The impacts of age and absolute threshold on binaural lateralization

Listeners varying in audiometric thresholds and in age discriminated the perceived location of complex tones relative to a diotic standard. Stimuli were composed of one, two, three, or six tones amplitude modulated at a rate of 75 Hz. Carrier frequencies were selected from a set of seven values (roughly log-spaced between 700 and 7000 Hz) and sensation levels were equated across listeners. Baseline performance was obtained for one- and two-component signals and the effects of diotic interference were assessed with two-, three-, and six-component signals. For all signals, the dichotic components contained interaural differences in time (whole-waveform delay), level, or both across a range of values. Listeners with higher audiometric thresholds performed more poorly in all conditions, despite the use of equivalent sensation levels. Diotic interferers reduced performance for all listeners. Statistical analyses indicated that the impacts of age were difficult to distinguish from to the co-occurring increases in pure-tone thresholds, suggesting that: (1) absolute threshold needs to be experimentally distinguished from age and (2) reduced sensitivity to binaural differences is not addressed by presenting stimuli at equivalent sensation levels.