Sound localization in subjects with impaired hearing. Spatial-discrimination and interaural-discrimination tests.

In order to get a systematic picture of how various hearing impairments and neurologic disorders may affect sound localization, psychophysical spatial- and lateralization-discrimination measurements were performed on 140 subjects, including 69 with different types of hearing impairments, 32 with neurological diseases, and 39 with normal hearing. The quantities measured were: in the freefield, the horizontal minimum audible angle (MAA) at eight reference azimuths around the head and the vertical MAA straight ahead; with headphones, the just-noticeable difference (JND) in interaural time delay and the JND in interaural intensity difference. The standard stimulus was broadband (0.25-10 kHz), pulsed (1-sec), noise presented at a suprathreshold level for both ears (65-100 dB SPL). The results show that there exist characteristic impairments of sound localization in the different types of hearing impairments tested. On a general level, the results are consistent with the concept that the localization of sound relies on a decision made by the central auditory system based on a number of cues present in the acoustic signal at the two ears. The cues tested in our study are: 1) the interaural time difference, 2) the interaural intensity difference, and 3) the spectrum of the received signal at each ear. At a more specific level, the sound localization impairments found in conductive hearing losses are interpreted as bone-conduction effects, the results found in sensorineural hearing losses are interpreted as consequences of impaired or preserved spectral processing, the results in neurinomas are interpreted as impaired signal transmission in the auditory nerves, and the results of subjects with central involvements suggest that separate processors exist at some level in the central auditory system for the different localization cues. Finally, comments are made about the practical clinical significance of sound localization tests in the audiological and neurological evaluation of patients.