Lateralization of tone complexes in noise : the role of monaural envelope processing in binaural hearing

The lateralization of tone complexes presented against a diotic band-pass noise background was measured as a function of level and Interaural Time Difference (ITD) of the tone complex. The bandwidths of the sine-phase tone complex and the noise were 600 Hz, centered at 550 Hz, the component spacing of the tones was 20 Hz. Due to this spectral configuration, none of the tonal components is spectrally resolved, and the binaural cues of the combined stimulus vary with the 20 Hz cycle of the stimulus. Results show that listeners can judge the lateral position of the tone complex down to levels of -5 dB SNR. A second experiment determined masked thresholds of the tone complex in the diotic noise as a function of ITD. Detection thresholds are as low as -20 dB SNR for ITDs of 0.7 ms, and about -7 dB for ITDs of 0 ms, a condition in which only monaural information could be used by the listeners. It seems that despite the audibility of the tone complex at rather low levels (-20 dB SNR) based on binaural cues, the ability to lateralize requires the audibility of monaural envelope information, which is restricted to levels above about -7 dB SNR. Thus, monaural envelope information may be a prerequisite for the readout of lateralization information corresponding to the tone complex from the binaural display.