Time course of the human acoustic stapedius reflex. A comparison of eight different measures in normal-hearing subjects

Different measures of the dynamic properties of the contralateral stapedius reflex were compared in normal-hearing subjects. The stapedius reflex was measured as change of the acoustic impedance at 800 Hz. The stimulus was a 1-s burst of pure tone at 500 or 2000 Hz with a rise time of 10 ms; a 25-ms burst of pure tone at 500 Hz, rise time 2 ms; an amplitude modulated 500 Hz pure tone (30% modulation depth); a continuous pure tone in which a 1-s pause was introduced after at least 5 s. Onset was measured as rise time to 50% of maximum response and offset as time from end of stimulus until 50% of response amplitude was reached. The impulse response was quantified by time from start of stimulus to peak of response. The frequency transfer function was characterized by the 6 dB (half amplitude) cut-off point and decay was measured in percentage during a 5-s period. Recovery was quantified as the relative regain of amplitude after a 1-s pause. It was found that impulse response correlated to rise time for response to tone burst. Onset and offset were not correlated. Amplitude modulation was a mixture of onset and offset properties and not correlated to either measure. The decay and recovery were (nearly significantly) correlated and are probably different measures of the same process. A simplified test procedure for different aspects of time course was suggested but it remains to be shown if the pattern in pathological cases follows the pattern found in the present group of normal subjects.