Dynamics of the Acoustic Reflex: Phenomenological Aspects

The acoustic reflex was studied by measuring the variations in the mechanical-input impedance to the ears of six normal human subjects. The auditory system was stimulated by wide-band noise at various intensity levels. The static magnitude and phase characteristics of the response were obtained at several frequencies. It was shown that reflex activity results in an increase in the magnitude and a decrease in the phase lag of the ear's input impedance. The system dynamics were studied with step stimuli. It was shown that at stimulus onset both the latency and speed of the response are related to stimulus magnitude. With higher intensities, shorter latency and faster response results. At the cessation of the stimulus, in general there was no clear functional relationship between latency and intensity. For some subjects, faster reflex relaxation occurred after low-intensity stimulation than after more-intense activity. For other subjects, there was no such clear trend. It was observed that, during prolonged stimulation with very high intensity noise, stable reflex activity resulted; but if the stimulus was weaker, then response adaptation occurred. In the vicinity of reflex threshold, the adaptation was usually quite rapid.