Auditory fusion: a critical interval test with implications in differential diagnosis.

Pulses of white noise each of uniform duration but adjustable in interpulse spacing were presented to subjects at a comfortable loudness level. Reduction of pulse spacing below a critical interval resulted in a subjective impression of continuous noise. Eighty-four per cent of patients with sensori-neural hearing loss had a longer critical interval than normal subjects. Tentative explanations for this phenomenon are put forward in terms of desynchronisation or impaired interaction between inner and outer hair cells. Longer critical intervals found predominately in neural cases are explicable in terms of a reduction in neural conduction velocities. Interesting similarities in respect of end organ and nerve fibre lesions are apparent between critical intervals and reported electrocochleographic responses.