The effects of amplitude modulation on acoustic reflex decay

The temporal decay of the acoustic reflex provides the basis for an objecti ve audiological test that differentiates cochlear from retrocochlear pathol ogies. The classic sign of a neural lesion is a rapid decay of the reflex u nder conditions of pure-tone stimulation for frequencies of 1,000 Hz and be low. This restriction to lower frequencies is due to the fact that even nor mal ears show decay for higher-frequency signals. At present, it is unclear whether the acoustic reflex decay (ARD) seen in normal ears is related to frequency-specific channels or whether the critical variable is the timing information coded within the channels. This study examined ARD in subjects with normal hearing and middle ear function. The degree of ARD was measured for both modulated and unmodulated carrier frequencies of 500, 1,000, 2,00 0 and 4,000 Hz with amplitude modulation rates of 50-400 Hz. The dependent variable was the half-life of the decaying reflex (ARD 50%) over a 20-secon d stimulation interval. Significant ARD was present for high-frequency unmo dulated carriers but not for low-frequency carriers. For all listeners, ARD was diminished for all modulated stimuli. The results of this study sugges t that resistance to ARD is mediated by both the temporal aspects and frequ ency of a stimulus.