Effects of age on the discrimination of modulation type for 2- and 10-Hz rates

Frequency modulation (FM) at a rate of 10 Hz may be detected via conversion of FM to amplitude modulation (AM) in the cochlea, while 2-Hz FM may be detected partly using temporal fine structure (TFS) information. Greater age may impair the processing of TFS information while sparing the processing of AM information. To test these ideas, while controlling for the effects of detection efficiency, a two-stage experiment was conducted. In stage 1, psychometric functions were measured for the detection of AM alone and FM alone imposed on a 1000-Hz carrier, using 2- and 10-Hz rates. In stage two, the task was to discriminate AM from FM at the same modulation rate when the detectability of the AM alone and FM alone was equated. For young normal-hearing subjects, discrimination was markedly better for the 2-Hz than for the 10-Hz rate, consistent with the idea that FM is coded in a different way from AM at 2 Hz. For older subjects with normal hearing at 1000 Hz, a similar pattern was obtained, but discrimination o...