Short latency disconjugate vestibulo-ocular responses to transient stimuli in the audio frequency range.

OBJECTIVE To determine whether unilateral activation of the vestibular labyrinth by brief air conducted tones can elicit disconjugate reflex eye movements in healthy subjects. METHODS 40 normal volunteers, one patient with bilateral congenital sensorineural deafness, and four patients with an acoustic neuromas were subjected to monoaural air conducted tones (125 to 6000 Hz; 132 dB SPL; 3-4 ms). Eye movements were recorded by averaged EOG. RESULTS The stimuli elicited bi- or triphasic transient EOG responses with a duration of about 10 ms and a 7-8 ms latency in 16 of 40 tested volunteers and in the patient with congenital deafness. In patients with acoustic neuromas the responses were induced only by stimuli to the healthy ear. The responses in the vertical EOG were recorded predominantly from the eye contralateral to the stimulated ear and were negligible ipsilaterally. These responses were similar to those found in patients with only one functioning labyrinth reported in a previous study. In the remaining subjects the responses were absent or barely discernible. CONCLUSIONS (1) Disconjugate eye movements in response to brief mechanical stimuli in this audio frequency range always indicated the side of the activated vestibular labyrinth. (2) In contrast to longer stimuli used by conventional vestibular activation methods, brief stimuli may activate only the direct monocular vestibulo-ocular pathway. This may be because the responses elicited by brief stimuli cease well before the slower indirect vestibulo-ocular subsystem can align the eyes.