Acoustic reflex measures of cochlear damage--a normative study

It has been argued that certain reflex measures may indicate the presence of end-organ hearing loss. It has been noticed that the difference between wide band noise and pure tone thresholds is diminished, that the reflex threshold duration function is flattened and that the reflex relaxation index, as specified by Norris et al. (1974b), is reduced in cases of end-organ hearing loss. These aspects of the acoustic stapedial reflex, together with its growth with stimulus intensity, have been investigated in 25 normally hearing subjects. The overriding feature of the measures taken of the acoustic stapedial reflex, was the high inter-subject variability. The differences between the acoustic reflex thresholds for pure tone and WBN was found to be normally distributed about a mean value of 12 dB. The threshold-duration functions again were highly variable and encompassed the entire range of results from normal and end-organ impaired groups as reported by Woodford et al. (1975). The variability encountered in the RRI measures was found to increase with frequency. The growth of the acoustic stapedial reflex with intensity was found to be frequency dependent: the gradient of the function increasing with frequency for the long duration (500 ms) pulses and decreasing with the short duration (50 ms) pulses. It is concluded that considerable caution must be exercised in extrapolation from any one of these measures in isolation.