Time course of acoustic intra-aural muscle reflex in non-anesthetized rabbits, normative data

The dynamic properties of the intra-aural reflex in response to tones of 2 000 Hz in non-anesthetized rabbits is described. Rise time (time from start of stimulus to response has reached half of its steady-state amplitude) decreased from about 300 msec at threshold to 54 msec at high level for the ipsilateral and 60 msec for the contralateral reflex. Offset time (time from end of stimulus to relaxation to 50% of steady-state amplitude) increased from 100-150 msec at threshold to 250-300 msec at high level, being longer for the crossed than for the ipsilateral reflex. Impulse response (response to 16 msec burst of pure tone) reach its peak after about 60 msec, a value independent of sound level. The ipsilateral reflex had a 5-8 msec shorter time lag than the contralateral reflex, after correction for difference in response amplitude. This difference is compatible with additional synapses and a longer neural pathway for the crossed reflex. It has been pointed out that the intensity dependence of rise time has to be carefully considered when evaluating 'normal ranges' of temporal parameters for reflex responses. The rise time is suggested as a suitable parameter for quantification of suprathreshold stapedius responses.