Age differences in the neuroelectric adaptation to meaningful sounds

Aging Impacts Basic Auditory and Timing Processes

Deterioration in the peripheral and central auditory systems is common in older adults and often leads to hearing and speech comprehension difficulties. Even when hearing remains intact, electrophysiological data of older adults frequently exhibit altered neural responses along the auditory pathway, reflected in variability in phase alignment of neural activity to speech sound onsets. However, it remains unclear whether challenges in speech processing in aging stem from more fundamental deficits in auditory and timing processes. Here, we investigated if and how aging individuals encoded temporal regularities in isochronous auditory sequences presented at 1.5Hz, and if they employed adaptive mechanisms of neural phase alignment in anticipation of next sound onsets. We recorded EEG in older and young individuals listening to simple isochronous tone sequences. We show that aging individuals displayed larger event-related neural responses, an increased 1/F slope, but reduced phase-coherence at the stimulation frequency (1.5Hz) and a reduced slope of phase-coherence over time in the delta and theta frequency-bands. These observations suggest altered top-down modulatory inhibition when processing repeated and predictable sounds in a sequence and altered mechanisms of continuous phase-alignment to expected sound onsets in aging. Given that deteriorations in these basic timing capacities may affect other higher-order cognitive processes (e.g., attention, perception, and action), these results underscore the need for future research examining the link between basic timing abilities and general cognition across the lifespan.

Short- and long-term neuroplasticity interact during the perceptual learning of concurrent speech

Plasticity from auditory experience shapes the brain's encoding and perception of sound. However, whether such long-term plasticity alters the trajectory of short-term plasticity during speech processing has yet to be investigated. Here, we explored the neural mechanisms and interplay between short- and long-term neuroplasticity for rapid auditory perceptual learning of concurrent speech sounds in young, normal-hearing musicians and nonmusicians. Participants learned to identify double-vowel mixtures during ~ 45 min training sessions recorded simultaneously with high-density electroencephalography (EEG). We analyzed frequency-following responses (FFRs) and event-related potentials (ERPs) to investigate neural correlates of learning at subcortical and cortical levels, respectively. Although both groups showed rapid perceptual learning, musicians showed faster behavioral decisions than nonmusicians overall. Learning-related changes were not apparent in brainstem FFRs. However, plasticity was highly evident in cortex, where ERPs revealed unique hemispheric asymmetries between groups suggestive of different neural strategies (musicians: right hemisphere bias; nonmusicians: left hemisphere). Source reconstruction and the early (150-200 ms) time course of these effects localized learning-induced cortical plasticity to auditory-sensory brain areas. Our findings reinforce the domain-general benefits of musicianship but reveal that successful speech sound learning is driven by a critical interplay between long- and short-term mechanisms of auditory plasticity, which first emerge at a cortical level.

Short- and long-term experience-dependent neuroplasticity interact during the perceptual learning of concurrent speech

Plasticity from auditory experiences shapes brain encoding and perception of sound. However, whether such long-term plasticity alters the trajectory of short-term plasticity during speech processing has yet to be investigated. Here, we explored the neural mechanisms and interplay between short- and long-term neuroplasticity for rapid auditory perceptual learning of concurrent speech sounds in young, normal-hearing musicians and nonmusicians. Participants learned to identify double-vowel mixtures during ∼45 minute training sessions recorded simultaneously with high-density EEG. We analyzed frequency-following responses (FFRs) and event-related potentials (ERPs) to investigate neural correlates of learning at subcortical and cortical levels, respectively. While both groups showed rapid perceptual learning, musicians showed faster behavioral decisions than nonmusicians overall. Learning-related changes were not apparent in brainstem FFRs. However, plasticity was highly evident in cortex, where ERPs revealed unique hemispheric asymmetries between groups suggestive of different neural strategies (musicians: right hemisphere bias; nonmusicians: left hemisphere). Source reconstruction and the early (150-200 ms) time course of these effects localized learning-induced cortical plasticity to auditory-sensory brain areas. Our findings confirm domain-general benefits for musicianship but reveal successful speech sound learning is driven by a critical interplay between long- and short-term mechanisms of auditory plasticity that first emerge at a cortical level.

Age differences in central auditory system responses to naturalistic music

Aging influences the central auditory system leading to difficulties in the decoding and understanding of overlapping sound signals, such as speech in noise or polyphonic music. Studies on central auditory system evoked responses (ERs) have found in older compared to young listeners increased amplitudes (less inhibition) of the P1 and N1 and decreased amplitudes of the P2, mismatch negativity (MMN), and P3a responses. While preceding research has focused on simplified auditory stimuli, we here tested whether the previously observed age-related differences could be replicated with sounds embedded in medium and highly naturalistic musical contexts. Older (age 55-77 years) and younger adults (age 21-31 years) listened to medium naturalistic (synthesized melody) and highly naturalistic (studio recording of a music piece) stimuli. For the medium naturalistic music, the age group differences on the P1, N1, P2, MMN, and P3a amplitudes were all replicated. The age group differences, however, appeared reduced with the highly compared to the medium naturalistic music. The finding of lower P2 amplitude in older than young was replicated for slow event rates (0.3-2.9Hz) in the highly naturalistic music. Moreover, the ER latencies suggested a gradual slowing of the auditory processing time course for highly compared to medium naturalistic stimuli irrespective of age. These results support that age-related differences on ERs can partly be observed with naturalistic stimuli. This opens new avenues for including naturalistic stimuli in the investigation of age-related central auditory system disorders.

Adaptation in auditory processing

Adaptation is an essential feature of auditory neurons, which reduces their responses to unchanging and recurring sounds and allows their response properties to be matched to the constantly changing statistics of sounds that reach the ears. As a consequence, processing in the auditory system highlights novel or unpredictable sounds and produces an efficient representation of the vast range of sounds that animals can perceive by continually adjusting the sensitivity and, to a lesser extent, the tuning properties of neurons to the most commonly encountered stimulus values. Together with attentional modulation, adaptation to sound statistics also helps to generate neural representations of sound that are tolerant to background noise and therefore plays a vital role in auditory scene analysis. In this review, we consider the diverse forms of adaptation that are found in the auditory system in terms of the processing levels at which they arise, the underlying neural mechanisms, and their impact on neural coding and perception. We also ask what the dynamics of adaptation, which can occur over multiple timescales, reveal about the statistical properties of the environment. Finally, we examine how adaptation to sound statistics is influenced by learning and experience and changes as a result of aging and hearing loss.

Central auditory system responses from children while listening to speech in noise

Cortical auditory evoked potentials (CAEPs) have been successfully used to explore the effects of noise on speech processing at the cortical level in adults and children. The purpose of this study was to determine whether +15 dB signal-to-noise ratios (SNRs), often recommended for optimal speech perception in children, elicit higher amplitude CAEPs than more realistic SNRs encountered by children during their daily lives (+10 dB SNR). Moreover, we aimed to investigate whether cortical speech categorization is observable in children in quiet and in noise and whether CAEPs to speech in noise are related to behavioral speech perception in noise performance in children. CAEPs were measured during a passive speech-syllable task in 51 normal hearing children aged 8 to 11 years. The speech syllables /da/ and /ga/ were presented in quiet and in the presence of a 4-talker-babble noise at +15 dB and +10 dB SNR. N1 latencies and P2 amplitudes and latencies varied as a function of SNR, with poorer SNRs (+10 dB) eliciting significantly smaller P2 amplitudes and delayed N1 and P2 latencies relative to the higher SNR (+15 dB). Finally, speech categorization was present at the cortical level in this group of children in quiet and at both SNRs; however, N1 and P2 amplitudes and latencies were not related to behavioral speech-in-noise perception of children.

N1 Repetition-Attenuation for Acoustically Variable Speech and Spectrally Rotated Speech

The amplitude of the event-related N1 wave decreases with repeated stimulation. This repetition-attenuation has not previously been investigated in response to variable auditory stimuli, nor has the relative impact of acoustic vs. perceptual category repetition been studied. In the present study, N1 repetition-attenuation was investigated for speech and spectrally rotated speech with varying degrees of acoustic and perceptual category variation. In the speech condition, participants (n = 19) listened to stimulus trains consisting of either the same vowel exemplar (no variability condition), different exemplars of the same vowel (low variability condition), or different exemplars of two different vowels (high variability condition). In the rotated speech condition, the spectrally rotated counterparts of the vowels were presented. Findings show N1 repetition-attenuation in the face of acoustic and perceptual category variability, but no impact of the degree of variability on the degree of N1 attenuation. Speech stimuli resulted in less attenuation than the acoustically matched non-speech stimuli, which is in line with previous findings. It remains unclear if the attenuation of the N1 wave is reduced as a result of stimuli being perceived as belonging to perceptual categories or as a result of some other characteristic of speech.

Nicotinic Receptor Subunit Distribution in Auditory Cortex: Impact of Aging on Receptor Number and Function

The presence of novel or degraded communication sounds likely results in activation of basal forebrain cholinergic neurons increasing release of ACh onto presynaptic and postsynaptic nAChRs in primary auditory cortex (A1). nAChR subtypes include high-affinity heteromeric nAChRs commonly composed of α4 and β2 subunits and low-affinity homomeric nAChRs composed of α7 subunits. In young male FBN rats, we detail the following: (1) the distribution/expression of nAChR subunit transcripts in excitatory (VGluT1) and inhibitory (VGAT) neurons across A1 layers; (2) heteromeric nAChR binding across A1 layers; and (3) nAChR excitability in A1 layer (L) 5 cells. In aged rats, we detailed the impact of aging on A1 nAChR subunit expression across layers, heteromeric nAChR receptor binding, and nAChR excitability of A1 L5 cells. A majority of A1 cells coexpressed transcripts for β2 and α4 with or without α7, while dispersed subpopulations expressed β2 and α7 or α7 alone. nAChR subunit transcripts were expressed in young excitatory and inhibitory neurons across L2-L6. Transcript abundance varied across layers, and was highest for β2 and α4. Significant age-related decreases in nAChR subunit transcript expression (message) and receptor binding (protein) were observed in L2-6, most pronounced in infragranular layers. In vitro patch-clamp recordings from L5B pyramidal output neurons showed age-related nAChR subunit-selective reductions in postsynaptic responses to ACh. Age-related losses of nAChR subunits likely impact ways in which A1 neurons respond to ACh release. While the elderly require additional resources to disambiguate degraded speech codes, resources mediated by nAChRs may be compromised with aging.SIGNIFICANCE STATEMENT When attention is required, cholinergic basal forebrain neurons may trigger increased release of ACh onto auditory neurons in primary auditory cortex (A1). Laminar and phenotypic differences in neuronal nAChR expression determine ways in which A1 neurons respond to release of ACh in challenging acoustic environments. This study detailed the distribution and expression of nAChR subunit transcript and protein across A1 layers in young and aged rats. Results showed a differential distribution of nAChR subunits across A1 layers. Age-related decreases in transcript/protein expression were reflected in age-related subunit specific functional loss of nAChR signaling to ACh application in A1 layer 5. Together, these findings could reflect the age-related decline in selective attention observed in the elderly.

Effects of Noise on the Behavioral and Neural Categorization of Speech

We investigated whether the categorical perception (CP) of speech might also provide a mechanism that aids its perception in noise. We varied signal-to-noise ratio (SNR) [clear, 0 dB, -5 dB] while listeners classified an acoustic-phonetic continuum (/u/ to /a/). Noise-related changes in behavioral categorization were only observed at the lowest SNR. Event-related brain potentials (ERPs) differentiated category vs. category-ambiguous speech by the P2 wave (~180-320 ms). Paralleling behavior, neural responses to speech with clear phonetic status (i.e., continuum endpoints) were robust to noise down to -5 dB SNR, whereas responses to ambiguous tokens declined with decreasing SNR. Results demonstrate that phonetic speech representations are more resistant to degradation than corresponding acoustic representations. Findings suggest the mere process of binning speech sounds into categories provides a robust mechanism to aid figure-ground speech perception by fortifying abstract categories from the acoustic signal and making the speech code more resistant to external interferences.

Neural correlates of auditory sensory memory dynamics in the aging brain

The auditory system allows us to monitor background environmental sound patterns and recognize deviations that may indicate opportunities or threats. The mismatch negativity and P3a potentials have generators in the auditory and inferior frontal cortex and index expected sound patterns (standards) and any aberrations (deviants). The mismatch negativity and P3a waveforms show increased positivity for consecutive standards and deviants preceded by more standards. We hypothesized attenuated repetition effects in older participants, potentially because of differences in prefrontal functions. Young (23 ± 5 years) and older (75 ± 5 years) adults were tested in 2 oddball paradigms with pitch or location deviants. Significant repetition effects were observed in the young standard and deviant waveforms at multiple time windows. Except the earliest time window (30-100 ms), repetition effects were absent in the older group. Repetition effects were significant at frontal but not temporal lobe sites and did not differ among pitch and location deviants. However, P3a repetition was evident in both ages. Findings suggest age differences in the dynamic updating of sensory memory for background sound patterns.

Top-down or bottom up: decreased stimulus salience increases responses to predictable stimuli of auditory thalamic neurons

KEY POINTS

Temporal imprecision leads to deficits in the comprehension of signals in cluttered acoustic environments, and the elderly are shown to use cognitive resources to disambiguate these signals. To mimic ageing in young rats, we delivered sound signals that are temporally degraded, which led to temporally imprecise neural codes. Instead of adaptation to repeated stimuli, with degraded signals, there was a relative increase in firing rates, similar to that seen in aged rats. We interpret this increase with repetition as a repair mechanism for strengthening the internal representations of degraded signals by the higher-order structures.

ABSTRACT

To better understand speech in challenging environments, older adults increasingly use top-down cognitive and contextual resources. The medial geniculate body (MGB) integrates ascending inputs with descending predictions to dynamically gate auditory representations based on salience and context. A previous MGB single-unit study found an increased preference for predictable sinusoidal amplitude modulated (SAM) stimuli in aged rats relative to young rats. The results suggested that the age-degraded/jittered up-stream acoustic code may engender an increased preference for predictable/repeating acoustic signals, possibly reflecting increased use of top-down resources. In the present study, we recorded from units in young-adult MGB, comparing responses to standard SAM with those evoked by less salient SAM (degraded) stimuli. We hypothesized that degrading the SAM stimulus would simulate the degraded ascending acoustic code seen in the elderly, increasing the preference for predictable stimuli. Single units were recorded from clusters of advanceable tetrodes implanted above the MGB of young-adult awake rats. Less salient SAM significantly increased the preference for predictable stimuli, especially at higher modulation frequencies. Rather than adaptation, higher modulation frequencies elicited increased numbers of spikes with each successive trial/repeat of the less salient SAM. These findings are consistent with previous findings obtained in aged rats suggesting that less salient acoustic signals engage the additional use of top-down resources, as reflected by an increased preference for repeating stimuli that enhance the representation of complex environmental/communication sounds.

Auditory-visual integration modulates location-specific repetition suppression of auditory responses

Space is a dimension shared by different modalities, but at what stage spatial encoding is affected by multisensory processes is unclear. Early studies observed attenuation of N1/P2 auditory evoked responses following repetition of sounds from the same location. Here, we asked whether this effect is modulated by audiovisual interactions. In two experiments, using a repetition-suppression paradigm, we presented pairs of tones in free field, where the test stimulus was a tone presented at a fixed lateral location. Experiment 1 established a neural index of auditory spatial sensitivity, by comparing the degree of attenuation of the response to test stimuli when they were preceded by an adapter sound at the same location versus 30° or 60° away. We found that the degree of attenuation at the P2 latency was inversely related to the spatial distance between the test stimulus and the adapter stimulus. In Experiment 2, the adapter stimulus was a tone presented from the same location or a more medial location than the test stimulus. The adapter stimulus was accompanied by a simultaneous flash displayed orthogonally from one of the two locations. Sound-flash incongruence reduced accuracy in a same-different location discrimination task (i.e., the ventriloquism effect) and reduced the location-specific repetition-suppression at the P2 latency. Importantly, this multisensory effect included topographic modulations, indicative of changes in the relative contribution of underlying sources across conditions. Our findings suggest that the auditory response at the P2 latency is affected by spatially selective brain activity, which is affected crossmodally by visual information.

Responses to Predictable versus Random Temporally Complex Stimuli from Single Units in Auditory Thalamus: Impact of Aging and Anesthesia

Human aging studies suggest that an increased use of top-down knowledge-based resources would compensate for degraded upstream acoustic information to accurately identify important temporally rich signals. Sinusoidal amplitude-modulated (SAM) stimuli have been used to mimic the fast-changing temporal features in speech and species-specific vocalizations. Single units were recorded from auditory thalamus [medial geniculate body (MGB)] of young awake, aged awake, young anesthetized, and aged anesthetized rats. SAM stimuli were modulated between 2 and 1024 Hz with the modulation frequency (fm) changed randomly (RAN) across trials or sequentially (SEQ) after several repeated trials. Units were found to be RAN-preferring, SEQ-preferring, or nonselective based on total firing rate. Significant anesthesia and age effects were found. The majority (86%) of young anesthetized units preferred RAN SAM stimuli; significantly fewer young awake units (51%, p < 0.0001) preferred RAN SAM signals with 16% preferring SEQ SAM. Compared with young awake units, there was a significant increase of aged awake units preferring SEQ SAM (30%, p < 0.05). We examined RAN versus SEQ differences across fms by measuring selective fm areas under the rate modulation transfer function curve. The largest age-related differences from awake animals were found for mid-to-high fms in MGB units, with young units preferring RAN SAM while aged units showed a greater preference for SEQ-presented SAM. Together, these findings suggest that aged MGB units/animals employ increased top-down mediated stimulus context to enhance processing of "expected" temporally rich stimuli, especially at more challenging higher fms.

SIGNIFICANCE STATEMENT

Older individuals compensate for impaired ascending acoustic information by increasing use of cortical cognitive and attentional resources. The interplay between ascending and descending influences in the thalamus may serve to enhance the salience of speech signals that are degraded as they ascend to the cortex. The present findings demonstrate that medial geniculate body units from awake rats show an age-related preference for predictable modulated signals relative to randomly presented signals, especially at higher, more challenging modulation frequencies. Conversely, units from anesthetized animals, with little top-down influences, strongly preferred randomly presented modulated sequences. These results suggest a neuronal substrate for an age-related increase in experience/attentional-based influences in processing temporally complex auditory information in the auditory thalamus.

Neural Correlates of Speech Segregation Based on Formant Frequencies of Adjacent Vowels

The neural substrates by which speech sounds are perceptually segregated into distinct streams are poorly understood. Here, we recorded high-density scalp event-related potentials (ERPs) while participants were presented with a cyclic pattern of three vowel sounds (/ee/-/ae/-/ee/). Each trial consisted of an adaptation sequence, which could have either a small, intermediate, or large difference in first formant (Δf₁) as well as a test sequence, in which Δf₁ was always intermediate. For the adaptation sequence, participants tended to hear two streams (“streaming”) when Δf₁ was intermediate or large compared to when it was small. For the test sequence, in which Δf₁ was always intermediate, the pattern was usually reversed, with participants hearing a single stream with increasing Δf₁ in the adaptation sequences. During the adaptation sequence, Δf₁-related brain activity was found between 100–250 ms after the /ae/ vowel over fronto-central and left temporal areas, consistent with generation in auditory cortex. For the test sequence, prior stimulus modulated ERP amplitude between 20–150 ms over left fronto-central scalp region. Our results demonstrate that the proximity of formants between adjacent vowels is an important factor in the perceptual organization of speech, and reveal a widely distributed neural network supporting perceptual grouping of speech sounds.

Increased Early Processing of Task-Irrelevant Auditory Stimuli in Older Adults

The inhibitory deficit hypothesis of cognitive aging posits that older adults' inability to adequately suppress processing of irrelevant information is a major source of cognitive decline. Prior research has demonstrated that in response to task-irrelevant auditory stimuli there is an age-associated increase in the amplitude of the N1 wave, an ERP marker of early perceptual processing. Here, we tested predictions derived from the inhibitory deficit hypothesis that the age-related increase in N1 would be 1) observed under an auditory-ignore, but not auditory-attend condition, 2) attenuated in individuals with high executive capacity (EC), and 3) augmented by increasing cognitive load of the primary visual task. ERPs were measured in 114 well-matched young, middle-aged, young-old, and old-old adults, designated as having high or average EC based on neuropsychological testing. Under the auditory-ignore (visual-attend) task, participants ignored auditory stimuli and responded to rare target letters under low and high load. Under the auditory-attend task, participants ignored visual stimuli and responded to rare target tones. Results confirmed an age-associated increase in N1 amplitude to auditory stimuli under the auditory-ignore but not auditory-attend task. Contrary to predictions, EC did not modulate the N1 response. The load effect was the opposite of expectation: the N1 to task-irrelevant auditory events was smaller under high load. Finally, older adults did not simply fail to suppress the N1 to auditory stimuli in the task-irrelevant modality; they generated a larger response than to identical stimuli in the task-relevant modality. In summary, several of the study's findings do not fit the inhibitory-deficit hypothesis of cognitive aging, which may need to be refined or supplemented by alternative accounts.

Early selectivity for vocal and musical sounds: electrophysiological evidence from an adaptation paradigm

There is growing interest in characterizing the neural basis of music perception and, in particular, assessing how similar, or not, it is to that of speech. To further explore this question, we employed an EEG adaptation paradigm in which we compared responses to short sounds belonging to the same category, either speech (pseudo-sentences) or music (piano or violin), depending on whether they were immediately preceded by a same- or different-category sound. We observed a larger reduction in the N100 component magnitude in response to musical sounds when they were preceded by music (either the same or different instrument) than by speech. In contrast, the N100 amplitude was not affected by the preceding stimulus category in the case of speech. For P200 component, we observed a diminution of amplitude when speech sounds were preceded speech, compared to music. No such decrease was found when we compared the responses to music sounds. These differences in the processing of speech and music are consistent with the proposal that some degree of category selectivity for these two classes of complex stimuli already occurs at early stages of auditory processing, possibly subserved by partly separated neuronal populations.

Similar sound intensity dependence of the N1 and P2 components of the auditory ERP: Averaged and single trial evidence

OBJECTIVE

The literature suggests that the N1 and P2 waves of the auditory ERP are dissociable at the developmental, experimental, and source levels. At the experimental level, inconsistent findings suggest different effects of intensity on the amplitudes of the auditory N1 and P2. Our main goal was to analyze the intensity dependence of the auditory N1 and P2 while controlling for habituation effects.

METHODS

We examined the intensity dependence of both averaged and single-trial auditory N1 and P2 waves elicited in a repeated-stimulation protocol.

RESULTS

N1 and P2 revealed similar intensity dependence on both standard and filter denoised ERP, with a linear tendency for higher intensities to elicit higher absolute peak amplitudes. At the single-trial level, both waves covary irrespective of stimulus intensity and trial order.

CONCLUSIONS

Our results suggest that stimulus intensity variation induces similar effects on both and N1 and P2 and partially contradict previous data that classified the P2 as a non-habituating component.

SIGNIFICANCE

Our findings contribute to the ongoing discussion on the functional significance of the auditory P2 deflection. In addition, the present work demonstrated the applicability of a filter denoising method for single-trial estimation in the analysis of the experimental effects on auditory ERP components.