the gain to 0.66 (far viewing) and 0.82 (near viewing) (22% difference). Although vergence also increased the gain for impulses to the left to the same degree before caloric stimulus, the caloric irrigation itself (applied to the right ear) did not influence the gain for contralateral impulses.

CONCLUSION: In our experiments vergence increased the gain of the human angular VOR during peripheral hyposensitivity elicited by cold thermal irrigation. These results suggest that cold irrigation does not abolish the function of the nonlinear/phasic vestibular afferent pathway.