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* To whom correspondence should be addressed. E-mail: d.mcalpine{at}ucl.ac.uk.
Interaural time differences (ITDs) are important cues for mammalian sound localization. At high-frequencies, sensitivity to ITDs, which are conveyed only by the envelope of the waveforms, has been shown to be poorer than sensitivity to ITDs at low-frequencies, which are conveyed primarily by the fine-structure of the waveforms. Recently, human psychophysical experiments have demonstrated that sensitivity to envelope-based ITDs in high-frequency transposed tones can be equivalent to low-frequency fine-structure-based ITD sensitivity. Transposed tones are designed to provide high-frequency auditory nerve fibers (ANFs) with similar temporal information to that provided by low-frequency tones. We investigated neural sensitivity to ITDs in high-frequency transposed and sinusoidally amplitude modulated (SAM) tones, in the inferior colliculus of the guinea pig. Neural sensitivity to ITDs in transposed tones was found to be greater than to ITDs in SAM tones; in response to transposed tones, neural firing rates were more modulated as a function of ITD and discrimination thresholds were found to be lower than in response to SAM tones. Similar to psychophysical findings, ITD discrimination of single neurons in response to transposed tones for rates of modulation less than 250-Hz was comparable to neural discrimination of ITDs in low-frequency tones. This suggests that the neural mechanisms that mediate sensitivity to ITDs at high- and low-frequencies are functionally equivalent provided that the stimuli result in appropriate temporal patterns of action potentials in ANFs.
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