Effects of Hearing Loss and Spectral Shaping on Identification and Neural Response Patterns of Stop-Consonant Stimuli in Young Adults

Abstract

The primary purpose of this study was to more clearly define the effects of hearing loss, separate from age, on perception, and neural response patterns of dynamic spectral cues. To do this, the study was designed to determine whether (1) hearing loss affects the neural representation and/or categorical perception of stop-consonant stimuli among young adults and (2) spectrally shaped amplification aimed at increasing the audibility of the F2 formant transition cue reduces any effects of hearing loss. It was predicted that (1) young adults with hearing loss would differ from young adults with normal hearing in their behavioral and neural responses to stop-consonant stimuli and (2) enhancing the audibility of the F2 formant transition cue relative to the rest of the stimulus would not overcome the effects of hearing loss on behavioral performance or neural response patterns. Behavioral identification and neural response patterns of stop-consonant stimuli varying along the /b-d-g/ place-of-articulation continuum were measured from seven young adults with mild-to-moderate hearing impairment (mean age = 21.4 yr) and compared with responses from 11 young adults with normal hearing (mean age = 27 yr). Psychometric functions and N1-P2 cortical-evoked responses were evoked by consonant-vowel (CV) stimuli without (unshaped) and with (shaped) frequency-dependent amplification that enhanced F2 relative to the rest of the stimulus. Behavioral identification and neural response patterns of stop-consonant CVs differed between the two groups. Specifically, to the unshaped stimuli, listeners with hearing loss tended to make low-frequency judgments more often (more /b/, fewer /g/) than listeners with normal hearing when categorizing along the /b-d-g/ continuum. Additionally, N1 amplitudes were larger and P2 latencies were longer to all phonemes in young adults with hearing impairment versus normal hearing. Enhancing the audibility of the F2 transition cue with spectrally shaped amplification did not alter the neural representation of the stop-consonant CVs in the young listeners with hearing loss. It did modify categorical perception such that listeners with hearing loss tended to make high-frequency judgments more often (more /g/, fewer /b/). However, shaping the stimuli did not make their psychometric functions more like those of the normal controls. Instead, young adults with hearing loss went from one extreme (low-frequency judgments with unshaped stimuli) to the other (high-frequency judgments with shaped stimuli), whereas judgments from the normal controls were more balanced. Hearing loss, separate from aging, seems to negatively impact identification and neural representation of time-varying spectral cues like the F2 formant transition. Enhancing the audibility of the F2 formant transition cue relative to the rest of the stimulus does not overcome the effects of hearing loss on behavioral performance or neural response patterns in young adults. Thus, the deleterious effects of hearing loss on stop-consonant perception along the place-of-articulation continuum may not only be due solely to decreased audibility but also due to improper coding by residual neurons, resulting in distortion of the time-varying spectral cue. This may explain, in part, why amplification cannot completely compensate for the effects of sensorineural hearing loss.