A hearing-aid signal-processing scheme based on the temporal aspects of compression

Abstract

Changes in gain associated with the basilar membrane compressive nonlinearity are accompanied by changes in the bandwidth of tuning. Filters with level-dependent bandwidth have level-dependent phase properties. These phase properties result in level-dependent timing of sustained phase-locked responses of auditory-nerve (AN) fibers at low frequencies and level-dependent latencies at high frequencies, where phase-locking rolls off. In the healthy ear, level-dependent temporal aspects of AN responses carry information about stimulus level and spectral properties. Loss of compression with hearing impairment thus results not only in a reduction of amplification, but also in distortion of the temporal response pattern of the AN. The temporal aspects of compression suggest that signal-processing schemes that attempt to correct sounds, or restore normal spatio-temporal response patterns, should include dynamic level-dependent phase shifts. A nonlinear signal-processing scheme will be presented which includes dynamic frequency- and level-dependent phase shifts, based on physiological models of the temporal response properties of AN fibers. Preliminary testing measured listeners preferences for sentences and intelligibility of vowel-consonant syllables with different degrees of nonlinear processing. Hearing-impaired listeners tended to prefer the dynamically corrected stimuli based on improved clarity. Correction also improved intelligibility for some phonemes. [Work supported by NIDCD R21-006057.]