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 perfor mance in listeners with normal hearing and impaired hearing for a numb er of different binaural tests. Experiment 1 measures the dependence o f binaural detection and discrimination performance on reference inter aural intensity differences (IID) in the range of +/-12 dB for listene rs with normal hearing. Experiment 2 extends these measures to include reference IIDs and interaural time differences (ITD) for two groups o f listeners with normal hearing with offsets in the range of +/-12 dB and +/-300 mu sec (group 1) and +/-24 dB and +/-600 mu sec (group 2). Experiment 3 includes the same tests and conditions as experiment 2 fo r listeners with various hearing impairments. Design: A set of psychop hysical measurements was completed on 11 listeners with sensorineural hearing losses and 9 listeners with clinically normal hearing. The pri mary measurements were a set of four binaural detection and interaural discrimination thresholds measured for two 1/3-octave bands of Gaussi an noise, one centered at 500 Hz and the other at 4000 Hz. Specificall y, we measured binaural (antiphasic) detection thresholds for tones ce ntered in the masking noise as well as the just-noticeable differences (JNDs) in IID, ITD, and interaural cross correlation (ICC) for each o f the noise-band stimuli. All measurements were done for a number of c ombinations of reference IID and ITD. In addition to these primary mea surements, several other measurements were made on each subject, inclu ding monaural absolute thresholds, monaural intensity discrimination, monaural masked thresholds, and intensity levels required for interaur ally balanced loudness and for a centered image. All measurements were made using a relatively quick, adaptive procedure. Results: For the s ubjects with normal hearing, measured dependencies of the IID and ITD JNDs using noise stimuli on reference ITD and IID are different from t hose previously reported for tonal stimuli. Binaural performance of th e listeners with impaired hearing varies widely across subjects and te sts and is generally poorer than that of listeners with normal hearing . Conclusions: On the basis of the results for subjects with hearing i mpairments, we have reached several conclusions. First, the results fo r 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 improve ment with any combinations in the reference values of ITD and IID, pro viding 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 discriminat ion are related, suggested by Durlach et al (1986), is generally suppo rted. Fourth, binaural detection performance is not simply explained i n terms of sensitivities to ITD and IID.