Age Effects in Temporal Envelope Processing: Speech Unmasking and Auditory Steady State Responses

Abstract

The purpose of this study was to determine whether temporal envelope processing is reduced in older listeners. Experiment 1 tested the hypothesis that older listeners exhibit reduced speech unmasking at higher modulation rates. Experiment 2 tested the hypothesis that auditory steady state response (ASSR) amplitudes are reduced in older listeners at high modulation rates. Two groups of observers with relatively normal hearing (younger, mean age = 25.0 years and older, mean age = 68.7 years) participated in two experiments. Experiment 1 examined speech unmasking in modulated noise as a function of masker modulation rate (16 and 32 Hz) and target speech rate (normal and 33% time compressed). Experiment 2 measured ASSR amplitudes as a function of modulation rate (32 and 128 Hz) and carrier frequency (500 and 2000 Hz). Experiment 1 indicated that older listeners show reduced speech unmasking for normal-rate speech and reduced recognition of rapid speech in steady noise. However, for rapid speech, there is no age effect for speech unmasking and no difference in the magnitude of masking release as a function of modulation rate. In general, effects of listener age and masker modulation rate on the magnitude of masking release are observed only for normal-rate speech. Experiment 2 showed that the ASSR amplitudes of older listeners are reduced for a 128-Hz modulation rate but not for a 32-Hz modulation rate, irrespective of carrier frequency. These results suggest that the reduced speech unmasking seen in older listeners for relatively slow modulation rates is not caused by deficits in envelope processing but rather is associated with the more constrained redundancy of the speech material available during the masker minima. Deficits in temporal envelope processing are evident in advanced age but only for relatively high envelope frequencies.