Volume 104, Issue 3, September 1998
Index of content:
- PSYCHOLOGICAL ACOUSTICS 
104(1998); http://dx.doi.org/10.1121/1.424369View Description Hide Description
Two experiments explored the limits of listeners’ abilities to interpret large interaural time delays (ITDs) in terms of laterality. In experiment 1, just-noticeable differences (jnd’s) were measured, using an adaptive procedure, for various reference ITDs of Gaussian noise between 0 and 3000 μs. The jnd’s increased gradually with reference ITD for reference ITDs between 0 μs and 700 μs, then rose sharply to plateau at much higher jnd’s for the remainder of the standard ITDs tested (1000–3000 μs). The second experiment tested left/right discrimination of Gaussian noise that was interaurally delayed up to 10 000 μs, and high-pass filtered to cutoff frequencies between 0 Hz (broadband) and 3000 Hz. There was good discrimination (62%; significantly above chance, ) for broadband and 500-Hz high-pass cutoff stimuli for all ITDs up to 10 000 μs, and for ITDs up to at least 3000 μs for higher high-pass cutoff frequencies. These results indicate that laterality cues are discriminable at much larger ITDs than are experienced in free-field listening, even in the absence of energy below 3 kHz.
104(1998); http://dx.doi.org/10.1121/1.424370View Description Hide Description
The relative contributions of various regions of the frequency spectrum to speech recognition were assessed with a correlational method [K. A. Doherty and C. W. Turner, J. Acoust. Soc. Am. 100, 3769–3773 (1996)]. The speech materials employed were the 258-item set of the Nonsense Syllable Test. The speech was filtered into four frequency bands and a random level of noise was added to each band on each trial. A point biserial correlation was computed between the signal-to-noise ratio in each band on the trials and the listener’s responses, and these correlations were then taken as estimates of the relative weights for each frequency band. When the four bands were presented separately, the correlations for each band were approximately equal; however, when the four bands were presented in combination, the correlations were quite different from one another, implying that in the broadband case listeners relied much more on some bands than on others. It is hypothesized that these differences reflect the way in which listeners combine and attend to speech information across various frequency regions. The frequency-weighting functions as determined by this method were highly similar across all subjects, suggesting that normal-hearing listeners use similar frequency-weighting strategies in recognizing speech.
Auditory-visual spatial integration: A new psychophysical approach using laser pointing to acoustic targets104(1998); http://dx.doi.org/10.1121/1.424371View Description Hide Description
The alignment of auditory and visual spatial perception was investigated in four experiments, employing a method of laser pointing toward acoustic targets in combination with various tasks of visual fixation in six subjects. Subjects had to fixate either a target LED or a laser spot projected on a screen in a dark, anechoic room and, while doing so, direct the laser beam toward the perceived azimuthal position of the sound stimulus (bandpass-filtered noise; bandwidth 1–3 kHz; 70 dB sound pressure level, duration 10 s). The sound was produced by one of nine loudspeakers, located behind the acoustically transparent screen between 22° to the left and 22° to the right of straight ahead. Systematic divergences between sound azimuth and laser adjustment were found, depending on the instructions given to the subjects. The eccentricity of acoustic targets was generally overestimated by up to 10.4° with an only slight influence of gaze direction on this effect. When the sound source was straight ahead, gaze direction had a substantial influence in that the laser adjustments deviated by up to 5.6° from sound azimuth, toward the side to which the gaze was directed. This effect of eye position decreased with increasing eccentricity of the sound. These results can be explained by the interactive effects of four distinct factors: the lateral overestimation of the auditory eccentricity, the effect of eye position on sound localization, the effect of the retinal eccentricity on visual localization, and the extraretinal effect of eye position on visual localization.