Observer weighting of interaural cues in positive and negative envelope slopes of amplitude-modulated waveforms

The auditory system can encode interaural delays in highpass-filtered complex sounds by phase locking
to their slowly modulating envelopes. Spectrotemporal analysis of interaurally time-delayed highpass
waveforms reveals the presence of a concomitant interaural level cue. The current study systematically
investigated the contribution of time and concomitant level cues carried by positive and negative
envelope slopes of a modified sinusoidally amplitude-modulated (SAM) high-frequency carrier. The
waveforms were generated from concatenation of individual modulation cycles whose envelope peaks
were extended by the desired interaural delay, allowing independent control of delays in the positive and
negative modulation slopes. In experiment 1, thresholds were measured using a 2-interval forced-choice
adaptive task for interaural delays in either the positive or negative modulation slopes. In a control
condition, thresholds were measured for a standard SAM tone. In experiment 2, decision weights were
estimated using a multiple-observation correlational method in a single-interval forced-choice task for
interaural delays carried simultaneously by the positive, and independently, negative slopes of the
modulation envelope. In experiment 3, decision weights were measured for groups of 3 modulation
cycles at the start, middle, and end of the waveform to determine the influence of onset dominance or
recency effects. Results were consistent across experiments: thresholds were equal for the positive and
negative modulation slopes. Decision weights were positive and equal for the time cue in the positive
and negative envelope slopes. Weights were also larger for modulations cycles near the waveform onset.
Weights estimated for the concomitant interaural level cue were positive for the positive envelope slope
and negative for the negative slope, consistent with exclusive use of time cues.
We thank Virginia M. Richards and Bruce G. Berg for helpful discussions. We also thank Brian C. J. Moore and an anonymous reviewer for their insightful comments on an earlier draft of the manuscript. Work supported by grants from the National Science Council, Taiwan NSC 98-2410-H-008-081-MY3 and NIH R01DC009659.