7 research outputs found
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
Learning to discriminate interaural time differences at low and high frequencies
This study investigated learning, in normal-hearing
adults, associated with training (i.e. repeated practice)
on the discrimination of ongoing interaural time difference
(ITD). Specifically, the study addressed an apparent
disparity in the conclusions of previous studies, which
reported training-induced learning at high frequencies
but not at low frequencies. Twenty normal-hearing adults
were trained with either low- or high-frequency stimuli,
associated with comparable asymptotic thresholds, or
served as untrained controls. Overall, trained listeners
learnt more than controls and over multiple sessions. The
magnitudes and time-courses of learning with the lowand
high-frequency stimuli were similar. While this is
inconsistent with the conclusion of a previous study with
low-frequency ITD, this previous conclusion may not be
justified by the results reported. Generalization of learning
across frequency was found, although more detailed
investigations of stimulus-specific learning are warranted.
Overall, the results are consistent with the notion that
ongoing ITD processing is functionally uniform across
frequency. These results may have implications for clinical
populations, such as users of bilateral cochlear implants