Measuring sensitivity to envelope interaural time differences by adapting modulation depth

Abstract

Listeners are sensitive to interaural time differences carried in the envelope of high-frequency sounds (ITDENV), but the salience of this cue depends on several envelope properties. Making use of the fact that sensitivity to ITDENV varies systematically with the depth of modulation of sinusoidally amplitude-modulated tones, we devised a task in which modulation depth is varied adaptively to measure ITDENV sensitivity. In our task, the stimulus is a 4 kHz tone modulated at rates of 32, 64, or 128 Hz, with a fixed ITD of ±500 μs. Modulation depth is adaptively varied (in dB) and listeners make left-right judgements. The data are used to obtain a psychometric function and threshold value. For listeners with normal hearing, we find that thresholds vary with modulation rate and with sensation level as expected. This task provides us with a convenient means for investigating ITDENV sensitivity in listeners with sensorineural hearing loss. Given that this population shows enhanced sensitivity to amplitude modulation under certain conditions, generally attributed to a loss of cochlear compression, we will test the hypothesis that this translates into superior ITDENV sensitivity under similar conditions.