Effects of reference interaural time and intensity differences on binaural performance in listeners with normal and impaired hearing.

OBJECTIVE: The purpose of this research was to measure the effects of reference interaural time and intensity differences on binaural performance in listeners with normal hearing and impaired hearing for a number of different binaural tests. Experiment 1 measures the dependence of binaural detection and discrimination performance on reference interaural intensity differences (IID) in the range of +/- 12 dB for listeners with normal hearing. Experiment 2 extends these measures to include reference IIDs and interaural time differences (ITD) for two groups of listeners with normal hearing with offsets in the range of +/- 12 dB and +/- 300 microseconds (group 1) and +/-24 dB and +/- 600 microseconds (group 2). Experiment 3 includes the same tests and conditions as experiment 2 for listeners with various hearing impairments.
DESIGN: A set of psychophysical measurements was completed on 11 listeners with sensorineural hearing losses and 9 listeners with clinically normal hearing. The primary measurements were a set of four binaural detection and interaural discrimination thresholds measured for two 1/3-octave bands of Gaussian noise, one centered at 500 Hz and the other at 4000 Hz. Specifically, we measured binaural (antiphasic) detection thresholds for tones centered in the masking noise as well as the just-noticeable differences (JNDs) in IID, ITD, and interaural cross-correlation (ICC) for each of the noise-band stimuli. All measurements were done for a number of combinations of reference IID and ITD. In addition to these primary measurements, several other measurements were made on each subject, including monaural absolute thresholds, monaural intensity discrimination, monaural masked thresholds, and intensity levels required for interaurally balanced loudness and for a centered image. All measurements were made using a relatively quick, adaptive procedure.
RESULTS: For the subjects with normal hearing, measured dependencies of the IID and ITD JNDs using noise stimuli on reference ITD and IID are different from those previously reported for tonal stimuli. Binaural performance of the listeners with impaired hearing varies widely across subjects and tests and is generally poorer than that of listeners with normal hearing.
CONCLUSIONS: On the basis of the results for subjects with hearing impairments, we have reached several conclusions. First, the results for the binaural measurements cannot be explained in terms of available monaural audiometric and psychophysical measurements on these subjects. Second, the subjects' binaural abilities show no significant improvement with any combinations in the reference values of ITD and IID, providing negative evidence for the hypothesis that degraded performance for some subjects may be due to internal interaural offsets in ITD or IID. Third, the hypothesis that binaural detection and ICC discrimination are related, suggested by Durlach et al (1986), is generally supported. Fourth, binaural detection performance is not simply explained in terms of sensitivities to ITD and IID.