Cross-modulation interference with lateralization of mixed-modulated waveforms

Purpose: This study investigated the ability to use spatial information in mixedmodulated
(MM) sounds containing concurrent frequency-modulated (FM) and
amplitude-modulated (AM) sounds by exploring patterns of interference when
different modulation types originated from different loci as may occur in a
multisource acoustic field.
Method: Interaural delay thresholds were measured from 5 normal-hearing adults
for an AM sound in the presence of interfering FM and vice versa as a function of
interferer modulation rate. In addition, the effects of near versus remote interferer
rates, and fixed versus randomized interferer interaural delay, were investigated.
Results: AM interfered with lateralization of FM at all modulation rates. However,
the FM interfered with AM lateralization only when the FM rate was higher than the
AM rate. This rate asymmetry was surprising given the prevalence of low-frequency
dominance in lateralization, but was predicted by a cross-correlation model of
binaural interaction. Effects were similar for fixed and randomized interferer
interaural delays.
Conclusions: The results suggest that in multisource environments, sources containing
different modulation types significantly interfere with localization in complex ways
that reveal interactions between modulation type and rate. These findings contribute
to the understanding of auditory object formation and localization.
This work was supported by Grant NSC 98-2410-H-008-
081-MY3 from the National Science Council, Taiwan; Grant
BCS0477984 from the National Science Foundation; and
Grant R01DC009659 from the National Institutes of Health.