Psychophysical and Electrophysiological Measures of Frequency Modulation Sensitivity: Effects of Age and Interaural Modulator Phase.

Objectives: The purpose of this study was to measure sensitivity to interaurally in-phase and out-of-phase frequency modulation (FM) using parallel behavioral and electrophysiological approaches. The broader goal was to apply these approaches to the assessment of age-related deficits in temporal fine-structure processing. The hypothesis was that the range of modulation rates over which FM detection is superior for out-of-phase modulation, as well as the magnitude of benefit, diminishes with age due to reduced fidelity of temporal fine-structure processing.

Design: Participants were 63 adults with good audiometric hearing for their age divided equally into 3 age groups of young (mean = 23 years), middle-aged (mean = 47 years), and older (mean = 70 years) listeners. FM detection thresholds for stimuli carried by a nominally 500-Hz tone were measured for interaurally in-phase and out-of-phase modulators having rates ranging from 4 to 32 Hz. The rationale for restricting carrier and modulator frequencies to low rates was to focus on the benefit provided by interaurally out-of-phase modulators as a gauge of temporal fine-structure processing. The electrophysiological acoustic change complex (ACC) was also measured in the same participants for a subset of these modulation rates where the depth of modulation was fixed at 3 Hz. The ACC was quantified using an intertrial phase coherence metric.

Results: For in-phase modulation, there was minimal change in FM detection threshold across the range of modulation rates, and no effects of age. For out-of-phase modulation, thresholds were markedly lower for low rates and increased (became poorer) monotonically as rate increased. For the 2 older age groups, thresholds for the 2 modulator phases converged by 32 Hz. Young participants performed consistently better than the participants in the 2 older groups for the out-of-phase configurations. The ACC was most robust for low-rate, out-of-phase modulation and diminished systematically as rate increased in all participants. No ACC was observed for the highest rate of 32 Hz. The older listeners had consistently poorer ACC responses across all rates. Correlations between behavioral and electrophysiological metrics were small, but significant, for rates of 8, 16, and 24 Hz.

Conclusions: The results support the hypothesis of diminishing temporal fine-structure processing as a function of age. For behavioral FM detection, middle-age and older adults had poorer thresholds for out-of-phase FM than young adults, with functions for in- and out-of-phase FM converging at a lower rate. For the ACC test, this was demonstrated by reduced intertrial phase coherences in the older participants. Although there were general similarities between the behavioral and electrophysiological data patterns, some differences were observed, and further study is required to clarify underlying mechanism(s). The study broadly demonstrates that objective measures can be used to gauge temporal processing, in parallel to behavioral measures, and so can be applied in situations where subjective assessment is challenging.