Do people with cognitive impairment benefit from cochlear implants? A scoping review.

PURPOSE: To identify and evaluate the evidence for the benefits of cochlear implants for people with cognitive impairment or dementia in terms of speech recognition, quality of life, behavioural and psychological symptoms of dementia, cognition, function in daily life, mental well-being, and caregiver burden. METHODS: Ten electronic databases were searched systematically from inception to December 2023 for studies reporting on outcomes for cochlear implants that included adults identified with cognitive impairment, mild cognitive impairment, or dementia. RESULTS: Thirteen studies were included in this review with a combined total of 222 cochlear implant patients with cognitive impairment, mild cognitive impairment. Two studies were non-randomised controlled design, the remainder were single group studies, case series or single case studies. Evidence suggested that people with cognitive impairment benefit in terms of improved speech recognition from cochlear implants, although they may benefit less than those with healthy cognition and the degree of benefit depends on the level of cognitive impairment. There was no evidence for increased adverse events among those with cognitive impairment. There was limited or no evidence for any other outcome. CONCLUSION: People with cognitive impairment or dementia do benefit from cochlear implants. To inform policy and clinical practice, further data are needed about the broader benefits of cochlear implants for people with cognitive impairment or dementia, and referral, eligibility, and cochlear implant support needs for people with cognitive impairment and their caregivers.

Divergent effects of listening demands and evaluative threat on listening effort in online and laboratory settings.

Objective: Listening effort (LE) varies as a function of listening demands, motivation and resource availability, among other things. Motivation is posited to have a greater influence on listening effort under high, compared to low, listening demands. Methods: To test this prediction, we manipulated the listening demands of a speech recognition task using tone vocoders to create moderate and high listening demand conditions. We manipulated motivation using evaluative threat, i.e., informing participants that they must reach a particular "score" for their results to be usable. Resource availability was assessed by means of working memory span and included as a fixed effects predictor. Outcome measures were indices of LE, including reaction times (RTs), self-rated work and self-rated tiredness, in addition to task performance (correct response rates). Given the recent popularity of online studies, we also wanted to examine the effect of experimental context (online vs. laboratory) on the efficacy of manipulations of listening demands and motivation. We carried out two highly similar experiments with two groups of 37 young adults, a laboratory experiment and an online experiment. To make listening demands comparable between the two studies, vocoder settings had to differ. All results were analysed using linear mixed models. Results: Results showed that under laboratory conditions, listening demands affected all outcomes, with significantly lower correct response rates, slower RTs and greater self-rated work with higher listening demands. In the online study, listening demands only affected RTs. In addition, motivation affected self-rated work. Resource availability was only a significant predictor for RTs in the online study. Discussion: These results show that the influence of motivation and listening demands on LE depends on the type of outcome measures used and the experimental context. It may also depend on the exact vocoder settings. A controlled laboratory settings and/or particular vocoder settings may be necessary to observe all expected effects of listening demands and motivation.

"We're just winging it". Identifying targets for intervention to improve the provision of hearing support for residents living with dementia in long-term care: an interview study with care staff.

PURPOSE: Hearing loss and dementia are common in long-term care home (LTCH) residents, causing communication difficulties and worsened behavioural symptoms. Hearing support provided to residents with dementia requires improvement. This study is the first to use the Behaviour Change Wheel (BCW) to identify barriers and propose interventions to improve the provision of hearing support by LTCH staff. METHODS: Semi-structured interviews with 10 staff members were conducted. Transcripts were analysed according to the BCW's Theoretical Domains Framework alongside reflective thematic analysis. Relevant intervention functions and exemplar interventions were proposed. RESULTS: Staff believed hearing support to be beneficial to residents (Beliefs about Consequences) but lacked knowledge of hearing loss management (Knowledge). Poor collaborations between LTCHs and audiology (Environmental Context and Resources), led to despondency, and apprehension about traditional hearing aids for residents (Optimism). Despite feeling responsible for hearing support, staff lacked personal accountability (Social/Professional Role and Identity). CONCLUSIONS: Future interventions should include staff Training (on hearing support), Education (on the consequences of unsupported hearing loss), Enablement (dementia-friendly hearing devices), Incentivisation and Modelling (of Hearing Champions) and Environmental Restructuring (flexible audiology appointments to take place within the LTCH). Interventions should be multi-faceted to boost the capabilities, opportunities and motivations of LTCH staff.

Hearing support for care home residents with dementia:Long-term care staff report inadequate knowledge and awareness of how to support residents' hearing needs and a lack of personal accountability for providing hearing support.They also report poor collaborations with audiologists and apprehension about traditional hearing aids.Barriers to hearing support stem from gaps in the capabilities, opportunities and motivations of staff, therefore, interventions should be designed to target all three constructs.Interventions to aid hearing support provision should target staffs' education, training, enablement, persuasion, modelling, incentivisation and environmental restructuring to boost staff capabilities, opportunities and motivations to provide hearing support.

Capabilities, opportunities and motivations of staff to provide hearing support to long-term care home residents with dementia.

OBJECTIVE: Many long-term care home (LTCH) residents have dementia and hearing loss, causing communication difficulties and agitation. Residents rely on staff for hearing support, but provision is often inconsistent. This study used the Behaviour Change Wheel's Capability, Opportunity and Motivation model to understand why LTCH staff do or do not, provide hearing support to residents with dementia who they believe could benefit from it. DESIGN: An online survey exploring hearing support provision, capabilities, opportunities, motivations and demographics. Data were analysed using descriptive statistics, within-participants ANOVA and multiple linear regression. STUDY SAMPLE: 165 LTCH staff. RESULTS: Staff provided hearing support to 50% of residents with dementia who they thought would benefit. Self-reported physical and psychological capabilities (skills/knowledge) were significantly higher than physical opportunity (having time/resources). The physical capability was significantly higher than social opportunity (collaborative working) and reflective motivation (feeling motivated). Lower levels of hearing support provision were predicted by LTCH funding (private vs. local authority), job role (care assistant vs. nurse) and fewer physical opportunities. CONCLUSIONS: Boosting capabilities through training alone may not be as effective as increasing opportunities via environmental restructuring. Opportunities may include strengthening working relationships with audiologists and ensuring hearing and communication aids are available within LTCHs.

Quantifying the Effects of Motivation on Listening Effort: A Systematic Review and Meta-Analysis.

Motivation influences the amount of listening effort (LE) exerted or experienced under challenging conditions, such as in high-noise environments. This systematic review and meta-analysis is the first to quantify the effects of motivation on LE. The review was pre-registered in PROSPERO, and performed in accordance with PRISMA guidelines. Eligible studies examined the influence of motivation or individual traits (related to motivation) on LE in adults. Motivational factors, coded as independent variables, included financial reward, evaluative threat, perceived competence, feedback, and individual traits. LE outcomes were categorized as subjective, behavioral, or physiological. The quality of evidence was assessed using an adaptation of the Cochrane Collaboration Risk of Bias Tool. Nested random-effects meta-analyses were performed to quantify and compare the influence of motivational factors across LE outcomes. After assessing 3,532 records, 48 studies met the inclusion criteria and 43 were included in the meta-analyses. Risk of bias was high, for example, many studies lacked sample size justification. Motivational factors had a small-to-medium effect (mean Cohen's d = 0.34, range: 0.11-0.72) on LE. When LE outcomes were considered collectively, an external manipulation of motivation (perceived competence) produced a larger mean effect size compared with individual traits. Some combinations of motivational factors and LE outcomes produced more robust effects than others, for example, evaluative threat and subjective LE outcomes. Although wide prediction intervals and high risk of bias mean that significant positive effects cannot be guaranteed, these findings provide useful guidance on the selection of motivational factors and LE outcomes for future research.

Associations between pre-stimulus alpha power, hearing level and performance in a digits-in-noise task.

OBJECTIVE: Baseline electroencephalography (EEG) alpha power, i.e. that measured prior to stimulus presentation, is a potential objective predictor of task performance. Here we assessed the predictive power of EEG alpha on performance accuracy in a digits-in-noise recognition task, factoring in hearing thresholds and age. DESIGN: EEG alpha power, recorded while participants listened to target digits presented in a noise background, was analysed during two different baseline periods: i) a pre-stimulus baseline (pre-STIM) free from any acoustic stimulus, and ii) a pre-target baseline (pre-TARG) recorded in background noise only. STUDY SAMPLE: Eighty-five participants with either normal hearing or aided hearing impairment (age range: 55-85 years old, 42 male). RESULTS: Hierarchical multiple regression analyses indicated that i) lower hearing thresholds and, to a lesser extent, higher pre-STIM alpha power were associated with improved performance accuracy ii) alpha power in pre-STIM and pre-TARG were highly correlated across individuals but pre-TARG alpha power was not a significant predictor of performance accuracy. CONCLUSION: Investigations of baseline EEG alpha power as a predictor of speech-in-noise performance accuracy should control for associations between hearing thresholds and measures of EEG baseline periods.

Effectiveness of Hearing Rehabilitation for Care Home Residents With Dementia: A Systematic Review.

OBJECTIVES: To report the effectiveness of, and barriers and facilitators to, hearing rehabilitation for care home residents with dementia. DESIGN: Systematic review. SETTING AND PARTICIPANTS: Care home residents with dementia and hearing loss. METHODS: No restrictions on publication date or language were set and gray literature was considered. Eligible studies were critically appraised and presented via a narrative review. RESULTS: Sixteen studies, most of low to moderate quality, were identified. Hearing rehabilitation, including hearing devices, communication techniques, and visual aids (eg, flashcards), was reported to improve residents' communication and quality of life and reduce agitation, with improvements in staff knowledge of hearing loss and job satisfaction. Residents' symptoms of dementia presented barriers, for example, losing or not tolerating hearing aids. Low staff prioritization of hearing loss due to time pressures and lack of hearing-related training for staff were further barriers, particularly for residents who required assistance with hearing devices. Adopting a person-centered approach based on residents' capabilities and preferences and involving family members facilitated hearing device use. CONCLUSIONS AND IMPLICATIONS: Residents with dementia can benefit from hearing rehabilitation. Identifying and implementing efficient, individualized hearing rehabilitation is necessary for those with complex cognitive needs. Increased funding and support for the social care sector is required to address systemic issues that pose barriers to hearing rehabilitation, including time pressures, lack of training for staff and access to audiology services for residents.

Financial reward has differential effects on behavioural and self-report measures of listening effort.

OBJECTIVES: To investigate the effects of listening demands and motivation on listening effort (LE) in a novel speech recognition task. DESIGN: We manipulated listening demands and motivation using vocoded speech and financial reward, respectively, and measured task performance (correct response rate) and indices of LE (response times (RTs), subjective ratings of LE and likelihood of giving up). Effects of inter-individual differences in cognitive skills and personality on task performance and LE were also assessed within the context of the Cognitive Energetics Theory (CET). STUDY SAMPLE: Twenty-four participants with normal-hearing (age range: 19 - 33 years, 6 male). RESULTS: High listening demands decreased the correct response rate and increased RTs, self-rated LE and self-rated likelihood of giving up. High financial reward increased subjective LE ratings only. Mixed-effects modelling showed small fixed effects for competitiveness on LE measured using RTs. Small fixed effects were found for cognitive skills (lexical decision RTs and backwards digit span) on LE measured using RTs and correct response rate, respectively. CONCLUSIONS: The effects of listening demands on LE in the speech recognition task aligned with CET, whereas predictions regarding the influence of motivation, cognitive skills and personality were only partially supported.

Dimensions of self-reported listening effort and fatigue on a digits-in-noise task, and association with baseline pupil size and performance accuracy.

OBJECTIVE: Pupillometry is sensitive to cognitive resource allocation and has been used as a potential measure of listening-related effort and fatigue. We investigated associations between peak pupil diameter, pre-stimulus pupil diameter, performance on a listening task, and the dimensionality of self-reported outcomes (task-related listening effort and fatigue). DESIGN: Pupillometry was recorded while participants performed a speech-in-noise task. Participants rated their experience of listening effort and fatigue using the NASA-Task Load Index (NASA-TLX) and the Visual Analogue Scale of Fatigue (VAS-F), respectively. The dimensionality of the NASA-TLX and the VAS-F was investigated using factor analysis. STUDY SAMPLE: 82 participants with either normal hearing or aided hearing impairment (age range: 55-85 years old, 43 male). RESULTS: Hierarchal linear regression analyses suggested that pre-stimulus pupil diameter predicts a dimension of self-reported fatigue, which we interpreted as tiredness/drowsiness, and listening task performance when controlling for hearing level and age: Larger pre-stimulus pupil diameter was associated with less tiredness/drowsiness and better task performance. CONCLUSION: Pre-stimulus pupil diameter is a potential index of listening fatigue associated with speech processing in challenging listening conditions. To our knowledge, this is the first investigation of the associations between pre-stimulus pupil diameter and self-reported ratings of listening effort and fatigue.

The association between subcortical and cortical fMRI and lifetime noise exposure in listeners with normal hearing thresholds.

In animal models, exposure to high noise levels can cause permanent damage to hair-cell synapses (cochlear synaptopathy) for high-threshold auditory nerve fibers without affecting sensitivity to quiet sounds. This has been confirmed in several mammalian species, but the hypothesis that lifetime noise exposure affects auditory function in humans with normal audiometric thresholds remains unconfirmed and current evidence from human electrophysiology is contradictory. Here we report the auditory brainstem response (ABR), and both transient (stimulus onset and offset) and sustained functional magnetic resonance imaging (fMRI) responses throughout the human central auditory pathway across lifetime noise exposure. Healthy young individuals aged 25-40 years were recruited into high (n = 32) and low (n = 30) lifetime noise exposure groups, stratified for age, and balanced for audiometric threshold up to 16 kHz fMRI demonstrated robust broadband noise-related activity throughout the auditory pathway (cochlear nucleus, superior olivary complex, nucleus of the lateral lemniscus, inferior colliculus, medial geniculate body and auditory cortex). fMRI responses in the auditory pathway to broadband noise onset were significantly enhanced in the high noise exposure group relative to the low exposure group, differences in sustained fMRI responses did not reach significance, and no significant group differences were found in the click-evoked ABR. Exploratory analyses found no significant relationships between the neural responses and self-reported tinnitus or reduced sound-level tolerance (symptoms associated with synaptopathy). In summary, although a small effect, these fMRI results suggest that lifetime noise exposure may be associated with central hyperactivity in young adults with normal hearing thresholds.

No Effect of Interstimulus Interval on Acoustic Reflex Thresholds.

The acoustic reflex (AR), a longstanding component of the audiological test battery, has received renewed attention in the context of noise-induced cochlear synaptopathy-the destruction of synapses between inner hair cells and auditory nerve fibers. Noninvasive proxy measures of synaptopathy are widely sought, and AR thresholds (ARTs) correlate closely with synaptic survival in rodents. However, measurement in humans at high stimulus frequencies-likely important when testing for noise-induced pathology-can be challenging; reflexes at 4 kHz are frequently absent or occur only at high stimulus levels, even in young people with clinically normal audiograms. This phenomenon may partly reflect differences across stimulus frequency in the temporal characteristics of the response; later onset of the response, earlier onset of adaptation, and higher rate of adaptation have been observed at 4 kHz than at 1 kHz. One temporal aspect of the response that has received little attention is the interstimulus interval (ISI); inadequate duration of ISI might lead to incomplete recovery of the response between successive presentations and consequent response fatigue. This research aimed to test for effects of ISI on ARTs in normally hearing young humans, measured at 1 and 4 kHz. Contrary to our hypotheses, increasing ISIs from 2.5 to 8.5 s did not reduce ART level, nor raise ART reliability. Results confirm that clinically measured ARTs-including those at 4 kHz-can exhibit excellent reliability and that relatively short (2.5 s) ISIs are adequate for the measurement of sensitive and reliable ARTs.

Effects of Age and Noise Exposure on Proxy Measures of Cochlear Synaptopathy.

Although there is strong histological evidence for age-related synaptopathy in humans, evidence for the existence of noise-induced cochlear synaptopathy in humans is inconclusive. Here, we sought to evaluate the relative contributions of age and noise exposure to cochlear synaptopathy using a series of electrophysiological and behavioral measures. We extended an existing cohort by including 33 adults in the age range 37 to 60, resulting in a total of 156 participants, with the additional older participants resulting in a weakening of the correlation between lifetime noise exposure and age. We used six independent regression models (corrected for multiple comparisons), in which age, lifetime noise exposure, and high-frequency audiometric thresholds were used to predict measures of synaptopathy, with a focus on differential measures. The models for auditory brainstem responses, envelope-following responses, interaural phase discrimination, and the co-ordinate response measure of speech perception were not statistically significant. However, both age and noise exposure were significant predictors of performance on the digit triplet test of speech perception in noise, with greater noise exposure (unexpectedly) predicting better performance in the 80 dB sound pressure level (SPL) condition and greater age predicting better performance in the 40 dB SPL condition. Amplitude modulation detection thresholds were also significantly predicted by age, with older listeners performing better than younger listeners at 80 dB SPL. Overall, the results are inconsistent with the predicted effects of synaptopathy.

Measures of Listening Effort Are Multidimensional.

OBJECTIVES: Listening effort can be defined as the cognitive resources required to perform a listening task. The literature on listening effort is as confusing as it is voluminous: measures of listening effort rarely correlate with each other and sometimes result in contradictory findings. Here, we directly compared simultaneously recorded multimodal measures of listening effort. After establishing the reliability of the measures, we investigated validity by quantifying correlations between measures and then grouping-related measures through factor analysis. DESIGN: One hundred and sixteen participants with audiometric thresholds ranging from normal to severe hearing loss took part in the study (age range: 55 to 85 years old, 50.3% male). We simultaneously measured pupil size, electroencephalographic alpha power, skin conductance, and self-report listening effort. One self-report measure of fatigue was also included. The signal to noise ratio (SNR) was adjusted at 71% criterion performance using sequences of 3 digits. The main listening task involved correct recall of a random digit from a sequence of six presented at a SNR where performance was around 82 to 93%. Test-retest reliability of the measures was established by retesting 30 participants 7 days after the initial session. RESULTS: With the exception of skin conductance and the self-report measure of fatigue, interclass correlation coefficients (ICC) revealed good test-retest reliability (minimum ICC: 0.71). Weak or nonsignificant correlations were identified between measures. Factor analysis, using only the reliable measures, revealed four underlying dimensions: factor 1 included SNR, hearing level, baseline alpha power, and performance accuracy; factor 2 included pupillometry; factor 3 included alpha power (during speech presentation and during retention); factor 4 included self-reported listening effort and baseline alpha power. CONCLUSIONS: The good ICC suggests that poor test reliability is not the reason for the lack of correlation between measures. We have demonstrated that measures traditionally used as indicators of listening effort tap into multiple underlying dimensions. We therefore propose that there is no "gold standard" measure of listening effort and that different measures of listening effort should not be used interchangeably. When choosing method(s) to measure listening effort, the nature of the task and aspects of increased listening demands that are of interest should be taken into account. The findings of this study provide a framework for understanding and interpreting listening effort measures.

Anterior paracingulate and cingulate cortex mediates the effects of cognitive load on speech sound discrimination.

Perceiving speech while performing another task is a common challenge in everyday life. How the brain controls resource allocation during speech perception remains poorly understood. Using functional magnetic resonance imaging (fMRI), we investigated the effect of cognitive load on speech perception by examining brain responses of participants performing a phoneme discrimination task and a visual working memory task simultaneously. The visual task involved holding either a single meaningless image in working memory (low cognitive load) or four different images (high cognitive load). Performing the speech task under high load, compared to low load, resulted in decreased activity in pSTG/pMTG and increased activity in visual occipital cortex and two regions known to contribute to visual attention regulation-the superior parietal lobule (SPL) and the paracingulate and anterior cingulate gyrus (PaCG, ACG). Critically, activity in PaCG/ACG was correlated with performance in the visual task and with activity in pSTG/pMTG: Increased activity in PaCG/ACG was observed for individuals with poorer visual performance and with decreased activity in pSTG/pMTG. Moreover, activity in a pSTG/pMTG seed region showed psychophysiological interactions with areas of the PaCG/ACG, with stronger interaction in the high-load than the low-load condition. These findings show that the acoustic analysis of speech is affected by the demands of a concurrent visual task and that the PaCG/ACG plays a role in allocating cognitive resources to concurrent auditory and visual information.

Impaired speech perception in noise with a normal audiogram: No evidence for cochlear synaptopathy and no relation to lifetime noise exposure.

In rodents, noise exposure can destroy synapses between inner hair cells and auditory nerve fibers ("cochlear synaptopathy") without causing hair cell loss. Noise-induced cochlear synaptopathy usually leaves cochlear thresholds unaltered, but is associated with long-term reductions in auditory brainstem response (ABR) amplitudes at medium-to-high sound levels. This pathophysiology has been suggested to degrade speech perception in noise (SPiN), perhaps explaining why SPiN ability varies so widely among audiometrically normal humans. The present study is the first to test for evidence of cochlear synaptopathy in humans with significant SPiN impairment. Individuals were recruited on the basis of self-reported SPiN difficulties and normal pure tone audiometric thresholds. Performance on a listening task identified a subset with "verified" SPiN impairment. This group was matched with controls on the basis of age, sex, and audiometric thresholds up to 14 kHz. ABRs and envelope-following responses (EFRs) were recorded at high stimulus levels, yielding both raw amplitude measures and within-subject difference measures. Past exposure to high sound levels was assessed by detailed structured interview. Impaired SPiN was not associated with greater lifetime noise exposure, nor with any electrophysiological measure. It is conceivable that retrospective self-report cannot reliably capture noise exposure, and that ABRs and EFRs offer limited sensitivity to synaptopathy in humans. Nevertheless, the results do not support the notion that noise-induced synaptopathy is a significant etiology of SPiN impairment with normal audiometric thresholds. It may be that synaptopathy alone does not have significant perceptual consequences, or is not widespread in humans with normal audiograms.

Supra-threshold auditory brainstem response amplitudes in humans: Test-retest reliability, electrode montage and noise exposure.

The auditory brainstem response (ABR) is a sub-cortical evoked potential in which a series of well-defined waves occur in the first 10 ms after the onset of an auditory stimulus. Wave V of the ABR, particularly wave V latency, has been shown to be remarkably stable over time in individual listeners. However, little attention has been paid to the reliability of wave I, which reflects auditory nerve activity. This ABR component has attracted interest recently, as wave I amplitude has been identified as a possible non-invasive measure of noise-induced cochlear synaptopathy. The current study aimed to determine whether ABR wave I amplitude has sufficient test-retest reliability to detect impaired auditory nerve function in an otherwise normal-hearing listener. Thirty normal-hearing females were tested, divided equally into low- and high-noise exposure groups. The stimulus was an 80 dB nHL click. ABR recordings were made from the ipsilateral mastoid and from the ear canal (using a tiptrode). Although there was some variability between listeners, wave I amplitude had high test-retest reliability, with an intraclass correlation coefficient (ICC) comparable to that for wave V amplitude. There were slight gains in reliability for wave I amplitude when recording from the ear canal (ICC of 0.88) compared to the mastoid (ICC of 0.85). The summating potential (SP) and ratio of SP to wave I were also quantified and found to be much less reliable than measures of wave I and V amplitude. Finally, we found no significant differences in the amplitude of any wave components between low- and high-noise exposure groups. We conclude that, if the other sources of between-subject variability can be controlled, wave I amplitude is sufficiently reliable to accurately characterize individual differences in auditory nerve function.

Dynamic semantic cognition: Characterising coherent and controlled conceptual retrieval through time using magnetoencephalography and chronometric transcranial magnetic stimulation.

Distinct neural processes are thought to support the retrieval of semantic information that is (i) coherent with strongly-encoded aspects of knowledge, and (ii) non-dominant yet relevant for the current task or context. While the brain regions that support readily coherent and more controlled patterns of semantic retrieval are relatively well-characterised, the temporal dynamics of these processes are not well-understood. This study used magnetoencephalography (MEG) and dual-pulse chronometric transcranial magnetic stimulation (cTMS) in two separate experiments to examine temporal dynamics during the retrieval of strong and weak associations. MEG results revealed a dissociation within left temporal cortex: anterior temporal lobe (ATL) showed greater oscillatory response for strong than weak associations, while posterior middle temporal gyrus (pMTG) showed the reverse pattern. Left inferior frontal gyrus (IFG), a site associated with semantic control and retrieval, showed both patterns at different time points. In the cTMS experiment, stimulation of ATL at ∼150 msec disrupted the efficient retrieval of strong associations, indicating a necessary role for ATL in coherent conceptual activations. Stimulation of pMTG at the onset of the second word disrupted the retrieval of weak associations, suggesting this site may maintain information about semantic context from the first word, allowing efficient engagement of semantic control. Together these studies provide converging evidence for a functional dissociation within the temporal lobe, across both tasks and time.

Task-based and resting-state fMRI reveal compensatory network changes following damage to left inferior frontal gyrus.

Damage to left inferior prefrontal cortex in stroke aphasia is associated with semantic deficits reflecting poor control over conceptual retrieval, as opposed to loss of knowledge. However, little is known about how functional recruitment within the semantic network changes in patients with executive-semantic deficits. The current study acquired functional magnetic resonance imaging (fMRI) data from 14 patients with semantic aphasia, who had difficulty with flexible semantic retrieval following left prefrontal damage, and 16 healthy age-matched controls, allowing us to examine activation and connectivity in the semantic network. We examined neural activity while participants listened to spoken sentences that varied in their levels of lexical ambiguity and during rest. We found group differences in two regions thought to be good candidates for functional compensation: ventral anterior temporal lobe (vATL), which is strongly implicated in comprehension, and posterior middle temporal gyrus (pMTG), which is hypothesized to work together with left inferior prefrontal cortex to support controlled aspects of semantic retrieval. The patients recruited both of these sites more than controls in response to meaningful sentences. Subsequent analysis identified that, in control participants, the recruitment of pMTG to ambiguous sentences was inversely related to functional coupling between pMTG and anterior superior temporal gyrus (aSTG) at rest, while the patients showed the opposite pattern. Moreover, stronger connectivity between pMTG and aSTG in patients was associated with better performance on a test of verbal semantic association, suggesting that this temporal lobe connection supports comprehension in the face of damage to left inferior prefrontal cortex. These results characterize network changes in patients with executive-semantic deficits and converge with studies of healthy participants in providing evidence for a distributed system underpinning semantic control that includes pMTG in addition to left inferior prefrontal cortex.

Causal cortical dynamics of a predictive enhancement of speech intelligibility.

Speech perception may be underpinned by a hierarchical cortical system, which attempts to match "external" incoming sensory inputs with "internal" top-down predictions. Prior knowledge modulates internal predictions of an upcoming stimulus and exerts its effects in temporal and inferior frontal cortex. Here, we used source-space magnetoencephalography (MEG) to study the spatiotemporal dynamics underpinning the integration of prior knowledge in the speech processing network. Prior knowledge was manipulated to i) increase the perceived intelligibility of speech sentences, and ii) dissociate the perceptual effects of changes in speech intelligibility from acoustical differences in speech stimuli. Cortical entrainment to the speech temporal envelope, which accounts for neural activity specifically related to sensory information, was affected by prior knowledge: This effect emerged early (∼50 ms) in left inferior frontal gyrus (IFG) and then (∼100 ms) in Heschl's gyrus (HG), and was sustained until latencies of ∼250 ms. Directed transfer function (DTF) measures were used for estimating direct Granger causal relations between locations of interest. In line with the cortical entrainment result, this analysis indicated that prior knowledge enhanced top-down connections from left IFG to all the left temporal areas of interest - namely HG, superior temporal sulcus (STS), and middle temporal gyrus (MTG). In addition, intelligible speech increased top-down information flow between left STS and left HG, and increased bottom-up flow in higher-order temporal cortex, specifically between STS and MTG. These results are compatible with theories that explain this mechanism as a result of both ascending and descending cortical interactions, such as predictive coding. Altogether, this study provides a detailed view of how, where and when prior knowledge influences continuous speech perception.

Effects of noise exposure on young adults with normal audiograms II: Behavioral measures.

An estimate of lifetime noise exposure was used as the primary predictor of performance on a range of behavioral tasks: frequency and intensity difference limens, amplitude modulation detection, interaural phase discrimination, the digit triplet speech test, the co-ordinate response speech measure, an auditory localization task, a musical consonance task and a subjective report of hearing ability. One hundred and thirty-eight participants (81 females) aged 18-36 years were tested, with a wide range of self-reported noise exposure. All had normal pure-tone audiograms up to 8 kHz. It was predicted that increased lifetime noise exposure, which we assume to be concordant with noise-induced cochlear synaptopathy, would elevate behavioral thresholds, in particular for stimuli with high levels in a high spectral region. However, the results showed little effect of noise exposure on performance. There were a number of weak relations with noise exposure across the test battery, although many of these were in the opposite direction to the predictions, and none were statistically significant after correction for multiple comparisons. There were also no strong correlations between electrophysiological measures of synaptopathy published previously and the behavioral measures reported here. Consistent with our previous electrophysiological results, the present results provide no evidence that noise exposure is related to significant perceptual deficits in young listeners with normal audiometric hearing. It is possible that the effects of noise-induced cochlear synaptopathy are only measurable in humans with extreme noise exposures, and that these effects always co-occur with a loss of audiometric sensitivity.