Cognition 133 (2014) 32–42

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Brief article

Working memory predicts semantic comprehension in dichotic listening in older adults Philip J. James 1, Saloni Krishnan, Jennifer Aydelott ⇑ Department of Psychological Sciences, Birkbeck, University of London, United Kingdom

a r t i c l e

i n f o

Article history: Received 19 December 2013 Revised 13 May 2014 Accepted 23 May 2014

Keywords: Aging Psycholinguistics Auditory language processing Dichotic listening Working memory

a b s t r a c t Older adults have difficulty understanding spoken language in the presence of competing voices. Everyday social situations involving multiple simultaneous talkers may become increasingly challenging in later life due to changes in the ability to focus attention. This study examined whether individual differences in cognitive function predict older adults’ ability to access sentence-level meanings in competing speech using a dichotic priming paradigm. Older listeners showed faster responses to words that matched the meaning of spoken sentences presented to the left or right ear, relative to a neutral baseline. However, older adults were more vulnerable than younger adults to interference from competing speech when the competing signal was presented to the right ear. This pattern of performance was strongly correlated with a non-auditory working memory measure, suggesting that cognitive factors play a key role in semantic comprehension in competing speech in healthy aging. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Understanding spoken language in competing speech is a particular challenge for older adults (Arlinger, Lunner, Lyxell, & Pichora-Fuller, 2009; Humes & Dubno, 2009; Schneider, Pichora-Fuller, & Daneman, 2010). Correlational studies indicate that pure tone hearing thresholds are the most effective predictor of speech identification performance in older listeners (Humes, 1996; Humes et al., 1994; van Rooij & Plomp, 1992). However, declines in higher-order executive functions such as working memory and inhibitory control may influence speech understanding in complex listening situations (Hasher, Lustig, & Zacks, 2007; Hasher & Zacks, 1988; Pichora-Fuller, Schneider, & ⇑ Corresponding author. Address: Department of Psychological Sciences, Birkbeck, University of London, Malet Street, London WC1E 7HX, United Kingdom. Tel.: +44 2076316368. E-mail address: [email protected] (J. Aydelott). 1 Present address: Bournemouth University, United Kingdom. http://dx.doi.org/10.1016/j.cognition.2014.05.014 0010-0277/Ó 2014 Elsevier B.V. All rights reserved.

Daneman, 1995; Schneider et al., 2010; Tun, O’Kane, & Wingfield, 2002; Wingfield & Stine-Morrow, 2000; Wingfield, Tun, & McCoy, 2005). A multitalker environment such as a cocktail party is likely to place increased demands on cognitive resources, as the listener must focus attention on a single talker and interpret the relevant linguistic message while ignoring competing voices. Indeed, older adults report increased comprehension difficulty in social settings in which there are multiple talkers (Committee on Hearing, Bioacoustics, and Biomechanics, 1988). Studies of dichotic listening, in which competing speech stimuli are presented simultaneously to separate auditory channels, indicate that older adults may be more vulnerable to interference from a secondary signal than younger adults in the identification of spoken words (Humes, Lee, & Coughlin, 2006). Further, older adults show a larger right-ear advantage (REA) than younger adults in dichotic word identification (Carter & Wilson, 2001; Roup, Wiley, & Wilson, 2006) and recall (Hommet et al., 2010), indicating an increased bias in favour of the dominant auditory

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pathway in older listeners. In addition, unlike younger adults, older listeners fail to demonstrate a reversal of the REA when focusing attention to the left ear (Andersson, Reinvang, Wehling, Hugdahl, & Lundervold, 2008; Hommet et al., 2010; Hällgren, Larsby, Lyxell, & Arlinger, 2001). Age-related hearing loss may contribute to difficulty with speech stream segregation under multitalker conditions, as reduced perceptual sensitivity may disrupt auditory object formation and selection in a complex auditory scene (Shinn-Cunningham & Best, 2008). However, although hearing sensitivity is a reliable predictor of speech identification performance under dichotic listening conditions in older adults (Jerger, Jerger, & Pirozzolo, 1991), correlations with memory measures have also been reported (Humes et al., 2006; Hällgren et al., 2001), suggesting that cognitive factors may influence older listeners’ ability to identify simultaneous competing speech signals. Cognitive aging is associated with a decline in working memory function, i.e., the ability to maintain information in memory while engaged in a secondary task (Dobbs & Rule, 1989). Working memory is traditionally characterized as a system of modality-specific stores subserved by a central executive processing component (Baddeley & Hitch, 1974), and is typically assessed using complex memory span tasks, in which verbal or visuo-spatial stimuli are encoded during the performance of an unrelated task and subsequently recalled (Conway et al., 2005). Recent theoretical approaches have emphasized the control of attention as a key determinant of working memory performance (Engle, 2002), with many accounts attributing individual differences in working memory capacity to the ability to direct attention to the processing and maintenance of task-relevant information and/or to inhibit processing of distracting or irrelevant stimuli (Gazzaley, Cooney, Rissman, & D’Esposito, 2005; Hasher & Zacks, 1988; Hasher et al., 2007; Kane, Conway, Hambrick, & Engle, 2007; Kane & Engle, 2002). The association of working memory function with attentional control has particular implications for comprehension in a multitalker environment, which requires both the direction of attention to a relevant speech stream and the inhibition of distracting information from one or more competing signals. Under this view, older adults with limited working memory capacity are likely to experience greater comprehension difficulties in complex listening situations with multiple simultaneous talkers. Despite the potential implications of individual differences in cognitive functioning for spoken language processing under adverse conditions, previous studies of speech understanding in noise in older adults have focused primarily on behavioural measures in which perceptual demands are high and cognitive demands are relatively low. ‘Speech understanding’ is defined in the literature in terms of intelligibility, i.e., the open-set recognition or the closed-set identification of speech stimuli in quiet or noise (see Humes & Dubno, 2009, for review). Paradigms such as the Coordinate Response Measure (Bolia, Nelson, Ericson, & Simpson, 2000; Humes et al., 2006) and the Speech Perception in Noise test (Kalikow, Stevens, & Elliott, 1977; Pichora-Fuller et al., 1995) require the listener to select

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a target item from a fixed array or to repeat a word or sentence after it is presented, with response accuracy serving as the dependent measure. Thus, in order to observe reliable group differences in performance on these measures, stimulus intelligibility must be sufficiently reduced to produce identification errors. However, spoken language comprehension may be adversely affected by perceptual and cognitive factors in older adults even when identification accuracy is at ceiling. Moreover, offline accuracy measures do not take into account differences in response time, so the extent to which performance reflects early perceptual identification processes or later-stage compensatory mechanisms may vary across individuals. Other studies have used memory tasks as an indirect measure of the processing demands imposed by complex listening situations. Free recall of spoken words is less accurate in both younger and older adults when stimuli are presented under adverse conditions (Murphy, Craik, Li, & Schneider, 2000; Rabbitt, 1968), with older listeners showing particular vulnerability to meaningful competing speech (Tun, O’Kane, & Wingfield, 2002). Mild-to-moderate hearing loss is also associated with poorer recall performance in older adults, indicating that speech identification is more challenging for hearing-impaired listeners (McCoy et al., 2005; Rabbitt, 1991; cf. Wingfield et al., 2005). However, although recall measures are sensitive to the cognitive demands imposed by the listening situation, it is not clear whether adverse listening conditions disrupt recall performance by interfering with the comprehension of the incoming speech stream, thereby reducing the set of linguistic representations that are available to memory processes, or by interfering with the encoding and maintenance of linguistic representations for subsequent retrieval. A comprehensive account of the comprehension difficulties experienced by older listeners must also consider the effect of competing speech on real-time, higher-level language processing, i.e., the online interpretation of linguistic messages conveyed by spoken words and sentences. As speech identification is critical to the initial retrieval of word meanings, reduced intelligibility is likely to have adverse effects on semantic comprehension. The increased demands imposed by complex listening situations may also interfere with the integration of semantic representations in the construction of sentence-level meaning. Psycholinguistic measures that tap into the implicit processing of semantic information may therefore offer further insight into the influence of perceptual and cognitive factors on spoken language comprehension in competing speech in healthy aging. Priming paradigms, which test whether online responses to a target word are influenced by the linguistic context in which the target occurs, have proven a useful tool for investigating semantic comprehension processes in younger adults (McNamara, 2005). Words presented in a sentence context are recognized more rapidly when they are congruent with the sentence meaning (Aydelott & Bates, 2004; Fischler & Bloom, 1979, 1980; Schuberth & Eimas, 1977; Stanovich & West, 1983). This facilitation effect is thought to reflect the contextual activation of compatible representations in semantic memory and the ease of integration of the target word into the ongoing discourse

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context (Kutas & Federmeier, 2000; Schwanenflugel & Lacount, 1988; Schwanenflugel & Shoben, 1985; Traxler & Foss, 2000). The magnitude of facilitation is influenced by the strength of the semantic bias (Traxler & Foss, 2000), indicating that word recognition is highly sensitive to the constraints imposed by the preceding linguistic message. Further, sentence priming in the auditory modality is modulated by the intelligibility of the sentence context, such that facilitation of intact target words is reduced in acoustically degraded sentences (Aydelott & Bates, 2004). The sentence priming paradigm therefore provides an implicit, real-time measure of listeners’ access to semantic information from a spoken sentence under adverse listening conditions. A recent study of auditory semantic processing in younger adults used dichotic sentence priming to assess the effect of competing speech on the comprehension of sentence-level meaning (Aydelott, Baer-Henney, Trzaskowski, Leech, & Dick, 2012). In a lexical decision paradigm, spoken word targets served as completions for strongly and weakly biasing and neutral sentence contexts. Sentence contexts were presented dichotically to a single auditory channel, either in isolation or with a competing speech signal of the same duration presented to the contralateral ear. Competing speech consisted of excised segments from an unrelated text passage read aloud by a single talker. Target words were always presented diotically with no competing speech, and were therefore acoustically intact in all experimental conditions, so that any observed differences in response time were attributable to the availability of semantic cues from the sentence context and not to the intelligibility of the target. Facilitation of congruent targets was measured relative to the neutral context condition, which served as a baseline. At a moderate signal-to-noise ratio (SNR) of 0 dB, competing speech presented to the right ear significantly reduced facilitation of congruent words in both strong and weak contexts presented to the left ear relative to the isolation condition, whereas competing speech presented to the left ear had no effect on priming, consistent with the REA for spoken language stimuli reported in previous studies (Kimura, 1961; Studdert-Kennedy & Shankweiler, 1970; Wada & Rasmussen, 1960; cf. Scott, Blank, Rosen, & Wise, 2000). However, at a more demanding SNR of 12 dB, younger listeners were able to override the REA by attending selectively to the left auditory channel, such that facilitation by strongly biasing contexts was unaffected by right-ear competing speech (Aydelott et al., 2012). Thus, in younger listeners, facilitation priming produced by strong contexts presented to the left ear is robust to right-ear interference under conditions of focused attention. To what extent do perceptual and attentional factors influence sentence-level semantic comprehension in competing speech in older adults? The present study used the dichotic sentence priming paradigm to address this question. This novel method allows us to assess the relationship of individual differences in both sensory and cognitive functioning to real-time, higher-level language comprehension. A group of 20 healthy individuals aged between 50 and 80 years completed the dichotic priming task with competing speech presented at a demanding SNR of

12 dB. Hearing sensitivity was assessed by pure tone audiometry at 0.25–8 kHz, and a non-auditory working memory measure, the Automated Operation Span task (AOSPAN) (Unsworth, Heitz, Schrock, & Engle, 2005), was administered. AOSPAN is a standardized complex span task that has been used extensively as an assessment of working memory capacity (Redick et al., 2012). Participants maintain visually-presented letter sequences in memory while simultaneously performing arithmetical calculations to a high degree of accuracy, and scores reflect the number of items in correctly remembered sequences. In addition to providing an index of individual differences in working memory resources, this measure has been shown to predict the performance of older adults on tasks requiring the strategic control of attention (Hayes, Kelly, & Smith, 2013). We predicted that older listeners’ ability to access semantic information from the speech stream would be associated with individual differences in hearing sensitivity and working memory capacity. As peripheral hearing loss reduces the intelligibility of the incoming signal so that words are less easily identified, semantic information may be less accessible to listeners with decreased hearing sensitivity; therefore, we predicted that hearing loss in the attended ear would reduce the facilitation produced by a biasing sentence context in this group relative to younger adults. Previous research has also suggested that bilateral hearing loss disrupts auditory stream segregation, resulting in failures of selective attention in multitalker situations (Shinn-Cunningham & Best, 2008). If this is the case, older listeners with reduced hearing sensitivity in both ears should also show increased interference from a competing speech signal. In addition, as performance on complex memory span tasks such as AOSPAN is highly correlated with measures of attentional control (Engle, 2002), we predicted that working memory capacity would constrain older participants’ ability to attend selectively to one auditory channel in the presence of competing speech. We predicted that this effect would be strongest in the most demanding condition, i.e., target speech presented to the left ear with right-ear competing speech. In this condition, listeners must override the REA for speech stimuli by focusing attention on the left auditory channel. Younger listeners are able use selective attention to access strong semantic cues in this condition, and show preserved facilitation priming for targets in strongly biasing contexts (Aydelott et al., 2012). We predicted that older adults with reduced working memory capacity would be more vulnerable to interference from right-ear competing speech, and would therefore show reduced facilitation of targets in strong contexts presented to the left ear.

2. Materials and methods 2.1. Stimuli Spoken word targets (e.g., desk) were presented in three auditory sentence context conditions: strong bias, in which the congruent target was the expected completion of the sentence (e.g., In the office, the computer is on my. . .); weak

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bias, in which the congruent target was one of many plausible completions (e.g., They took all of the furniture in the office except for the. . .); and neutral, in which there was no semantic bias (The next item is. . .). Targets (e.g., desk) were either congruent or incongruent with the biasing contexts. Excised segments of recorded speech read aloud from an unrelated textbook passage served as the competing speech signal. All targets, sentence contexts, and competing speech segments were recorded onto digital audio tape in an Industrial Acoustics Corporation 403-A audiometric chamber with a Tascam DA-P1 tape recorder and a Sennheiser ME65/K6 supercardioid microphone and pre-amp at gain levels between 6 and 12 dB. The recorded stimuli were transferred via digital-to-digital sampling onto a Macintosh G4 computer with a Digidesign audio card using ProTools LE software at a sampling rate of 44.1 kHz with a 16-bit quantization. The waveform of each target item, sentence context, and competing speech segment was edited and saved in its own mono audio file in WAV format for subsequent manipulation using Praat software (Boersma, 2001).

sentences contained significantly more words that were semantically related to the target item than weak bias sentences (strong: M = 1.15, SD = 0.61; weak: M = 0.87, SD = 0.83; paired t(59) = 3.43, p < 0.01). A sentence context without a semantic bias in favour of a particular target served as the neutral baseline. Distractor sentence contexts were also generated for the nonword targets. These were strongly or weakly biased in favour of words that were not included in the target set (cloze probability of expected word, strong: M = 71%, SD = 24%; weak: M = 41%, SD = 31%), although they were always presented with nonword targets. Distractor sentence contexts did not differ significantly from test sentence contexts in number of syllables (strong: M = 10.13, SD = 2.83; weak: M = 9.73, SD = 2.67), duration in msec (strong: M = 2188, SD = 513; weak: M = 1995, SD = 485), or number of content words (strong: M = 3.80, SD = 1.39; weak: M = 3.47, SD = 1.23). All sentence contexts were produced by a female native speaker of Southern British English, and the resulting waveforms were scaled to a nominal average intensity level of 60 dB in Praat.

2.1.1. Target items As the experimental conditions were rotated (see Design, below), all word and nonword targets appeared in all conditions. Word targets were 60 one-syllable English words containing three to five phonemes (M = 3.30, SD = 0.65) with a mean duration of 643 ms (SD = 87), a mean Kucera-Francis print frequency of 139 (SD = 104) (Kucera & Francis, 1967), a mean London-Lund spoken frequency of 17 (SD = 25) (Brown, 1984), and a mean concreteness rating of 536 (SD = 87) (all values obtained from the MRC Psycholinguistic Database; Coltheart, 1981). To avoid possible morphological and morpho-phonological constraints on determiners (a/an, the), mass nouns (e.g., blood, dust) were excluded, and all targets were consonant-initial. Nonword distractor targets consisted of 60 phonologically permissible one-syllable nonsense items that did not differ significantly from the word targets in terms of number of phonemes (M = 3.38, SD = 0.58) or duration in msec (M = 666, SD = 112). All target items were produced by a male native speaker of Southern British English, and the resulting waveforms were scaled to a nominal average intensity level of 72 dB in Praat.

2.1.3. Competing speech A passage from the economics textbook Profit Patterns (Slywotzky, 1999) was read aloud by a different female native speaker of Southern British English. Segments of the same duration as each of the sentence contexts were excised at random from this recorded passage, with onset and offset cuts made at zero crossings of the waveform. These excised portions were scaled to a nominal average intensity level of 72 dB in Praat. 2.1.4. Dichotic sentence stimuli For each of the sentence contexts, a stereo sound file was generated with the sentence waveform inserted into the left channel and silence in the right channel. These stereo files served as the isolation condition. In the competing speech condition, the duration-matched excised portions of the competing speech signal were inserted into the right channel of the stereo sound file. All stimuli (dichotic sentence contexts and targets) were converted in SoundEdit 16 to System 7 format for presentation via SuperLab software. 2.2. Design

2.1.2. Sentence contexts Two sentence contexts were created for each target word: one with a strong semantic bias in favour of the target, and one with a weak semantic bias in favour of the target. Twenty-two native speakers of British English evaluated the predictability of the intended target word in each of these sentence contexts using the cloze procedure. Target words had a mean cloze probability of 94% (SD = 7%) in strong bias contexts, and 28% (SD = 19%) in weak bias contexts. Strong and weak bias sentences did not differ significantly in number of syllables (strong: M = 10.03, SD = 2.70; weak: M = 9.98, SD = 2.80), duration in msec (strong: M = 2036, SD = 477; weak: M = 2065, SD = 470), or number of content words (strong: M = 3.65, SD = 1.31; weak: M = 3.40, SD = 1.17); however, strong bias

Strong, weak, and neutral sentence contexts were paired with congruent word targets. Targets also appeared in incongruent contexts (strong and weak) so that the experimental conditions would be identical to Aydelott et al. (2012), to allow for comparison with the performance of younger adults tested using the same paradigm; however, as the present study examined facilitation of congruent targets, incongruent context-target pairings were treated as distractor trials. Thus, targets appeared in a total of five bias conditions, with 12 trials in each condition. For the incongruent conditions, the same sentence contexts were used as in the congruent conditions, paired with different (unexpected) targets. All sentence contexts (biased and neutral) were either presented in isolation

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(half of trials), or with competing speech. Nonword targets were presented in matched experimental conditions. For each participant, each context and target appeared only once during an experimental block, but the conditions in which each context and target appeared were counterbalanced across participants. Thus, all items were presented in all conditions, and all participants received all items. 2.3. Procedure Participants completed pure-tone audiometry and AOSPAN testing prior to the dichotic priming task. The AOSPAN task is a standardized complex span measure in which participants memorize series of letters while performing simultaneous mental calculations (Unsworth et al., 2005). All stimuli are presented visually on a computer screen, and all responses are made by pointing to a region of the screen and clicking with the computer mouse. In each trial sequence, participants see an arithmetic equation, indicate whether the answer shown is true or false, and then see the first letter in the series. This is followed by another equation, and then the next letter in the series is presented. After a sequence of between three and seven encoding trials, participants are asked to reproduce the series of letters in the correct order. Participants are required to maintain an accuracy level of 85% or above on the arithmetic task, and feedback is provided after every trial sequence for both the arithmetic and memory tasks. Scores reflect the total number of items in all correctly reproduced letter series. The procedure for the dichotic priming task was identical to that used in Aydelott et al. (2012), with the exception that participants received both the right and left ear-ofpresentation conditions. These conditions were presented in separate blocks; order of presentation of blocks was counterbalanced across participants, and each participant received different context-target pairings for each ear of presentation. The structure of experimental trials is illustrated in Fig. 1. Sentence contexts were presented to the left or right auditory channel, either in isolation or with competing speech presented to the other channel. Targets

were then presented diotically to both channels without competing speech; this ensured that any observed effects of competing speech reflected the availability of semantic cues from the contexts, rather than the intelligibility of the targets. Order of presentation of trials was randomized within each block. Participants were instructed to attend to the ear in which the sentence context was presented while ignoring the other channel, and to make a lexical decision response (Is the target a real word? YES/NO) to the target item by pressing one of two buttons on a response pad. Reaction times (RTs) and accuracy were recorded. Harmonic mean RTs were calculated for congruent targets in strong bias, weak bias, and neutral contexts. The harmonic mean was used to minimize the influence of RT outliers (Ratcliff, 1993). As in Aydelott et al. (2012), inverse efficiency (IE) scores were calculated by dividing the harmonic mean RT by the proportion correct for each participant in each condition, resulting in a combined value reflecting both RT and accuracy. The IE calculation was performed so that the results could be compared statistically with the published data from younger adults, which were analyzed using this method. It is worth noting, however, that older adults’ performance on the lexical decision task was highly accurate for congruent targets (99.93%); thus, IE scores reflected the same pattern of results observed in the raw RTs. Mean raw RTs are displayed in Table 1. To control for individual variation in RT, facilitation was calculated relative to the neutral baseline condition for each participant. This is particularly important when comparing groups whose overall RTs are likely to differ (i.e., younger and older adults; Salthouse, 2000), as the magnitude of priming can be overestimated in populations with slower RTs when absolute difference values are used (Chapman, Chapman, Curran, & Miller, 1994). As in Aydelott et al. (2012), proportion priming in strong and weak bias conditions was computed by subtracting IE scores in the biasing conditions from the neutral condition and dividing by the neutral condition. Facilitation priming is therefore reflected in negative values, which represent faster and more accurate performance in biasing conditions relative to the neutral baseline.

Fig. 1. Structure of experimental trials in the dichotic sentence priming paradigm. Sentence contexts were presented to the left or right auditory channel, either in isolation or with competing speech presented to the other channel. Targets were then presented diotically to both channels without competing speech.

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P.J. James et al. / Cognition 133 (2014) 32–42 Table 1 Mean (and standard deviation) RTs for older adults in the dichotic priming paradigm. Ear of presentation

Bias

Competing speech

Mean RT (SD)

Left

Strong

Isolation Competing signal Isolation Competing signal Isolation Competing signal

1167 1250 1226 1301 1338 1338

(103) (182) (102) (147) (120) (129)

Isolation Competing signal Isolation Competing signal Isolation Competing signal

1173 1241 1252 1265 1340 1343

(169) (174) (136) (169) (165) (139)

Weak Neutral Right

Strong Weak Neutral

2.4. Apparatus Audiometric screening and AOSPAN testing were administered on a Dell Precision T3500 PC running Windows 7. An Otovation OTOPod M2 audiometer with TDH39 headphones and Amplivox Audiocups was managed wirelessly using Symphony NOAH software. AOSPAN was run as an E-Prime 2.0 script via a Dell 2009W 20-in. flat panel monitor and an ATI FirePro 2260 graphics card. The dichotic priming task was administered on an Apple eMac using SuperLab Pro 1.77 software, a Cedrus RB-730 response pad, and Sennheiser HD25-1 II headphones. All tests were completed in a sound-treated room. 2.5. Participants Volunteers were recruited from the Greater London area via Birkbeck’s SONA experiment management system, online classified advertisements, emails to managers of distribution lists relevant to older adults, and fliers posted at local community centres. Twenty right-handed native British English speakers without history of neurological illness between the ages of 50 and 80 years were paid for their participation. Participants’ education levels ranged from 10–23 years of formal study (M = 15.63, SD = 3.31; N = 4 educated to secondary level, N = 9 to university level, N = 7 to postgraduate level). The participant group had an average age at time of testing of 61.96 years (SD = 7.43, range 50.28–78.08), and an average AOSPAN score of 40.50 (SD = 16.73, range 13–69), consistent with the broad range of working memory capacity observed in the normative data for this measure (Redick et al., 2012). Pure tone average hearing loss (PTA HL) in the speech frequencies (0.5, 1, and 2 kHz) in this group was 13.08 dB in the left ear (SD = 9.71, range 5.00–36.67), and 12.75 dB in the right ear (SD = 9.34, range 1.67–33.33). Chronological age was positively correlated with PTA HL in both the left ear, Pearson r(18) = .608, p = .004, and the right ear, r(18) = .583, p = .007. AOSPAN score was not significantly correlated with age, years of education, or PTA HL in either ear, all p > .30. 3. Results Mean proportion priming scores in each experimental condition are shown in Table 2. A four-way repeated

measures analysis of variance (ANOVA) was conducted on the facilitation priming data with Ear of Presentation of the sentence context (right versus left), Bias Strength (strong versus weak), and Competing Speech (isolation versus competing signal) as within-subjects factors; Order of Presentation of ear conditions (right-ear condition first versus left-ear condition first) was included as a between-subjects factor to rule out the possible influence of this variable on the within-subjects effects. A significant main effect of Bias Strength emerged, F(1, 19) = 7.02, p = .016, partial g2 = .28, reflecting greater facilitation priming in strong contexts than in weak contexts (strong: M = 0.09, SD = 0.08; weak: M = 0.05, SD = 0.06). A significant main effect of Competing Speech was also observed, F(1, 19) = 7.08, p = .016, partial g2 = .28, indicating that the presence of a competing signal reduced facilitation overall (isolation: M = 0.09, SD = 0.08; competing signal: M = 0.05, SD = 0.07). No significant main effects of Ear of Presentation (F(1, 18) = 0.01, p = .931, partial g2 < .01) or Order of Presentation (F(1, 18) = 0.12, p = .738, partial g2 = .06) emerged, and there were no significant interactions (all p > .13). This pattern of results differs from the performance of younger adults aged 18 to 40 years reported in Aydelott et al. (2012), who showed significant ear-of-presentation effects on the magnitude of facilitation priming in competing speech. The original data from the two ear of presentation groups in the Aydelott et al. (2012) study were compared to the present results in separate three-way ANOVAs, with Bias Strength and Competing Speech as within-subjects variables and Age Group (older adults versus younger adults) as a between-subjects variable. Data from this analysis are plotted in Fig. 2. When sentence contexts were presented to the right ear, significant main effects of Bias Strength, F(1, 27) = 5.87, p = .022, partial g2 = .18, and Competing Speech, F(1, 27) = 5.29, p = .029, partial g2 = .16, were observed, with no significant interaction of Age Group x Bias Strength x Competing Speech, F(1, 27) = 0.05, p = .82, partial g2 = .002, and no two-way interactions. However, when sentence contexts were presented to the left ear, the interaction of Age Group  Bias Strength  Competing Speech was at the threshold of significance, F(1, 27) = 4.11, p = .052, partial g2 = .13. Separate two-way ANOVAs conducted for each age group revealed a significant Bias Strength  Competing Speech interaction for younger adults, F(1, 8) = 5.30, p = .041, partial g2 = .43, but not for older adults, F(1, 19) = 0.23, p = .637, partial Table 2 Mean (and standard deviation) proportion facilitation priming scores for older adults in the dichotic priming paradigm. Ear of presentation

Bias

Competing speech

Left

Strong

Isolation Competing signal Isolation Competing signal

0.13 0.06 0.07 0.02

(0.08) (0.14) (0.09) (0.10)

Isolation Competing signal Isolation Competing signal

0.12 0.07 0.06 0.05

(0.15) (0.14) (0.14) (0.13)

Weak Right

Strong Weak

Mean proportion priming (SD)

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Fig. 2. Facilitation priming of targets in biasing sentence contexts presented to the left ear, either in isolation or with right-ear competing speech, in younger adults (aged 18–40 years; Aydelott et al., 2012) and older adults (aged 50–80 years). Error bars represent standard error. Note that values on the yaxis are plotted in reverse order, as more negative scores represent increased facilitation priming (i.e., faster and more accurate responses relative to the neutral baseline condition).

g2 = .01 (see Fig. 2). No significant main effect of Age Group emerged in either analysis. To assess the contributions of hearing sensitivity and memory span to the pattern of performance observed in older adults, Pearson correlations of the magnitude of facilitation priming in each experimental condition with chronological age, PTA HL in each ear, and AOSPAN score were conducted. A strong and highly significant correlation of AOSPAN with facilitation in right-ear competing speech was observed for strong contexts presented to the left ear, r(18) = .741, p < .001. These data are plotted in Fig. 3. As more negative values represent increased facilitation (i.e., faster and more accurate responses relative to the neutral baseline), this negative correlation reflects greater facilitation priming with increasing AOSPAN scores. A moderate, marginally significant correlation was also

observed for weak contexts under the same listening conditions, r(18) = .409, p = .073. No significant correlations of chronological age or PTA HL in either ear with facilitation were observed in any experimental condition, and no other significant correlations with AOSPAN emerged, all p > .12. A set of regression models were then constructed to determine the unique contributions of each predictor. To avoid model overfitting, only those predictors that singly could account for at least a moderate amount of variance in each condition (defined here as a correlation of r P 0.30) were included in the model. All combinations of predictors were used to create the set of models, e.g., for predictors A & B, possible models were (i) A & B, (ii) A, and (iii) B. Results of these models are reported ranked by proportion of variance accounted for; all analyses report

Fig. 3. Correlation of AOSPAN with facilitation priming of targets in strongly biasing sentence contexts presented to the left ear with right-ear competing speech in older adults. Note that values on the y-axis are plotted in reverse order, as more negative scores represent increased facilitation priming (i.e., faster and more accurate responses relative to the neutral baseline condition).

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adjusted-R2 (henceforth adj-R2) to allow for model comparison across a different number of predictors (Leach & Henson, 2007); individual predictors that contributed significant unique variance to each model are also indicated. All analyses were conducted in Stata/SE 11.1 using distribution-independent bootstrap techniques to estimate probability and variance measures (10,000 replication samples per model). Bootstrapped regression analysis (Efron, 1979; Efron & Tibshirani, 1986; cf. Howell, 2007) has the advantage of being robust to false positive results that may emerge in smaller and non-normally distributed samples. In the condition in which strong contexts were presented to the left ear in right-ear competing speech, two predictors (left ear PTA HL and AOSPAN) met our criterion. The model with both these predictors accounted for the most variance (adj-R2 = 0.5332, p < .0001) with only AOSPAN scores accounting for unique variance (z = 4.36, p < .001). AOSPAN scores alone accounted for significant variance in this condition (adj-R2 = 0.5242, p < .0001), whereas LE PTA scores did not (p > .12). In all other conditions, either the criterion for predictors was not met, or none of the models accounted for significant variance (all p > .11). 4. Discussion This study used a dichotic sentence priming paradigm to examine the effects of competing speech on sentencelevel semantic comprehension in older adults. Older listeners showed a different pattern of performance from younger listeners on the dichotic sentence priming task, and this difference was driven by one experimental condition: unlike younger adults, older listeners failed to demonstrate preserved facilitation priming for strong contexts presented to the left ear when competing speech was presented to the right ear. This finding is consistent with our theoretical predictions. In this condition, listeners must direct attention to the left auditory channel and override the REA typically observed for language stimuli (cf. Hugdahl & Andersson, 1986). In younger adults, selective attention to the left channel in the presence of right-ear competing speech allows sentence-level semantic integration processes to influence responses to target words, resulting in spared facilitation of congruent targets (Aydelott et al., 2012). The results of the present study demonstrate that older adults are more vulnerable than younger adults to interference from a right-ear competing signal, resulting in reduced access to sentence-level semantic cues. The results also supported our prediction that facilitation in this condition would be associated with individual differences in cognitive function. Older adults’ performance on a measure of working memory capacity, AOSPAN, was strongly correlated with facilitation by strong contexts presented to the left ear in right-ear competing speech. Recent accounts of working memory attribute performance on complex span tasks to attentional control (Engle, 2002; Gazzaley, Cooney, Rissman, & D’Esposito, 2005; Hasher & Zacks, 1988; Hasher et al., 2007; Kane & Engle, 2002; Kane et al., 2007). Importantly,

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AOSPAN did not predict facilitation in older adults when the context sentences were presented in isolation, or when a competing signal was presented to the left ear, for either strong or weak contexts. Thus, greater working memory capacity was not associated with an increased use of semantic cues to anticipate upcoming words in a sentence, nor did it predict priming performance in competing speech when the attended sentence benefitted from the REA. Instead, working memory predicted facilitation only when competing speech was presented to the right ear, such that processing of the attended sentence was at a disadvantage relative to the competing signal – the critical condition that differentiated the performance of older and younger adults in the present study. As noted above, reversal of the REA depends upon the capacity to direct attention toward the relevant signal in the left channel and to ignore the irrelevant signal in the right channel. This result indicates that older adults with reduced working memory capacity are more vulnerable to interference from competing speech when the processing of the attended signal requires the allocation of attention away from the prepotent auditory channel. These findings provide strong evidence that cognitive factors play a key role in spoken language comprehension in older adults under complex listening conditions, and demonstrate that individual differences in cognitive ability are relevant to real-time spoken language comprehension. It is worth noting that AOSPAN performance was not significantly correlated with chronological age in our sample. Thus, for this group of older individuals, increasing age from 50 to 80 years was not associated with a general decline in working memory capacity. This result may appear counterintuitive given the extensive literature on age-related decrements in executive function. However, whereas an overall decline in performance on working memory measures is typically observed across the lifespan, there is also considerable individual variability within the population of older adults (Carmichael et al., 2012; cf. Christensen et al., 1999; Wilson et al., 2002). Further, prefrontal cortical volume has been shown to be a better predictor of executive function than chronological age in healthy, active older adults over 60 (Elderkin-Thompson, Ballmaier, Hellemann, Pham, & Kumar, 2008). Our sample of healthy older adults shows substantial variability in working memory performance, which likely reflects influences such as differences in grey matter volume that are more directly related to cognitive functioning than chronological age. Moreover, our findings are consistent with previous findings that higher-level cognitive function is relatively preserved in some older adults (Rowe & Kahn, 1997). Factors contributing to spared cognitive ability in older adults include greater levels of physical and intellectual activity and cardiovascular health, with some studies indicating enhanced prefrontal brain activation in highperforming individuals (see Daffner, 2010, for review). High-functioning older adults can draw upon these relatively intact cognitive resources to mitigate the perceptual and attentional demands of listening situations involving multiple simultaneous talkers. The present findings suggest that preserved working memory capacity may be one indicator of resilience to comprehension difficulties

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in competing speech in older listeners. If this is the case, our results may have important clinical implications for the everyday management and remediation of sentence comprehension difficulties in noisy listening situations (cf. Aydelott, Leech, & Crinion, 2010). For example, recent studies have indicated that intensive adaptive training can lead to increases in working memory capacity (see Klingberg, 2010, for review), which raises the possibility that such training could improve speech-in-speech understanding in older listeners (cf. Moore & Füllgrabe, 2012). However, the extent to which training-induced enhancements in working memory performance transfer to other cognitive skills remains a subject of debate (Shipstead, Redick, & Engle, 2012). Future research is needed to determine whether cognitive interventions will be effective in mitigating the comprehension difficulties of older listeners in complex auditory environments. We also predicted that declines in hearing sensitivity would be associated with reduced facilitation priming for sentences presented in isolation and increased interference from competing speech. However, no correlation between PTA HL and facilitation was observed in any of the experimental conditions in the present study. Thus, age-related hearing loss did not influence listeners’ ability to access sentence-level semantic information from speech presented in isolation, nor did it increase the level of interference produced by dichotically presented competing speech. The degree of hearing loss for this group of older listeners ranged from normal (PTA < 15 dB) to mildly impaired (PTA 26–40 dB) (Clark, 1981). Mild hearing impairment is associated with decrements in recall performance for lists of spoken words, even when hearingimpaired listeners are able to repeat the to-be-recalled words correctly on first presentation (McCoy et al., 2005; Rabbitt, 1991). This effect has been attributed to the increased cognitive demand associated with interpreting a degraded sensory input, which may contribute to the auditory comprehension difficulties experienced by older listeners (the ‘effortfulness’ hypothesis; McCoy et al., 2005; Pichora-Fuller, 2008; Schneider & Pichora-Fuller, 2000; Wingfield et al., 2005). The present results suggest that implicit, online access to sentence-level semantic information is not disrupted by mild hearing loss in older listeners. However, the majority of participants in the present study had normal hearing thresholds in both ears (N = 9) or slight hearing loss (PTA 16–25 dB) in one or both ears (N = 8), with only three participants classified as having a hearing impairment in the mild range. Comparison with a sample of older participants with mild-to-moderate hearing impairments may reveal more substantial effects of hearing sensitivity on the processing of contextual cues. In summary, the present results demonstrate that a non-auditory measure of executive control predicts older listeners’ ability to access sentence-level semantic information in competing speech when the availability of contextual cues depends upon the allocation of attention to the left auditory channel. These findings highlight the importance of cognitive factors in spoken language comprehension under complex listening conditions, and suggest that working memory capacity limits older adults’

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