Age-related changes in consonant and sentence processing

INTRODUCTION: EXPERIMENT 1 Although it is well established that aging impairs speech comprehension in noise in older subjects [1-4], the relative contribution of peripheral and central changes to presbycusic deterioration remains an area of active investigation. Some researchers hypothesize that age-related changes in speech comprehension primarily reflect deterioration in cochlear function [5], whereas others ascribe an important role to deficits in central auditory processing [6-7]. Central deficits may reflect impaired phonological processing [8] or higher-level lexical and semantic impairments that are most clearly revealed in sentence testing [9]. A primary goal of audiological rehabilitation of older subjects with hearing impairment (OHI) is to restore speech understanding to normal levels with hearing aids [10] and/or with perceptual training [11]. The appropriate target for OHI subjects is to restore their speech comprehension to age-normal performance levels observed in older subjects with normal hearing (ONH). Hence, the careful measurement of speech comprehension in noise in ONH subjects is necessary to provide a reliable benchmark against which the success of audiological rehabilitation can be measured. A number of studies have suggested that phonological processing in noise is impaired by advancing age [12-14]. For example, Gelfand et al. used the City University of New York Nonsense Syllable Test to examine consonant discrimination in younger and older subjects with audiometric thresholds better than 25 dB at 250 to 8,000 Hz [15]. They found significant differences in performance between the younger (20-49 yr) and older (60-69 yr) subject groups despite similar patterns of consonant confusions. The differences persisted after factoring out individual pure-tone thresholds in the 250 to 4,000 Hz range but were correlated with thresholds at 8,000 Hz, suggesting that subtle changes in hearing acuity contributed to consonant identification deficits, even in older adults with virtually normal thresholds. In experiment 1, we used the California Syllable Test (CaST) [16-17] to compare consonant identification in younger subjects with normal hearing (YNH) and ONH subjects. The CaST measures the signal-to-noise ratio (SNR) required to identify consonants in consonantvowel-consonant (CVC) syllables presented in speech spectrum noise. It uses the signal detection measure d' to quantify the SNRs necessary to identify each of 20 leading and 20 trailing consonants. In previous studies, we demonstrated that consonants differ substantially in the SNRs required for their identification [16]. Group A consonants (/[integral]/ /tf/, /t/, /s/, /z/, /[??]/, and /r/) in isolated syllables are accurately identified at SNRs near 0 dB, i.e., similar to the SNRs obtained in sentence reception threshold (SeRT) testing of YNH subjects. Group B consonants (/d/, /g/, /l/, /m/, /n/, /f/, and /k/) are accurately identified at SNRs that are 4 to 10 dB above SeRTs, suggesting that the identification of these consonants may make some contribution to sentence comprehension at SNRs that characterize SeRTs. Finally, group C consonants (/b/, /v/, /[??]/, /[eta]/, /[theta]/, /p/, and /h/) are identified at SNRs that are 10 to 40 dB above SeRT SNRs and therefore likely make minimal contributions to sentence comprehension using existing SeRT instruments. Previous studies have found age-related decrements in consonant identification when consonants are presented in nonsense syllables [8,12,15,18]. However, Dubno et al. found no overall age-related changes in word recognition in audiometrically matched subjects ranging in age from 55 to 84 yr, suggesting that age-related deficits in consonant identification may not be evident for lexically meaningful tokens [19]. Because the CaST contains both words and nonsense syllables, it permits a comparison of the effects of lexicality on consonant identification in ONH and YNH subjects. …