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Time-Shift Dichotic Pitch
*University of British Columbia, Vancouver, BC, Canada
#British Columbia's Children's Hospital, Vancouver, BC, Canada
Two patterns of appropriately filtered acoustic white noise can be binaurally fused by the human auditory system to extract pitch and location information that is not available to either ear alone. This phenomenon is called dichotic pitch. We present a new method for generating dichotic pitch stimuli which allows the psychophysical assessment of dichotic pitch detection thresholds and also allows manipulation of several key parameters of the dichotic pitch stimulus. These experiments measure the effects of interaural coherence and interaural time difference on the dichotic pitch percept.
In the first experiment, we show that dichotic pitch perception is crucially dependent on the background noise interaural coherence but is relatively unaffected by the interaural coherence of the pitch-producing noise band. In the second experiment, we demonstrate that for discrimination between rising and falling dichotic pitch sequences, the threshold interaural time difference (ITD) between the background noise and the pitch-producing noise band is 100 to 120-µsec. The third experiment shows that the threshold ITD change to detect the direction of movement of the dichotic pitch is 66 to 97-µsec. However, both listeners were reliably above chance on this task at an ITD change of 23-µsec, the smallest change possible for our 44.1 kHz digital audio hardware.
These results further characterize the dichotic pitch percept and deepen understanding of this curious perceptual phenomenon. These results also have important implications for models of dichotic pitch perception. Finally, the pitch sensations produced by these stimuli bypass the more peripheral pitch processing mechanisms of the cochlea and cochlear nucleus, so dichotic pitch may be useful for revealing structural aspects of the auditory system.
Keywords: dichotic pitch, binaural masking level difference, interaural time difference, sound localization, auditory perception
Supported by March of Dimes grant FY96-0479 & BC-MSF
grant 95-36(95) to D.E.G.
This work was performed in the Department of Ophthalmology
at British Columbia's Children's Hospital, Vancouver, BC, V6H 3V4 Canada
For more on dichotic pitch and examples of dichotic pitch stimuli, see Dichotic Pitch Demos.