Dynamic Properties of the Intra-Aural Reflex in Lesions of the Lower Auditory Pathway:An Experimental Study in Rabbits

The dynamic properties of the acoustic middle ear muscle reflex were studied in rabbits with chronic experimental lesions of the inner ear, the eighth nerve, the cochlear nucleus and the trapezoid body. The time course of the reflex in response to 2.0 kHz pure tones (duration 1.0 sec, rise time 2 msec) was quantified as the time (T1) from the start of sound stimulus to the point where the response amplitude increased to 50% of its individual steady state value. The T1 was compared in the same ear before and after lesion at equal sound pressure level, equal level above reflex threshold and at equal reflex response amplitude. In general, rise time was prolonged by all types of lesions for responses at equal sound pressure level. A lesion to the inner ear (noise) was followed by a prolongation of T1 near threshold. At 10, 15 and 20 dB above reflex threshold no significant differences were seen. The change of time course was as a rule well explained by the change in response amplitude, but at high amplitude there was a tendency to shortening of rise time. Lesions to the eighth nerve and the cochlear nucleus produced a prolongation of rise time (at equal dB SPL). At equal level.above reflex threshold and at the same steady-state amplitude T1 was shorter after lesion than before. Animals with lesions to the trapezoid body showed a prolongation of T1 of the crossed reflex, independent of response amplitude. It was concluded that all lesions of the reflex pathway causing a change of response amplitude also caused a prolongation of rise time. Only in lesions to the trapezoid body was there an effect on time course (a prolongation) exceeding that expected from the pure decline in the response amplitude. The good correlation seen in the majority of lesions between rise time (T1) and response amplitude implies that T1 being independent of ear canal volume and static middle ear impedance, is a suitable parameter for quantification of the intra-aural reflex at suprathreshold level.