Noise-induced increase in human auditory evoked neuromagnetic fields

Adding noise is a confounded nuisance

A wide variety of research and clinical assessments involve presenting speech stimuli in the presence of some kind of noise. Here, I selectively review two theoretical perspectives and discuss ways in which these perspectives may help researchers understand the consequences for listeners of adding noise to a speech signal. I argue that adding noise changes more about the listening task than merely making the signal more difficult to perceive. To fully understand the effects of an added noise on speech perception, we must consider not just how much the noise affects task difficulty, but also how it affects all of the systems involved in understanding speech: increasing message uncertainty, modifying attentional demand, altering affective response, and changing motivation to perform the task.

[Effects of background noise on auditory response characteristics of primary auditory cortex neurons in awake mice]

OBJECTIVE

To study the effects of different continuous background noises on auditory response characteristics of primary auditory cortex (A1) neurons in awake mice.

METHODS

We performed in vivo cell-attached recordings in layer 4 neurons of the A1 of awake mice to investigate how continuous background noises of different levels affected the intensity tuning, frequency tuning and time characteristics of individual A1 neurons. According to the intensity tuning characteristics and types of stimulation, 44 neurons were devided into 4 groups: monotonic-intensity group (20 monotonic neurons), nonmonotonic-intensity group (6 nonmonotonic neurons), monotonic-frequency group (25 monotonic neurons) and monotonic-latency group (15 monotonic neurons).

RESULTS

The A1 neurons only had transient spike response within 10 to 40 ms after the onset of continuous wild-band noise stimulation. The noise intensity had no significant effects on the background firing rates of the A1 neurons (P>0.05). The increase of background noise resulted in a significant linear elevation of the intensity threshold of monotonic and nonmonotonic neurons for tone-evoked response (R²>0.90, P < 0.05). No significant difference was observed in the slopes of threshold changes between monotonic and nonmonotonic neurons (P>0.05). The best intensity of nonmonotonic neurons increased along with the intensity of the background noise, and the variation of the best intensity was positively correlated with the change of the threshold of the same neuron (r=0.944, P < 0.001). The frequency response bandwidth and the firing rate of the A1 neurons decreased as the noise intensity increased (P < 0.001), but the best frequency almost remained unchanged (P < 0.001). The increase of background noise intensity resulted in an increased first spike latency of the neurons to the same tone stimulus (P < 0.05) without affecting the time accuracy of the first action potential (P>0.05).

CONCLUSION

The acoustic response threshold of the A1 neurons increases linearly with the increase of background noise intensity. An increased background noise leads to compressed frequency band-width, a decreased firing rate and a prolonged spike latency, but the frequency selectivity and the time accuracy of auditory response to the same noise remain stable.

Binaural Background Noise Enhances Neuromagnetic Responses from Auditory Cortex

The presence of binaural low-level background noise has been shown to enhance the transient evoked N1 response at about 100 ms after sound onset. This increase in N1 amplitude is thought to reflect noise-mediated efferent feedback facilitation from the auditory cortex to lower auditory centers. To test this hypothesis, we recorded auditory-evoked fields using magnetoencephalography while participants were presented with binaural harmonic complex tones embedded in binaural or monaural background noise at signal-to-noise ratios of 25 dB (low noise) or 5 dB (higher noise). Half of the stimuli contained a gap in the middle of the sound. The source activities were measured in bilateral auditory cortices. The onset and gap N1 response increased with low binaural noise, but high binaural and low monaural noise did not affect the N1 amplitudes. P1 and P2 onset and gap responses were consistently attenuated by background noise, and noise level and binaural/monaural presentation showed distinct effects. Moreover, the evoked gamma synchronization was also reduced by background noise, and it showed a lateralized reduction for monaural noise. The effects of noise on the N1 amplitude follow a bell-shaped characteristic that could reflect an optimal representation of acoustic information for transient events embedded in noise.

Effect of Auditory Distraction on Working Memory, Attention Switching, and Listening Comprehension

The effect of non-informational speech spectrum noise as a distractor on cognitive and listening comprehension ability was examined in fifty-three young, normal hearing adults. Time-controlled tasks were used to measure auditory working memory (WM) capacity and attention switching (AS) ability. Listening comprehension was measured using a lecture, interview, and spoken narratives test. Noise level was individually set to achieve at least 90% or higher speech intelligibility. Participants' listening comprehension in the presence of distracting noise was better on inference questions compared to listening in quiet. Their speed of information processing was also significantly faster in WM and AS tasks in noise. These results were consistent with the view that noise may enhance arousal levels leading to faster information processing during cognitive tasks. Whereas the speed of AS was faster in noise, this rapid switching of attention resulted in more errors in updating items. Participants who processed information faster in noise and did so accurately, more effectively switched their attention to refresh/rehearse recall items within WM. More efficient processing deployed in the presence of noise appeared to have led to improvements in WM performance and making inferences in a listening comprehension task. Additional research is required to examine these findings using background noise that can cause informational masking.

An examination of electrophysiological release from masking in young and older adults

The effect of age on release from masking (RFM) was examined using cortical auditory evoked potentials (CAEPs). Two speech-in-noise paradigms [i.e., fixed speech with varying signal-to-noise ratios (SNRs) and fixed noise with varying speech levels], similar to those used in behavioral measures of RFM, were employed with competing continuous and interrupted noises. Young and older normal-hearing adults participated (N = 36). Cortical responses were evoked in the fixed speech paradigm at SNRs of -10, 0, and 10 dB. In the fixed noise paradigm, the CAEP SNR threshold was determined in both noises as the lowest SNR that yielded a measurable response. RFM was demonstrated in the fixed speech paradigm with a significant amount of missing responses, longer P1 and N1 latencies, and smaller N1 response amplitudes in continuous noise at the poorest -10 dB SNR. In the fixed noise paradigm, RFM was demonstrated with significantly lower CAEP SNR thresholds in interrupted noise. Older participants demonstrated significantly longer P2 latencies and reduced P1 and N1 amplitudes. There was no evidence of a group difference in RFM in either paradigm.

White Noise Background Improves Tone Discrimination by Suppressing Cortical Tuning Curves

The brain faces the difficult task of maintaining a stable representation of key features of the outside world in noisy sensory surroundings. How does the sensory representation change with noise, and how does the brain make sense of it? We investigated the effect of background white noise (WN) on tuning properties of neurons in mouse A1 and its impact on discrimination performance in a go/no-go task. We find that WN suppresses the activity of A1 neurons, which surprisingly increases the discriminability of tones spectrally close to each other. To confirm the involvement of A1, we optogenetically excited parvalbumin-positive (PV⁺) neurons in A1, which have similar effects as WN on both tuning properties and frequency discrimination. A population model suggests that the suppression of A1 tuning curves increases frequency selectivity and thereby improves discrimination. Our findings demonstrate that the cortical representation of pure tones adapts during noise to improve sensory acuity.

Cortical Auditory Event-Related Potentials and Categorical Perception of Voice Onset Time in Children With an Auditory Neuropathy Spectrum Disorder

Objective: This study evaluated cortical encoding of voice onset time (VOT) in quiet and noise, and their potential associations with the behavioral categorical perception of VOT in children with auditory neuropathy spectrum disorder (ANSD). Design: Subjects were 11 children with ANSD ranging in age between 6.4 and 16.2 years. The stimulus was an /aba/-/apa/ vowel-consonant-vowel continuum comprising eight tokens with VOTs ranging from 0 ms (voiced endpoint) to 88 ms (voiceless endpoint). For speech in noise, speech tokens were mixed with the speech-shaped noise from the Hearing In Noise Test at a signal-to-noise ratio (SNR) of +5 dB. Speech-evoked auditory event-related potentials (ERPs) and behavioral categorization perception of VOT were measured in quiet in all subjects, and at an SNR of +5 dB in seven subjects. The stimuli were presented at 35 dB SL (re: pure tone average) or 115 dB SPL if this limit was less than 35 dB SL. In addition to the onset response, the auditory change complex (ACC) elicited by VOT was recorded in eight subjects. Results: Speech evoked ERPs recorded in all subjects consisted of a vertex positive peak (i.e., P1), followed by a trough occurring approximately 100 ms later (i.e., N2). For results measured in quiet, there was no significant difference in categorical boundaries estimated using ERP measures and behavioral procedures. Categorical boundaries estimated in quiet using both ERP and behavioral measures closely correlated with the most-recently measured Phonetically Balanced Kindergarten (PBK) scores. Adding a competing background noise did not affect categorical boundaries estimated using either behavioral or ERP procedures in three subjects. For the other four subjects, categorical boundaries estimated in noise using behavioral measures were prolonged. However, adding background noise only increased categorical boundaries measured using ERPs in three out of these four subjects. Conclusions: VCV continuum can be used to evaluate behavioral identification and the neural encoding of VOT in children with ANSD. In quiet, categorical boundaries of VOT estimated using behavioral measures and ERP recordings are closely associated with speech recognition performance in children with ANSD. Underlying mechanisms for excessive speech perception deficits in noise may vary for individual patients with ANSD.

Noise-induced enhancement of envelope following responses in normal-hearing adults

Measures of signal-in-noise neural encoding may improve understanding of the hearing-in-noise difficulties experienced by many individuals in everyday life. Usually noise results in weaker envelope following responses (EFRs); however, some studies demonstrate EFR enhancements. This experiment tested whether noise-induced enhancements in EFRs are demonstrated with simple 500- and 1000-Hz pure tones amplitude modulated at 110 Hz. Most of the 12 young normal-hearing participants demonstrated enhanced encoding of the 110-Hz fundamental in a noise background compared to quiet; in contrast, responses at the harmonics were decreased in noise relative to quiet conditions. Possible mechanisms of such an enhancement are discussed.

Auditory event-related potentials and function of the medial olivocochlear efferent system in children with auditory processing disorders

OBJECTIVE

The objectives were to investigate the function of central auditory pathways and of the medial efferent olivocochlear system (MOCS).

DESIGN

Event-related potentials (ERP) were recorded following the delivery of the stimulus /da/ in quiet and in ipsilateral, contralateral, and binaural noise conditions and correlated to the results of the auditory processing disorders (APD) diagnostic test battery. MOCS function was investigated by adding ipsilateral, contralateral, and binaural noise to transient evoked otoacoustic emission recordings. Auditory brainstem responses and pure tone audiogram were also evaluated.

STUDY SAMPLE

Nineteen children (7 to 12 years old) with APD were compared with 24 age-matched controls.

RESULTS

Otoacoustic emissions and ABR characteristics did not differ between groups, whereas ERP latencies were significantly longer and of higher amplitudes in APD children than in controls, in both quiet and noise conditions. The MOCS suppression was higher in APD children.

CONCLUSIONS

Findings indicate that children with APD present with neural deficiencies in both challenging and nonchallenging environments with an increase in the timing of several central auditory processes correlated to their behavioural performances. Meanwhile, their modulation of the auditory periphery under noisy conditions differs from control children with higher suppression.

Multisensory Integration Strategy for Modality-Specific Loss of Inhibition Control in Older Adults

Older adults are known to have lesser cognitive control capability and greater susceptibility to distraction than young adults. Previous studies have reported age-related problems in selective attention and inhibitory control, yielding mixed results depending on modality and context in which stimuli and tasks were presented. The purpose of the study was to empirically demonstrate a modality-specific loss of inhibitory control in processing audio-visual information with ageing. A group of 30 young adults (mean age = 25.23, Standard Deviation (SD) = 1.86) and 22 older adults (mean age = 55.91, SD = 4.92) performed the audio-visual contour identification task (AV-CIT). We compared performance of visual/auditory identification (Uni-V, Uni-A) with that of visual/auditory identification in the presence of distraction in counterpart modality (Multi-V, Multi-A). The findings showed a modality-specific effect on inhibitory control. Uni-V performance was significantly better than Multi-V, indicating that auditory distraction significantly hampered visual target identification. However, Multi-A performance was significantly enhanced compared to Uni-A, indicating that auditory target performance was significantly enhanced by visual distraction. Additional analysis showed an age-specific effect on enhancement between Uni-A and Multi-A depending on the level of visual inhibition. Together, our findings indicated that the loss of visual inhibitory control was beneficial for the auditory target identification presented in a multimodal context in older adults. A likely multisensory information processing strategy in the older adults was further discussed in relation to aged cognition.

Subcortical sources dominate the neuroelectric auditory frequency-following response to speech

Frequency-following responses (FFRs) are neurophonic potentials that provide a window into the encoding of complex sounds (e.g., speech/music), auditory disorders, and neuroplasticity. While the neural origins of the FFR remain debated, renewed controversy has reemerged after demonstration that FFRs recorded via magnetoencephalography (MEG) are dominated by cortical rather than brainstem structures as previously assumed. Here, we recorded high-density (64 ch) FFRs via EEG and applied state-of-the art source imaging techniques to multichannel data (discrete dipole modeling, distributed imaging, independent component analysis, computational simulations). Our data confirm a mixture of generators localized to bilateral auditory nerve (AN), brainstem inferior colliculus (BS), and bilateral primary auditory cortex (PAC). However, frequency-specific scrutiny of source waveforms showed the relative contribution of these nuclei to the aggregate FFR varied across stimulus frequencies. Whereas AN and BS sources produced robust FFRs up to ∼700 Hz, PAC showed weak phase-locking with little FFR energy above the speech fundamental (100 Hz). Notably, CLARA imaging further showed PAC activation was eradicated for FFRs >150 Hz, above which only subcortical sources remained active. Our results show (i) the site of FFR generation varies critically with stimulus frequency; and (ii) opposite the pattern observed in MEG, subcortical structures make the largest contribution to electrically recorded FFRs (AN ≥ BS > PAC). We infer that cortical dominance observed in previous neuromagnetic data is likely due to the bias of MEG to superficial brain tissue, underestimating subcortical structures that drive most of the speech-FFR. Cleanly separating subcortical from cortical FFRs can be achieved by ensuring stimulus frequencies are >150-200 Hz, above the phase-locking limit of cortical neurons.

Background Suppression and its Relation to Foreground Processing of Speech Versus Non-speech Streams

Since sound perception takes place against a background with a certain amount of noise, both speech and non-speech processing involve extraction of target signals and suppression of background noise. Previous works on early processing of speech phonemes largely neglected how background noise is encoded and suppressed. This study aimed to fill in this gap. We adopted an oddball paradigm where speech (vowels) or non-speech stimuli (complex tones) were presented with or without a background of amplitude-modulated noise and analyzed cortical responses related to foreground stimulus processing, including mismatch negativity (MMN), N2b, and P300, as well as neural representations of the background noise, that is, auditory steady-state response (ASSR). We found that speech deviants elicited later and weaker MMN, later N2b, and later P300 than non-speech ones, but N2b and P300 had similar strength, suggesting more complex processing of certain acoustic features in speech. Only for vowels, background noise enhanced N2b strength relative to silence, suggesting an attention-related speech-specific process to improve perception of foreground targets. In addition, noise suppression in speech contexts, quantified by ASSR amplitude reduction after stimulus onset, was lateralized towards the left hemisphere. The left-lateralized suppression following N2b was associated with the N2b enhancement in noise for speech, indicating that foreground processing may interact with background suppression, particularly during speech processing. Together, our findings indicate that the differences between perception of speech and non-speech sounds involve not only the processing of target information in the foreground but also the suppression of irrelevant aspects in the background.

Noise and pitch interact during the cortical segregation of concurrent speech

Behavioral studies reveal listeners exploit intrinsic differences in voice fundamental frequency (F0) to segregate concurrent speech sounds-the so-called "F0-benefit." More favorable signal-to-noise ratio (SNR) in the environment, an extrinsic acoustic factor, similarly benefits the parsing of simultaneous speech. Here, we examined the neurobiological substrates of these two cues in the perceptual segregation of concurrent speech mixtures. We recorded event-related brain potentials (ERPs) while listeners performed a speeded double-vowel identification task. Listeners heard two concurrent vowels whose F0 differed by zero or four semitones presented in either clean (no noise) or noise-degraded (+5 dB SNR) conditions. Behaviorally, listeners were more accurate in correctly identifying both vowels for larger F0 separations but F0-benefit was more pronounced at more favorable SNRs (i.e., pitch × SNR interaction). Analysis of the ERPs revealed that only the P2 wave (∼200 ms) showed a similar F0 x SNR interaction as behavior and was correlated with listeners' perceptual F0-benefit. Neural classifiers applied to the ERPs further suggested that speech sounds are segregated neurally within 200 ms based on SNR whereas segregation based on pitch occurs later in time (400-700 ms). The earlier timing of extrinsic SNR compared to intrinsic F0-based segregation implies that the cortical extraction of speech from noise is more efficient than differentiating speech based on pitch cues alone, which may recruit additional cortical processes. Findings indicate that noise and pitch differences interact relatively early in cerebral cortex and that the brain arrives at the identities of concurrent speech mixtures as early as ∼200 ms.

Melodic Contour Identification Reflects the Cognitive Threshold of Aging

Cognitive decline is a natural phenomenon of aging. Although there exists a consensus that sensitivity to acoustic features of music is associated with such decline, no solid evidence has yet shown that structural elements and contexts of music explain this loss of cognitive performance. This study examined the extent and the type of cognitive decline that is related to the contour identification task (CIT) using tones with different pitches (i.e., melodic contours). Both younger and older adult groups participated in the CIT given in three listening conditions (i.e., focused, selective, and alternating). Behavioral data (accuracy and response times) and hemodynamic reactions were measured using functional near-infrared spectroscopy (fNIRS). Our findings showed cognitive declines in the older adult group but with a subtle difference from the younger adult group. The accuracy of the melodic CITs given in the target-like distraction task (CIT2) was significantly lower than that in the environmental noise (CIT1) condition in the older adult group, indicating that CIT2 may be a benchmark test for age-specific cognitive decline. The fNIRS findings also agreed with this interpretation, revealing significant increases in oxygenated hemoglobin (oxyHb) concentration in the younger (p < 0.05 for Δpre - on task; p < 0.01 for Δon – post task) rather than the older adult group (n.s for Δpre - on task; n.s for Δon – post task). We further concluded that the oxyHb difference was present in the brain regions near the right dorsolateral prefrontal cortex. Taken together, these findings suggest that CIT2 (i.e., the melodic contour task in the target-like distraction) is an optimized task that could indicate the degree and type of age-related cognitive decline.

Functional changes in inter- and intra-hemispheric cortical processing underlying degraded speech perception

Previous studies suggest that at poorer signal-to-noise ratios (SNRs), auditory cortical event-related potentials are weakened, prolonged, and show a shift in the functional lateralization of cerebral processing from left to right hemisphere. Increased right hemisphere involvement during speech-in-noise (SIN) processing may reflect the recruitment of additional brain resources to aid speech recognition or alternatively, the progressive loss of involvement from left linguistic brain areas as speech becomes more impoverished (i.e., nonspeech-like). To better elucidate the brain basis of SIN perception, we recorded neuroelectric activity in normal hearing listeners to speech sounds presented at various SNRs. Behaviorally, listeners obtained superior SIN performance for speech presented to the right compared to the left ear (i.e., right ear advantage). Source analysis of neural data assessed the relative contribution of region-specific neural generators (linguistic and auditory brain areas) to SIN processing. We found that left inferior frontal brain areas (e.g., Broca's areas) partially disengage at poorer SNRs but responses do not right lateralize with increasing noise. In contrast, auditory sources showed more resilience to noise in left compared to right primary auditory cortex but also a progressive shift in dominance from left to right hemisphere at lower SNRs. Region- and ear-specific correlations revealed that listeners' right ear SIN advantage was predicted by source activity emitted from inferior frontal gyrus (but not primary auditory cortex). Our findings demonstrate changes in the functional asymmetry of cortical speech processing during adverse acoustic conditions and suggest that "cocktail party" listening skills depend on the quality of speech representations in the left cerebral hemisphere rather than compensatory recruitment of right hemisphere mechanisms.

Thresholding of auditory cortical representation by background noise

It is generally thought that background noise can mask auditory information. However, how the noise specifically transforms neuronal auditory processing in a level-dependent manner remains to be carefully determined. Here, with in vivo loose-patch cell-attached recordings in layer 4 of the rat primary auditory cortex (A1), we systematically examined how continuous wideband noise of different levels affected receptive field properties of individual neurons. We found that the background noise, when above a certain critical/effective level, resulted in an elevation of intensity threshold for tone-evoked responses. This increase of threshold was linearly dependent on the noise intensity above the critical level. As such, the tonal receptive field (TRF) of individual neurons was translated upward as an entirety toward high intensities along the intensity domain. This resulted in preserved preferred characteristic frequency (CF) and the overall shape of TRF, but reduced frequency responding range and an enhanced frequency selectivity for the same stimulus intensity. Such translational effects on intensity threshold were observed in both excitatory and fast-spiking inhibitory neurons, as well as in both monotonic and nonmonotonic (intensity-tuned) A1 neurons. Our results suggest that in a noise background, fundamental auditory representations are modulated through a background level-dependent linear shifting along intensity domain, which is equivalent to reducing stimulus intensity.

Background noise can enhance cortical auditory evoked potentials under certain conditions

OBJECTIVE

To use cortical auditory evoked potentials (CAEPs) to understand neural encoding in background noise and the conditions under which noise enhances CAEP responses.

METHODS

CAEPs from 16 normal-hearing listeners were recorded using the speech syllable/ba/presented in quiet and speech-shaped noise at signal-to-noise ratios of 10 and 30dB. The syllable was presented binaurally and monaurally at two presentation rates.

RESULTS

The amplitudes of N1 and N2 peaks were often significantly enhanced in the presence of low-level background noise relative to quiet conditions, while P1 and P2 amplitudes were consistently reduced in noise. P1 and P2 amplitudes were significantly larger during binaural compared to monaural presentations, while N1 and N2 peaks were similar between binaural and monaural conditions.

CONCLUSIONS

Methodological choices impact CAEP peaks in very different ways. Negative peaks can be enhanced by background noise in certain conditions, while positive peaks are generally enhanced by binaural presentations.

SIGNIFICANCE

Methodological choices significantly impact CAEPs acquired in quiet and in noise. If CAEPs are to be used as a tool to explore signal encoding in noise, scientists must be cognizant of how differences in acquisition and processing protocols selectively shape CAEP responses.

Effects of age-related hearing loss and background noise on neuromagnetic activity from auditory cortex

Aging is often accompanied by hearing loss, which impacts how sounds are processed and represented along the ascending auditory pathways and within the auditory cortices. Here, we assess the impact of mild binaural hearing loss on the older adults’ ability to both process complex sounds embedded in noise and to segregate a mistuned harmonic in an otherwise periodic stimulus. We measured auditory evoked fields (AEFs) using magnetoencephalography while participants were presented with complex tones that had either all harmonics in tune or had the third harmonic mistuned by 4 or 16% of its original value. The tones (75 dB sound pressure level, SPL) were presented without, with low (45 dBA SPL), or with moderate (65 dBA SPL) Gaussian noise. For each participant, we modeled the AEFs with a pair of dipoles in the superior temporal plane. We then examined the effects of hearing loss and noise on the amplitude and latency of the resulting source waveforms. In the present study, results revealed that similar noise-induced increases in N1m were present in older adults with and without hearing loss. Our results also showed that the P1m amplitude was larger in the hearing impaired than in the normal-hearing adults. In addition, the object-related negativity (ORN) elicited by the mistuned harmonic was larger in hearing impaired listeners. The enhanced P1m and ORN amplitude in the hearing impaired older adults suggests that hearing loss increased neural excitability in auditory cortices, which could be related to deficits in inhibitory control.

Differential effects of temporal regularity on auditory-evoked response amplitude: a decrease in silence and increase in noise

Background

In daily life, we are continuously exposed to temporally regular and irregular sounds. Previous studies have demonstrated that the temporal regularity of sound sequences influences neural activity. However, it remains unresolved how temporal regularity affects neural activity in noisy environments, when attention of the listener is not focused on the sound input.

Methods

In the present study, using magnetoencephalography we investigated the effects of temporal regularity in sound signal sequencing (regular vs. irregular) in silent versus noisy environments during distracted listening.

Results

The results demonstrated that temporal regularity differentially affected the auditory-evoked N1m response depending on the background acoustic environment: the N1m amplitudes elicited by the temporally regular sounds were smaller in silence and larger in noise than those elicited by the temporally irregular sounds.

Conclusions

Our results indicate that the human auditory system is able to involuntarily utilize temporal regularity in sound signals to modulate the neural activity in the auditory cortex in accordance with the surrounding acoustic environment.

Low-arousal speech noise improves performance in N-back task: an ERP study

The relationship between noise and human performance is a crucial topic in ergonomic research. However, the brain dynamics of the emotional arousal effects of background noises are still unclear. The current study employed meaningless speech noises in the n-back working memory task to explore the changes of event-related potentials (ERPs) elicited by the noises with low arousal level vs. high arousal level. We found that the memory performance in low arousal condition were improved compared with the silent and the high arousal conditions; participants responded more quickly and had larger P2 and P3 amplitudes in low arousal condition while the performance and ERP components showed no significant difference between high arousal and silent conditions. These findings suggested that the emotional arousal dimension of background noises had a significant influence on human working memory performance, and that this effect was independent of the acoustic characteristics of noises (e.g., intensity) and the meaning of speech materials. The current findings improve our understanding of background noise effects on human performance and lay the ground for the investigation of patients with attention deficits.

Involuntary monitoring of sound signals in noise is reflected in the human auditory evoked N1m response

Constant sound sequencing as operationalized by repeated stimulation with tones of the same frequency has multiple effects. On the one hand, it activates mechanisms of habituation and refractoriness, which are reflected in the decrease of response amplitude of evoked responses. On the other hand, the constant sequencing acts as spectral cueing, resulting in tones being detected faster and more accurately. With the present study, by means of magnetoencephalography, we investigated the impact of repeated tone stimulation on the N1m auditory evoked fields, while listeners were distracted from the test sounds. We stimulated subjects with trains of either four tones of the same frequency, or with trains of randomly assigned frequencies. The trains were presented either in a silent or in a noisy background. In silence, the patterns of source strength decline originating from repeated stimulation suggested both, refractoriness as well as habituation as underlying mechanisms. In noise, in contrast, there was no indication of source strength decline. Furthermore, we found facilitating effects of constant sequencing regarding the detection of the single tones as indexed by a shortening of N1m latency. We interpret our findings as a correlate of a bottom-up mechanism that is constantly monitoring the incoming auditory information, even when voluntary attention is directed to a different modality.

Effects of age and background noise on processing a mistuned harmonic in an otherwise periodic complex sound

Older adults presented with short (i.e., 40 ms) harmonic complex tones show a reduced likelihood of hearing the mistuned harmonic as a separate sound. Here, we examined whether this age difference for the mistuned harmonic would generalize to a longer signal duration (i.e., 200 ms). We measured auditory evoked fields (AEFs) using magnetoencephalography while young and older adults were presented with harmonic complex tones that either had all partials of the tones in tune (single sound object) or contained a 4 or 16% mistuned harmonic (dual sound objects). The auditory stimuli were presented in isolation or embedded in low or moderate levels of continuous white noise. For each participant, we modeled the AEFs with a pair of dipoles in the superior temporal plane and examined the effects of age and noise on the amplitude and latency of the resulting source waveforms. The present study reveals similar noise-induced increases in N1m and object-related negativity in young and older adults which may be mediated via efferent feedback connections and/or changes in the temporal window of integration. We observed less age-related differences in concurrent sound segregation for stimuli that matched the duration of the temporal integration window of auditory perception (i.e., ∼200 ms) than for short duration sounds (i.e., 40 ms). Possible explanations for this duration-dependent age-related decline in concurrent sound perception are a general slowing in auditory processing and/or lengthening of the temporal integration window.

Neural generators underlying concurrent sound segregation

Although an object-based account of auditory attention has become an increasingly popular model for understanding how temporally overlapping sounds are segregated, relatively little is known about the cortical circuit that supports such ability. In the present study, we applied a beamformer spatial filter to magnetoencephalography (MEG) data recorded during an auditory paradigm that used inharmonicity to promote the formation of multiple auditory objects. Using this unconstrained, data-driven approach, the evoked field component linked with the perception of multiple auditory objects (i.e., the object-related negativity; ORNm), was found to be associated with bilateral auditory cortex sources that were distinct from those coinciding with the P1m, N1m, and P2m responses elicited by sound onset. The right hemispheric ORNm source in particular was consistently positioned anterior to the other sources across two experiments. These findings are consistent with earlier proposals of multiple auditory object detection being associated with generators in the auditory cortex and further suggest that these neural populations are distinct from the long latency evoked responses reflecting the detection of sound onset.

Cortical-evoked potentials reflect speech-in-noise perception in children

Children are known to be particularly vulnerable to the effects of noise on speech perception, and it is commonly acknowledged that failure of central auditory processes can lead to these difficulties with speech-in-noise (SIN) perception. However, little is known about the mechanistic relationship between central processes and the perception of SIN. Our aims were twofold: to examine the effects of noise on the central encoding of speech through measurement of cortical event-related potentials and to examine the relationship between cortical processing and behavioral indices of SIN perception. We recorded cortical responses to the speech syllable [da] in quiet and multi-talker babble noise in 32 children with a broad range of SIN perception. Outcomes suggest inordinate effects of noise on auditory function in the bottom SIN perceivers compared with the top perceivers. The cortical amplitudes in the top SIN group remained stable between conditions, whereas amplitudes increased significantly in the bottom SIN group, suggesting a developmental central processing impairment in the bottom perceivers that may contribute to difficulties in encoding and perceiving speech in challenging listening environments.

What subcortical-cortical relationships tell us about processing speech in noise

To advance our understanding of the biological basis of speech-in-noise perception, we investigated the effects of background noise on both subcortical- and cortical-evoked responses, and the relationships between them, in normal hearing young adults. The addition of background noise modulated subcortical and cortical response morphology. In noise, subcortical responses were later, smaller in amplitude and demonstrated decreased neural precision in encoding the speech sound. Cortical responses were also delayed by noise, yet the amplitudes of the major peaks (N1, P2) were affected differently, with N1 increasing and P2 decreasing. Relationships between neural measures and speech-in-noise ability were identified, with earlier subcortical responses, higher subcortical response fidelity and greater cortical N1 response magnitude all relating to better speech-in-noise perception. Furthermore, it was only with the addition of background noise that relationships between subcortical and cortical encoding of speech and the behavioral measures of speech in noise emerged. Results illustrate that human brainstem responses and N1 cortical response amplitude reflect coordinated processes with regards to the perception of speech in noise, thereby acting as a functional index of speech-in-noise perception.