The Role of AgeRelated Declines in Subcortical Au

Authors: Tim Schoof Stuart Rosen

Publish Date: 2016/05/23

Volume: 17, Issue: 5, Pages: 441-460

PDF Link

Abstract

Older adults even those without hearing impairment often experience increased difficulties understanding speech in the presence of background noise This study examined the role of agerelated declines in subcortical auditory processing in the perception of speech in different types of background noise Participants included normalhearing young 19 – 29 years and older 60 – 72 years adults Normal hearing was defined as puretone thresholds of 25 dB HL or better at octave frequencies from 025 to 4 kHz in both ears and at 6 kHz in at least one ear Speech reception thresholds SRTs to sentences were measured in steadystate SS and 10Hz amplitudemodulated AM speechshaped noise as well as twotalker babble In addition clickevoked auditory brainstem responses ABRs and envelope following responses EFRs in response to the vowel /ɑ/ in quiet SS and AM noise were measured Of primary interest was the relationship between the SRTs and EFRs SRTs were significantly higher ie worse by about 15 dB for older adults in twotalker babble but not in AM and SS noise In addition the EFRs of the older adults were less robust compared to the younger participants in quiet AM and SS noise Both young and older adults showed a “neural masking release” indicated by a more robust EFR at the trough compared to the peak of the AM masker The amount of neural masking release did not differ between the two age groups Variability in SRTs was best accounted for by audiometric thresholds puretone average across 05–4 kHz and not by the EFR in quiet or noise Aging is thus associated with a degradation of the EFR both in quiet and noise However these declines in subcortical neural speech encoding are not necessarily associated with impaired perception of speech in noise as measured by the SRT in normalhearing older adultsOlder adults typically experience increased difficulties understanding speech in noisy environments even in the absence of hearing impairment Dubno et al 2002 Helfer and Freyman 2008 This has often been attributed to an agerelated decline in auditory temporal processing eg Frisina and Frisina 1997 CHABA 1988 PichoraFuller and Souza 2003 PichoraFuller et al 2007 Normalhearing older adults perform more poorly on behavioral measures of temporal processing eg Snell 1997 He et al 2007 Similarly neurophysiological studies have shown agerelated declines in the temporal precision of subcortical neural sound encoding eg Burkard and Sims 2001 Purcell et al 2004 Anderson et al 2012 While the focus of this paper is on the role of agerelated declines in subcortical auditory processing it should be noted that cognitive declines associated with aging may also impact on the ability to understand speech in noise eg Akeroyd 2008 Füllgrabe et al 2015 but see Schoof and Rosen 2014The temporal precision or fidelity of subcortical neural coding of complex sounds such as speech is perhaps best assessed by measuring the scalprecorded frequency following response FFR which reflects sustained synchronous neural firing in the brainstem in response to periodic auditory stimuli Worden and Marsh 1968 Moushegian et al 1973 Adding FFRs recorded to stimuli of positive and negative polarities is thought to eliminate the cochlear microphonic and any linear stimulus artifacts and accentuate the envelope of the FFR at its fundamental frequency F0 Gorga et al 1985 Subtracting opposite polarity responses on the other hand is assumed to reflect phaselocked activity to the temporal fine structure TFS Aiken and Picton 2008 However because envelope cues can be reconstructed from TFS information at the output of peripheral auditory filters it is difficult to determine to what extent the subtracted polarity FFR indeed reflects TFS information in the stimulus Ghitza 2001 Heinz and Swaminathan 2009 The focus in this paper is therefore on the added polarity FFR henceforth referred to as the envelope following response EFRSeveral studies have shown that EFRs and FFRs are less robust for older compared to younger adults eg Clinard et al 2010 Vander Werff and Burns 2011 ParberyClark et al 2012 Anderson et al 2012 Clinard and Tremblay 2013 Marmel et al 2013 Agerelated changes in subcortical processing have been shown for example in response to the syllable /dɑ/ Vander Werff and Burns 2011 Anderson et al 2012 Clinard and Tremblay 2013 Both Vander Werff and Burns 2011 and Clinard and Tremblay 2013 only found group differences for peaks at the onset and offset of the response Similarly Anderson et al 2012 found increased peak latencies for the older adults only for the onset and formant transition parts of the response and not the steadystate vowel part However they also found that several other response measures such as the responsetoresponse correlation phaselocking factor and rms amplitude showed age effects both for the transition and steadystate portions of the response Similarly agerelated changes in subcortical processing have been shown in response to pure tones Clinard et al 2010 Clinard and Tremblay 2013 Marmel et al 2013 Marmel et al 2013 for example measured FFRs to pure tones at various frequencies for participants with a wide range of ages and audiometric thresholds They found that age was significantly correlated with the FFR even when accounting for individual differences in audiometric thresholdsPrevious literature has indicated a relationship between speech perception in noise and the EFR within a group of older adults Anderson et al 2011 2013 Anderson et al 2011 for instance showed that older adults with more robust EFRs performed better on a speechinnoise task than older adults with less robust EFRs However while withingroup differences in speechinnoise performance may be attributed in part to differences in subcortical auditory processing it remains unclear whether the EFR can similarly predict differences in speechinnoise performance between young and older listenersAnother question that remains open is whether the EFR can predict the benefit a listener derives when perceiving speech in the presence of a fluctuating compared to a steadystate masker the fluctuating masker benefit FMB Cooke 2006 When a masker fluctuates in amplitude over time it can be expected that the degrading effect of the noise on the EFR will also vary over time The degree of “neural masking release” at the troughs of the fluctuating masker may relate to listeners’ abilities to listen in the dips of fluctuating maskersThe primary aim of this study was to determine the role of agerelated declines in subcortical auditory processing in the perception of speech in different types of background noise Speech perception abilities were assessed in steadystate and amplitudemodulated speechshaped noise as well as twotalker babble In addition click auditory brainstem responses ABRs and EFRs in response to a vowel /ɑ/ in quiet steadystate and amplitudemodulated speechshaped noise were measuredThis experiment was part of a larger study that looked at the relative contribution of agerelated declines in both lowlevel auditory processing and higher level cognitive processing to difficulties in understanding speech in noise typically experienced by older adults Schoof and Rosen 2014

Keywords: