FREQUENCY SUMMATION OBSERVED IN THE HUMAN ACOUSTIC REFLEX

It is known that the threshold of an acoustically induced middle-ear-m uscle (MEM) reflex can be lowered by the simultaneous presentation of a second tone (facilitator), which is presented to the ipsilateral or contralateral ear at a level below the acoustic reflex threshold (ART) of the facilitator itself (Sesterhenn and Breuninger, 1976; Blood and Greenberg, 1981). In the present study, a primary elicitor and a faci litator were presented to the ear contralateral to that used for measu rement of the acoustic reflex (AR), and the effects of changing freque ncies and sound levels of the facilitator were investigated in human s ubjects with normal ears. The sound levels of facilitators, which caus ed a significant reduction of ART for the primary elicitors (facilitat ion thresholds), showed an asymmetrical pattern as a function of frequ ency of the facilitators. The facilitation thresholds tended to be low er when a facilitator with a frequency lower than the frequency of the elicitor (1 kHz) was used. In addition, effects of the elicitor on th e masked thresholds of the facilitator were examined to observe the po ssible interaction between elicitor and facilitator from the viewpoint of 'spread of excitation.' The underlying mechanism of summation effe cts of two tones are discussed based on the possible input mechanism i nvolved in the acoustically induced MEM reflex arc.