Eye blink reflexes to sudden free falls: a clinical test of otolith function.

Conventional neuro-otological tests measure only semi-circular canal function and not otolith function. A clinically acceptable test of otolith function was developed. Previous studies suggested that muscle responses which occur less than 100 ms after release into free-fall are part of a startle reflex originating in the otoliths. With a couch capable of producing sudden, safe, comfortable free-falls it was shown that in normal subjects orbicularis oculi muscle responses (that is eyeblinks) invariably began within 45 ms of this stimulus and confirmed that these eyeblink reflexes are part of a generalised startle reflex, as they cross-habituate with eyeblink reflexes elicited by supraorbital nerve stimulation. Eyeblink reflexes to free-fall may prove a useful clinical test of otolith function.