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Activity of middle ear muscles under certain pure tone and noise conditions Hatch, Marion Beryl


This investigation was concerned with measurements of middle ear muscle activity. Specifically, the investigation was designed: (a) to compare reflex thresholds for pure tone(PT), octave band noise(OBN), and one-third octave band noise(TQBN); (b) to study middle ear muscle activity during a five minute exposure to these stimuli at 114 dB SPL; and (c) to compare middle ear muscle activity for stimuli of different frequencies. Eighteen subjects were divided into two groups. Group I subjects were exposed to a 700 Hz PT, to OBN with center frequency 500 Hz, and TOBN with center frequency 630 Hz; Group II subjects were exposed to a 1400 Hz PT, to OBN with center frequency 1000 Hz, and TOBN with center frequency 1250 Hz. A Madsen Electroacoustic Impedance Bridge and graphic recorder were used to record impedance changes during stimulation with the above stimuli. These impedance changes were assumed to reflect muscle activity. Thresholds for the acoustic reflex were determined using standard procedures. Analysis of results indicated muscle activity decreased progressively during stimulation. Reflex decay was significantly greater for stimulation with PT than for stimulation with OBN(Groups I and II) and TOBN(Group II). It was noted that reflex decay was less during stimulation with a 700 Hz PT than during stimulation with a 1400 Hz PT. Finally, it was observed that reflex thresholds were higher for PT than for OBN and TOBN. Decrease in muscle activity during acoustic stimulation was discussed in terms of adaptation and possibly central mechanisms; the characteristics of the stimuli were compared and discussed, as a possible explanation for differences in results obtained for tones and noise.

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