Age-related auditory deficits in temporal processing in F-344 rats

Neurometric amplitude modulation detection in the inferior colliculus of Young and Aged rats

Amplitude modulation is an important acoustic cue for sound discrimination, and humans and animals are able to detect small modulation depths behaviorally. In the inferior colliculus (IC), both firing rate and phase-locking may be used to detect amplitude modulation. How neural representations that detect modulation change with age are poorly understood, including the extent to which age-related changes may be attributed to the inherited properties of ascending inputs to IC neurons. Here, simultaneous measures of local field potentials (LFPs) and single-unit responses were made from the inferior colliculus of Young and Aged rats using both noise and tone carriers in response to sinusoidally amplitude-modulated sounds of varying depths. We found that Young units had higher firing rates than Aged for noise carriers, whereas Aged units had higher phase-locking (vector strength), especially for tone carriers. Sustained LFPs were larger in Young animals for modulation frequencies 8-16 Hz and comparable at higher modulation frequencies. Onset LFP amplitudes were much larger in Young animals and were correlated with the evoked firing rates, while LFP onset latencies were shorter in Aged animals. Unit neurometric thresholds by synchrony or firing rate measures did not differ significantly across age and were comparable to behavioral thresholds in previous studies whereas LFP thresholds were lower than behavior.

Age-related upregulation of dense core vesicles in the central inferior colliculus

Presbycusis is one of the most prevalent disabilities in aged populations of industrialized countries. As we age less excitation reaches the central auditory system from the periphery. To compensate, the central auditory system [e.g., the inferior colliculus (IC)], downregulates GABAergic inhibition to maintain homeostatic balance. However, the continued downregulation of GABA in the IC causes a disruption in temporal precision related to presbycusis. Many studies of age-related changes to neurotransmission in the IC have therefore focused on GABAergic systems. However, we have discovered that dense core vesicles (DCVs) are significantly upregulated with age in the IC. DCVs can carry neuropeptides, co-transmitters, neurotrophic factors, and proteins destined for the presynaptic zone to participate in synaptogenesis. We used immuno transmission electron microscopy across four age groups (3-month; 19-month; 24-month; and 28-month) of Fisher Brown Norway rats to examine the ultrastructure of DCVs in the IC. Tissue was stained post-embedding for GABA immunoreactivity. DCVs were characterized by diameter and by the neurochemical profile (GABAergic/non-GABAergic) of their location (bouton, axon, soma, and dendrite). Our data was collected across the dorsolateral to ventromedial axis of the central IC. After quantification, we had three primary findings. First, the age-related increase of DCVs occurred most robustly in non-GABAergic dendrites in the middle and low frequency regions of the central IC during middle age. Second, the likelihood of a bouton having more than one DCV increased with age. Lastly, although there was an age-related loss of terminals throughout the IC, the proportion of terminals that contained at least one DCV did not decline. We interpret this finding to mean that terminals carrying proteins packaged in DCVs are spared with age. Several recent studies have demonstrated a role for neuropeptides in the IC in defining cell types and regulating inhibitory and excitatory neurotransmission. Given the age-related increase of DCVs in the IC, it will be critical that future studies determine whether (1) specific neuropeptides are altered with age in the IC and (2) if these neuropeptides contribute to the loss of inhibition and/or increase of excitability that occurs during presbycusis and tinnitus.

Neural hyperactivity and altered envelope encoding in the central auditory system: Changes with advanced age and hearing loss

Temporal modulations are ubiquitous features of sound signals that are important for auditory perception. The perception of temporal modulations, or temporal processing, is known to decline with aging and hearing loss and negatively impact auditory perception in general and speech recognition specifically. However, neurophysiological literature also provides evidence of exaggerated or enhanced encoding of specifically temporal envelopes in aging and hearing loss, which may arise from changes in inhibitory neurotransmission and neuronal hyperactivity. This review paper describes the physiological changes to the neural encoding of temporal envelopes that have been shown to occur with age and hearing loss and discusses the role of disinhibition and neural hyperactivity in contributing to these changes. Studies in both humans and animal models suggest that aging and hearing loss are associated with stronger neural representations of both periodic amplitude modulation envelopes and of naturalistic speech envelopes, but primarily for low-frequency modulations (<80 Hz). Although the frequency dependence of these results is generally taken as evidence of amplified envelope encoding at the cortex and impoverished encoding at the midbrain and brainstem, there is additional evidence to suggest that exaggerated envelope encoding may also occur subcortically, though only for envelopes with low modulation rates. A better understanding of how temporal envelope encoding is altered in aging and hearing loss, and the contexts in which neural responses are exaggerated/diminished, may aid in the development of interventions, assistive devices, and treatment strategies that work to ameliorate age- and hearing-loss-related auditory perceptual deficits.

Rapid and objective assessment of auditory temporal processing using dynamic amplitude-modulated stimuli

Auditory neural coding of speech-relevant temporal cues can be noninvasively probed using envelope following responses (EFRs), neural ensemble responses phase-locked to the stimulus amplitude envelope. EFRs emphasize different neural generators, such as the auditory brainstem or auditory cortex, by altering the temporal modulation rate of the stimulus. EFRs can be an important diagnostic tool to assess auditory neural coding deficits that go beyond traditional audiometric estimations. Existing approaches to measure EFRs use discrete amplitude modulated (AM) tones of varying modulation frequencies, which is time consuming and inefficient, impeding clinical translation. Here we present a faster and more efficient framework to measure EFRs across a range of AM frequencies using stimuli that dynamically vary in modulation rates, combined with spectrally specific analyses that offer optimal spectrotemporal resolution. EFRs obtained from several species (humans, Mongolian gerbils, Fischer-344 rats, and Cba/CaJ mice) showed robust, high-SNR tracking of dynamic AM trajectories (up to 800Hz in humans, and 1.4 kHz in rodents), with a fivefold decrease in recording time and thirtyfold increase in spectrotemporal resolution. EFR amplitudes between dynamic AM stimuli and traditional discrete AM tokens within the same subjects were highly correlated (94% variance explained) across species. Hence, we establish a time-efficient and spectrally specific approach to measure EFRs. These results could yield novel clinical diagnostics for precision audiology approaches by enabling rapid, objective assessment of temporal processing along the entire auditory neuraxis.

Age-related upregulation of perineuronal nets on inferior collicular cells that project to the cochlear nucleus

Introduction

Disruptions to the balance of excitation and inhibition in the inferior colliculus (IC) occur during aging and underlie various aspects of hearing loss. Specifically, the age-related alteration to GABAergic neurotransmission in the IC likely contributes to the poorer temporal precision characteristic of presbycusis. Perineuronal nets (PNs), a specialized form of the extracellular matrix, maintain excitatory/inhibitory synaptic environments and reduce structural plasticity. We sought to determine whether PNs increasingly surround cell populations in the aged IC that comprise excitatory descending projections to the cochlear nucleus.

Method

We combined Wisteria floribunda agglutinin (WFA) staining for PNs with retrograde tract-tracing in three age groups of Fischer Brown Norway (FBN) rats.

Results

The data demonstrate that the percentage of IC-CN cells with a PN doubles from ~10% at young age to ~20% at old age. This was true in both lemniscal and non-lemniscal IC.

Discussion

Furthermore, the increase of PNs occurred on IC cells that make both ipsilateral and contralateral descending projections to the CN. These results indicate that reduced structural plasticity in the elderly IC-CN pathway, affecting excitatory/inhibitory balance and, potentially, may lead to reduced temporal precision associated with presbycusis.

Age-related Changes of GAD1 mRNA Expression in the Central Inferior Colliculus

Encoding sounds with a high degree of temporal precision is an essential task for the inferior colliculus (IC) to perform and maintain the accurate processing of sounds and speech. However, the age-related reduction of GABAergic neurotransmission in the IC interrupts temporal precision and likely contributes to presbycusis. As presbycusis often manifests at high or low frequencies specifically, we sought to determine if the expression of mRNA for glutamic decarboxylase 1 (GAD1) is downregulated non-uniformly across the tonotopic axis or cell size range in the aging IC. Using single molecule in situ fluorescent hybridization across young, middle age and old Fisher Brown Norway rats (an aging model that acquires low frequency presbycusis) we quantified individual GAD1 mRNA in small, medium and large GABAergic cells. Our results demonstrate that small GABAergic cells in low frequency regions had ~58% less GAD1 in middle age and continued to decline into old age. In contrast, the amount of GAD1 mRNA in large cells in low frequency regions significantly increased with age. As several studies have shown that downregulation of GAD1 decreases the release of GABA, we interpret our results in two ways. First, the onset of presbycusis may be driven by small GABAergic cells downregulating GAD1. Second, as previous studies demonstrate that GAD67 expression is broadly downregulated in the old IC, perhaps the translation of GAD1 to GAD67 is interrupted in large GABAergic IC cells during aging. These results point to a potential genetic mechanism explaining reduced temporal precision in the aging IC, and in turn, presbycusis.

Cortical-brainstem interplay during speech perception in older adults with and without hearing loss

Introduction

Real time modulation of brainstem frequency-following responses (FFRs) by online changes in cortical arousal state via the corticofugal (top-down) pathway has been demonstrated previously in young adults and is more prominent in the presence of background noise. FFRs during high cortical arousal states also have a stronger relationship with speech perception. Aging is associated with increased auditory brain responses, which might reflect degraded inhibitory processing within the peripheral and ascending pathways, or changes in attentional control regulation via descending auditory pathways. Here, we tested the hypothesis that online corticofugal interplay is impacted by age-related hearing loss.

Methods

We measured EEG in older adults with normal-hearing (NH) and mild to moderate hearing-loss (HL) while they performed speech identification tasks in different noise backgrounds. We measured α power to index online cortical arousal states during task engagement. Subsequently, we split brainstem speech-FFRs, on a trial-by-trial basis, according to fluctuations in concomitant cortical α power into low or high α FFRs to index cortical-brainstem modulation.

Results

We found cortical α power was smaller in the HL than the NH group. In NH listeners, α-FFRs modulation for clear speech (i.e., without noise) also resembled that previously observed in younger adults for speech in noise. Cortical-brainstem modulation was further diminished in HL older adults in the clear condition and by noise in NH older adults. Machine learning classification showed low α FFR frequency spectra yielded higher accuracy for classifying listeners' perceptual performance in both NH and HL participants. Moreover, low α FFRs decreased with increased hearing thresholds at 0.5-2 kHz for clear speech but noise generally reduced low α FFRs in the HL group.

Discussion

Collectively, our study reveals cortical arousal state actively shapes brainstem speech representations and provides a potential new mechanism for older listeners' difficulties perceiving speech in cocktail party-like listening situations in the form of a miss-coordination between cortical and subcortical levels of auditory processing.

Age-related ultrastructural changes in the lateral cortex of the inferior colliculus

Temporal precision, a key component of sound and speech processing in the inferior colliculus (IC), depends on a balance of inhibition and excitation, and this balance degrades during aging. The cause of disrupted excitatory-inhibitory balance in aging is unknown, however changes at the synapse are a likely candidate. We sought to determine whether synaptic changes occur in the lateral cortex of the IC (IClc), a multimodal nucleus that processes lemniscal, intrinsic, somatosensory, and descending auditory input. Using electron microscopic techniques across young, middle age and old Fisher Brown Norway rats, our results demonstrate minimal loss of synapses in middle age, but significant (∼28%) loss during old age. However, in middle age, targeting of GABAergic dendrites by GABAergic synapses is increased and the active zones of excitatory synapses (that predominantly target GABA-negative dendrites) are lengthened. These synaptic changes likely result in a net increase of excitation in the IClc during middle age. Thus, disruption of excitatory-inhibitory balance in the aging IClc may be due to synaptic changes that begin in middle age.

Corticothalamic projections deliver enhanced responses to medial geniculate body as a function of the temporal reliability of the stimulus

Ageing and challenging signal-in-noise conditions are known to engage the use of cortical resources to help maintain speech understanding. Extensive corticothalamic projections are thought to provide attentional, mnemonic and cognitive-related inputs in support of sensory inferior colliculus (IC) inputs to the medial geniculate body (MGB). Here we show that a decrease in modulation depth, a temporally less distinct periodic acoustic signal, leads to a jittered ascending temporal code, changing MGB unit responses from adapting responses to responses showing repetition enhancement, posited to aid identification of important communication and environmental sounds. Young-adult male Fischer Brown Norway rats, injected with the inhibitory opsin archaerhodopsin T (ArchT) into the primary auditory cortex (A1), were subsequently studied using optetrodes to record single-units in MGB. Decreasing the modulation depth of acoustic stimuli significantly increased repetition enhancement. Repetition enhancement was blocked by optical inactivation of corticothalamic terminals in MGB. These data support a role for corticothalamic projections in repetition enhancement, implying that predictive anticipation could be used to improve neural representation of weakly modulated sounds. KEY POINTS: In response to a less temporally distinct repeating sound with low modulation depth, medial geniculate body (MGB) single units show a switch from adaptation towards repetition enhancement. Repetition enhancement was reversed by blockade of MGB inputs from the auditory cortex. Collectively, these data argue that diminished acoustic temporal cues such as weak modulation engage cortical processes to enhance coding of those cues in auditory thalamus.

Longitudinal auditory pathophysiology following mild blast-induced trauma

Blast-induced hearing difficulties affect thousands of veterans and civilians. The long-term impact of even a mild blast exposure on the central auditory system is hypothesized to contribute to lasting behavioral complaints associated with mild blast traumatic brain injury (bTBI). Although recovery from mild blast has been studied separately over brief or long time windows, few, if any, studies have investigated recovery longitudinally over short-term and longer-term (months) time windows. Specifically, many peripheral measures of auditory function either recover or exhibit subclinical deficits, masking deficits in processing complex, real-world stimuli that may recover differently. Thus, examining the acute time course and pattern of neurophysiological impairment using appropriate stimuli is critical to better understanding and intervening in bTBI-induced auditory system impairments. Here, we compared auditory brainstem response, middle-latency auditory-evoked potentials, and envelope following responses. Stimuli were clicks, tone pips, amplitude-modulated tones in quiet and in noise, and speech-like stimuli (iterated rippled noise pitch contours) in adult male rats subjected to mild blast and sham exposure over the course of 2 mo. We found that blast animals demonstrated drastic threshold increases and auditory transmission deficits immediately after blast exposure, followed by substantial recovery during the window of 7-14 days postblast, although with some deficits remaining even after 2 mo. Challenging conditions and speech-like stimuli can better elucidate mild bTBI-induced auditory deficit during this period. Our results suggest multiphasic recovery and therefore potentially different time windows for treatment, and deficits can be best observed using a small battery of sound stimuli.NEW & NOTEWORTHY Few studies on blast-induced hearing deficits go beyond simple sounds and sparsely track postexposure. Therefore, the recovery arc for potential therapies and real-world listening is poorly understood. Evidence suggested multiple recovery phases over 2 mo postexposure. Hearing thresholds largely recovered within 14 days and partially explained recovery. However, midlatency responses, responses to amplitude modulation in noise, and speech-like pitch sweeps exhibited extended changes, implying persistent central auditory deficits and the importance of subclinical threshold shifts.

Inferior collicular cells that project to the auditory thalamus are increasingly surrounded by perineuronal nets with age

The age-related loss of GABA in the inferior colliculus (IC) likely plays a role in the development of age-related hearing loss. Perineuronal nets (PNs), specialized aggregates of extracellular matrix, increase with age in the IC. PNs, associated with GABAergic neurotransmission, can stabilize synapses and inhibit structural plasticity. We sought to determine whether PN expression increased on GABAergic and non-GABAergic IC cells that project to the medial geniculate body (MG). We used retrograde tract-tracing in combination with immunohistochemistry for glutamic acid decarboxylase and Wisteria floribunda agglutinin across three age groups of Fischer Brown Norway rats. Results demonstrate that PNs increase with age on lemniscal and non-lemniscal IC-MG cells, however two key differences exist. First, PNs increased on non-lemniscal IC-MG cells during middle-age, but not until old age on lemniscal IC-MG cells. Second, increases of PNs on lemniscal IC-MG cells occurred on non-GABAergic cells rather than on GABAergic cells. These results suggest that synaptic stabilization and reduced plasticity likely occur at different ages on a subset of the IC-MG pathway.

Effect of Stimulation Rate on Speech Understanding in Older Cochlear-Implant Users

OBJECTIVES

Cochlear implants (CIs) are considered a safe and effective intervention for more severe degrees of hearing loss in adults of all ages. Although older CI users ≥65 years of age can obtain large benefits in speech understanding from a CI, there is a growing body of literature suggesting that older CI users may not perform as well as younger CI users. One reason for this potential age-related limitation could be that default CI stimulation settings are not optimal for older CI users. The goal of this study was to determine whether improvements in speech understanding were possible when CI users were programmed with nondefault stimulation rates and to determine whether lower-than-default stimulation rates improved older CI users' speech understanding.

DESIGN

Sentence recognition was measured acutely using different stimulation rates in 37 CI users ranging in age from 22 to 87 years. Maps were created using rates of 500, 720, 900, and 1200 pulses per second (pps) for each subject. An additional map using a rate higher than 1200 pps was also created for individuals who used a higher rate in their clinical processors. Thus, the clinical rate of each subject was also tested, including non-default rates above 1200 pps for Cochlear users and higher rates consistent with the manufacturer defaults for subjects implanted with Advanced Bionics and Med-El devices. Speech understanding performance was evaluated at each stimulation rate using AzBio and Perceptually Robust English Sentence Test Open-set (PRESTO) sentence materials tested in quiet and in noise.

RESULTS

For Cochlear-brand users, speech understanding performance using non-default rates was slightly poorer when compared with the default rate (900 pps). However, this effect was offset somewhat by age, in which older subjects were able to maintain comparable performance using a 500-pps map compared with the default rate map when listening to the more difficult PRESTO sentence material. Advanced Bionics and Med-El users showed modest improvements in their overall performance using 720 pps compared with the default rate (>1200 pps). On the individual-subject level, 10 subjects (11 ears) showed a significant effect of stimulation rate, with 8 of those ears performing best with a lower-than-default rate.

CONCLUSIONS

Results suggest that default stimulation rates are likely sufficient for many CI users, but some CI users at any age can benefit from a lower-than-default rate. Future work that provides experience with novel rates in everyday life has the potential to identify more individuals whose performance could be improved with changes to stimulation rate.

Effect of Hearing Aid Acclimatization on Speech-in-Noise Perception and Its Relationship With Changes in Auditory Long Latency Responses

Objective The study attempted to track speech-in-noise perception and auditory long latency responses (ALLRs) over a period of hearing aid use in naïve hearing aid users. The primary aim was to investigate the relationship of change in speech-in-noise perception with the change in ALLRs. Method Thirty adults with mild-to-moderate sensorineural hearing loss (clinical group) and 17 adults with normal hearing (control group) in the age range of 23-60 years participated in the study. Syllable identification in noise (SIN) and ALLRs in noise were measured three times (three sessions) over a period of 2 months of hearing aid use. Results Results showed a significant increase in SIN and a decrease in the latency of ALLRs in the later sessions compared to the baseline session in the clinical group. However, the changes seen across the three sessions in the control group were not statistically significant. The magnitude of change in ALLRs seen in the clinical group did not significantly correlate with the change in SIN scores seen in them. Conclusions The study provides evidence for improvements in speech perception in noise and in processing time of auditory cortical areas with hearing aid acclimatization. However, it is important to note that the improvement in ALLRs does not assure improvement in speech perception in noise.

Age- and movement-related modulation of cortical oscillations in a mouse model of presbycusis

Brain oscillations are associated with specific cognitive and sensory processes. How age-related hearing loss (presbycusis) alters cortical oscillations is unclear. Altered inhibitory neurotransmission and temporal processing deficits contribute to speech recognition impairments in presbycusis. Specifically, age-related reduction in parvalbumin positive interneurons and perineuronal nets in the auditory cortex predicts a reduction in gamma oscillations that may lead to a decline in temporal precision and attention. To test the hypothesis that resting and evoked gamma oscillations decline with presbycusis, EEGs were recorded from the auditory and frontal cortex of awake, freely moving C57BL/6 J mice at three ages (3, 14 and 24 months). Resting EEG data were analyzed according to movement state (move versus still). Evoked responses were recorded following presentation of noise bursts or amplitude modulated noise with time varying modulation frequencies. We report an age-related decrease in resting gamma power, a decline in gamma-range synchrony to time varying stimuli, and an increase in noise evoked and induced gamma power. A decline in temporal processing is seen in aged mice that exhibit robust auditory-evoked potentials, dissociating hearing loss from temporal processing deficits. We also report an increase in gamma power when mice moved compared to the still state. However, the movement-related modulation of gamma oscillations did not change with age. Together, these data identify a number of novel markers of presbycusis-related changes in auditory and frontal cortex. Because EEGs are commonly recorded in humans, the mouse data may serve as translation relevant preclinical biomarkers to facilitate the development of therapeutics to delay or reverse central auditory processing deficits in presbycusis.

Age-Related Compensation Mechanism Revealed in the Cortical Representation of Degraded Speech

Older adults understand speech with comparative ease in quiet, but signal degradation can hinder speech understanding much more than it does in younger adults. This difficulty may result, in part, from temporal processing deficits related to the aging process and/or high-frequency hearing loss that can occur in listeners who have normal- or near-normal-hearing thresholds in the speech frequency range. Temporal processing deficits may manifest as degraded neural representation in peripheral and brainstem/midbrain structures that lead to compensation, or changes in response strength in auditory cortex. Little is understood about the process by which the neural representation of signals is improved or restored by age-related cortical compensation mechanisms. Therefore, we used vocoding to simulate spectral degradation to compare the behavioral and neural representation of words that contrast on a temporal dimension. Specifically, we used the closure duration of the silent interval between the vowel and the final affricate /t∫/ or fricative /ʃ/ of the words DITCH and DISH, respectively. We obtained perceptual identification functions and electrophysiological neural measures (frequency-following responses (FFR) and cortical auditory-evoked potentials (CAEPs)) to unprocessed and vocoded versions of these words in young normal-hearing (YNH), older normal- or near-normal-hearing (ONH), and older hearing-impaired (OHI) listeners. We found that vocoding significantly reduced the slope of the perceptual identification function in only the OHI listeners. In contrast to the limited effects of vocoding on perceptual performance, vocoding had robust effects on the FFRs across age groups, such that stimulus-to-response correlations and envelope magnitudes were significantly lower for vocoded vs. unprocessed conditions. Increases in the P1 peak amplitude for vocoded stimuli were found for both ONH and OHI listeners, but not for the YNH listeners. These results suggest that while vocoding substantially degrades early neural representation of speech stimuli in the midbrain, there may be cortical compensation in older listeners that is not seen in younger listeners.

The Density of Perineuronal Nets Increases With Age in the Inferior Colliculus in the Fischer Brown Norway Rat

Age-related hearing loss, one of the most frequently diagnosed disabilities in industrialized countries, may result from declining levels of GABA in the aging inferior colliculus (IC). However, the mechanisms of aging and subsequent disruptions of temporal processing in elderly hearing abilities are still being investigated. Perineuronal nets (PNs) are a specialized form of the extracellular matrix and have been linked to GABAergic neurotransmission and to the regulation of structural and synaptic plasticity. We sought to determine whether the density of PNs in the IC changes with age. We combined Wisteria floribunda agglutinin (WFA) staining with immunohistochemistry to glutamic acid decarboxylase in three age groups of Fischer Brown Norway (FBN) rats. The density of PNs on GABAergic and non-GABAergic cells in the three major subdivisions of the IC was quantified. Results first demonstrate that the density of PNs in the FBN IC increase with age. The greatest increases of PN density from young to old age occurred in the central IC (67% increase) and dorsal IC (117% increase). Second, in the young IC, PNs surround non-GABAergic and GABAergic cells with the majority of PNs surrounding the former. The increase of PNs with age in the IC occurred on both non-GABAergic and GABAergic populations. The average density of PN-surrounded non-GABAergic cells increased from 84.9 PNs/mm² in the young to 134.2 PNs/mm² in the old. While the density of PN-surrounded GABAergic cells increased from 26 PNs/mm² in the young to 40.6 PNs/mm² in the old. The causality is unclear, but increases in PN density in old age may play a role in altered auditory processing in the elderly, or may lead to further changes in IC plasticity.

Temporal Coding of Single Auditory Nerve Fibers Is Not Degraded in Aging Gerbils

People suffering from age-related hearing loss typically present with deficits in temporal processing tasks. Temporal processing deficits have also been shown in single-unit studies at the level of the auditory brainstem, midbrain, and cortex of aged animals. In this study, we explored whether temporal coding is already affected at the level of the input to the central auditory system. Single-unit auditory nerve fiber recordings were obtained from 41 Mongolian gerbils of either sex, divided between young, middle-aged, and old gerbils. Temporal coding quality was evaluated as vector strength in response to tones at best frequency, and by constructing shuffled and cross-stimulus autocorrelograms, and reverse correlations, from responses to 1 s noise bursts at 10-30 dB sensation level (dB above threshold). At comparable sensation levels, all measures showed that temporal coding was not altered in auditory nerve fibers of aging gerbils. Furthermore, both temporal fine structure and envelope coding remained unaffected. However, spontaneous rates were decreased in aging gerbils. Importantly, despite elevated pure tone thresholds, the frequency tuning of auditory nerve fibers was not affected. These results suggest that age-related temporal coding deficits arise more centrally, possibly due to a loss of auditory nerve fibers (or their peripheral synapses) but not due to qualitative changes in the responses of remaining auditory nerve fibers. The reduced spontaneous rate and elevated thresholds, but normal frequency tuning, of aged auditory nerve fibers can be explained by the well known reduction of endocochlear potential due to strial dysfunction in aged gerbils.SIGNIFICANCE STATEMENT As our society ages, age-related hearing deficits become ever more prevalent. Apart from decreased hearing sensitivity, elderly people often suffer from a reduced ability to communicate in daily settings, which is thought to be caused by known age-related deficits in auditory temporal processing. The current study demonstrated, using several different stimuli and analysis techniques, that these putative temporal processing deficits are not apparent in responses of single-unit auditory nerve fibers of quiet-aged gerbils. This suggests that age-related temporal processing deficits may develop more central to the auditory nerve, possibly due to a reduced population of active auditory nerve fibers, which will be of importance for the development of treatments for age-related hearing disorders.

Temporal Coding of Single Auditory Nerve Fibers Is Not Degraded in Aging Gerbils

People suffering from age-related hearing loss typically present with deficits in temporal processing tasks. Temporal processing deficits have also been shown in single-unit studies at the level of the auditory brainstem, midbrain, and cortex of aged animals. In this study, we explored whether temporal coding is already affected at the level of the input to the central auditory system. Single-unit auditory nerve fiber recordings were obtained from 41 Mongolian gerbils of either sex, divided between young, middle-aged, and old gerbils. Temporal coding quality was evaluated as vector strength in response to tones at best frequency, and by constructing shuffled and cross-stimulus autocorrelograms, and reverse correlations, from responses to 1 s noise bursts at 10-30 dB sensation level (dB above threshold). At comparable sensation levels, all measures showed that temporal coding was not altered in auditory nerve fibers of aging gerbils. Furthermore, both temporal fine structure and envelope coding remained unaffected. However, spontaneous rates were decreased in aging gerbils. Importantly, despite elevated pure tone thresholds, the frequency tuning of auditory nerve fibers was not affected. These results suggest that age-related temporal coding deficits arise more centrally, possibly due to a loss of auditory nerve fibers (or their peripheral synapses) but not due to qualitative changes in the responses of remaining auditory nerve fibers. The reduced spontaneous rate and elevated thresholds, but normal frequency tuning, of aged auditory nerve fibers can be explained by the well known reduction of endocochlear potential due to strial dysfunction in aged gerbils.SIGNIFICANCE STATEMENT As our society ages, age-related hearing deficits become ever more prevalent. Apart from decreased hearing sensitivity, elderly people often suffer from a reduced ability to communicate in daily settings, which is thought to be caused by known age-related deficits in auditory temporal processing. The current study demonstrated, using several different stimuli and analysis techniques, that these putative temporal processing deficits are not apparent in responses of single-unit auditory nerve fibers of quiet-aged gerbils. This suggests that age-related temporal processing deficits may develop more central to the auditory nerve, possibly due to a reduced population of active auditory nerve fibers, which will be of importance for the development of treatments for age-related hearing disorders.

Age-related hearing loss increases full-brain connectivity while reversing directed signaling within the dorsal-ventral pathway for speech

Speech comprehension difficulties are ubiquitous to aging and hearing loss, particularly in noisy environments. Older adults' poorer speech-in-noise (SIN) comprehension has been related to abnormal neural representations within various nodes (regions) of the speech network, but how senescent changes in hearing alter the transmission of brain signals remains unspecified. We measured electroencephalograms in older adults with and without mild hearing loss during a SIN identification task. Using functional connectivity and graph-theoretic analyses, we show that hearing-impaired (HI) listeners have more extended (less integrated) communication pathways and less efficient information exchange among widespread brain regions (larger network eccentricity) than their normal-hearing (NH) peers. Parameter optimized support vector machine classifiers applied to EEG connectivity data showed hearing status could be decoded (> 85% accuracy) solely using network-level descriptions of brain activity, but classification was particularly robust using left hemisphere connections. Notably, we found a reversal in directed neural signaling in left hemisphere dependent on hearing status among specific connections within the dorsal-ventral speech pathways. NH listeners showed an overall net "bottom-up" signaling directed from auditory cortex (A1) to inferior frontal gyrus (IFG; Broca's area), whereas the HI group showed the reverse signal (i.e., "top-down" Broca's → A1). A similar flow reversal was noted between left IFG and motor cortex. Our full-brain connectivity results demonstrate that even mild forms of hearing loss alter how the brain routes information within the auditory-linguistic-motor loop.

Mutual information analysis of neural representations of speech in noise in the aging midbrain

Younger adults with normal hearing can typically understand speech in the presence of a competing speaker without much effort, but this ability to understand speech in challenging conditions deteriorates with age. Older adults, even with clinically normal hearing, often have problems understanding speech in noise. Earlier auditory studies using the frequency-following response (FFR), primarily believed to be generated by the midbrain, demonstrated age-related neural deficits when analyzed with traditional measures. Here we use a mutual information paradigm to analyze the FFR to speech (masked by a competing speech signal) by estimating the amount of stimulus information contained in the FFR. Our results show, first, a broadband informational loss associated with aging for both FFR amplitude and phase. Second, this age-related loss of information is more severe in higher-frequency FFR bands (several hundred hertz). Third, the mutual information between the FFR and the stimulus decreases as noise level increases for both age groups. Fourth, older adults benefit neurally, i.e., show a reduction in loss of information, when the speech masker is changed from meaningful (talker speaking a language that they can comprehend, such as English) to meaningless (talker speaking a language that they cannot comprehend, such as Dutch). This benefit is not seen in younger listeners, which suggests that age-related informational loss may be more severe when the speech masker is meaningful than when it is meaningless. In summary, as a method, mutual information analysis can unveil new results that traditional measures may not have enough statistical power to assess.NEW & NOTEWORTHY Older adults, even with clinically normal hearing, often have problems understanding speech in noise. Auditory studies using the frequency-following response (FFR) have demonstrated age-related neural deficits with traditional methods. Here we use a mutual information paradigm to analyze the FFR to speech masked by competing speech. Results confirm those from traditional analysis but additionally show that older adults benefit neurally when the masker changes from a language that they comprehend to a language they cannot.

Auditory temporal acuity improves with age in the male mouse auditory thalamus: A role for perineuronal nets?

The ability to perceive and interpret environmental sound accurately is conserved across many species and is fundamental for understanding communication via vocalizations. Auditory acuity and temporally controlled neuronal firing underpin this ability. Deterioration in neuronal firing precision likely contributes to poorer hearing performance, yet the role of neural processing by key nuclei in the central auditory pathways is not fully understood. Here, we record from the auditory thalamus (medial geniculate body [MGB]) of young and middle-aged, normally hearing male CBA/Ca mice. We report changes in temporal processing of auditory stimuli, with neurons recorded from ventral and medial MGB subdivisions of older animals more likely to synchronize to rapid temporally varying stimuli. MGB subdivisions also showed increased probability of neuronal firing and shorter response latencies to clicks in older animals. Histological investigation of neuronal extracellular specializations, perineuronal nets (PNNs) and axonal coats, in the MGB identified greater organization of PNNs around MGB neurons and the presence of axonal coats within older animals. This supports the observation that neural responses recorded from ventral and medial MGB of older mice were more likely to synchronize to temporally varying stimuli presented at faster repetition rates than those recorded from young adult animals. These changes are observed in animals with normal hearing thresholds, confirming that neural processing differs between the MGB subdivisions and such processing is associated with age-related changes to PNNs. Understanding these age-related changes and how they occur have important implications for the design of effective therapeutic interventions to improve speech intelligibility into later life.

Effects of Age, Cognition, and Neural Encoding on the Perception of Temporal Speech Cues

Older adults commonly report difficulty understanding speech, particularly in adverse listening environments. These communication difficulties may exist in the absence of peripheral hearing loss. Older adults, both with normal hearing and with hearing loss, demonstrate temporal processing deficits that affect speech perception. The purpose of the present study is to investigate aging, cognition, and neural processing factors that may lead to deficits on perceptual tasks that rely on phoneme identification based on a temporal cue - vowel duration. A better understanding of the neural and cognitive impairments underlying temporal processing deficits could lead to more focused aural rehabilitation for improved speech understanding for older adults. This investigation was conducted in younger (YNH) and older normal-hearing (ONH) participants who completed three measures of cognitive functioning known to decline with age: working memory, processing speed, and inhibitory control. To evaluate perceptual and neural processing of auditory temporal contrasts, identification functions for the contrasting word-pair WHEAT and WEED were obtained on a nine-step continuum of vowel duration, and frequency-following responses (FFRs) and cortical auditory-evoked potentials (CAEPs) were recorded to the two endpoints of the continuum. Multiple linear regression analyses were conducted to determine the cognitive, peripheral, and/or central mechanisms that may contribute to perceptual performance. YNH participants demonstrated higher cognitive functioning on all three measures compared to ONH participants. The slope of the identification function was steeper in YNH than in ONH participants, suggesting a clearer distinction between the contrasting words in the YNH participants. FFRs revealed better response waveform morphology and more robust phase-locking in YNH compared to ONH participants. ONH participants also exhibited earlier latencies for CAEP components compared to the YNH participants. Linear regression analyses revealed that cortical processing significantly contributed to the variance in perceptual performance in the WHEAT/WEED identification functions. These results suggest that reduced neural precision contributes to age-related speech perception difficulties that arise from temporal processing deficits.

Age Effects on Neural Representation and Perception of Silence Duration Cues in Speech

Purpose Degraded temporal processing associated with aging may be a contributing factor to older adults' hearing difficulties, especially in adverse listening environments. This degraded processing may affect the ability to distinguish between words based on temporal duration cues. The current study investigates the effects of aging and hearing loss on cortical and subcortical representation of temporal speech components and on the perception of silent interval duration cues in speech. Method Identification functions for the words DISH and DITCH were obtained on a 7-step continuum of silence duration (0-60 ms) prior to the final fricative in participants who are younger with normal hearing (YNH), older with normal hearing (ONH), and older with hearing impairment (OHI). Frequency-following responses and cortical auditory-evoked potentials were recorded to the 2 end points of the continuum. Auditory brainstem responses to clicks were obtained to verify neural integrity and to compare group differences in auditory nerve function. A multiple linear regression analysis was conducted to determine the peripheral or central factors that contributed to perceptual performance. Results ONH and OHI participants required longer silence durations to identify DITCH than did YNH participants. Frequency-following responses showed reduced phase locking and poorer morphology, and cortical auditory-evoked potentials showed prolonged latencies in ONH and OHI participants compared with YNH participants. No group differences were noted for auditory brainstem response Wave I amplitude or Wave V/I ratio. After accounting for the possible effects of hearing loss, linear regression analysis revealed that both midbrain and cortical processing contributed to the variance in the DISH-DITCH perceptual identification functions. Conclusions These results suggest that age-related deficits in the ability to encode silence duration cues may be a contributing factor in degraded speech perception. In particular, degraded response morphology relates to performance on perceptual tasks based on silence duration contrasts between words.

GABAergic and glutamatergic cells in the inferior colliculus dynamically express the GABAAR γ1 subunit during aging

Age-related hearing loss may result, in part, from declining levels of γ-amino butyric acid (GABA) in the aging inferior colliculus (IC). An upregulation of the GABA_AR γ₁ subunit, which has been shown to increase sensitivity to GABA, occurs in the aging IC. We sought to determine whether the upregulation of the GABA_AR γ₁ subunit was specific to GABAergic or glutamatergic IC cells. We used immunohistochemistry for glutamic acid decarboxylase and the GABA_AR γ₁ subunit at 4 age groups in the IC of Fisher Brown Norway rats. The percentage of somas that expressed the γ₁ subunit and the number of subunits on each soma were quantified. Our results show that GABAergic and glutamatergic IC cells increasingly expressed the γ₁ subunit from young age until expression peaked during middle age. At old age (∼77% of life span), the number of GABA_AR γ₁ subunits per cell sharply decreased for both cell types. These results, along with previous studies, suggest inhibitory and excitatory IC circuits may express the GABA_AR γ₁ subunit in response to the age-related decline of available GABA.

Speech-in-noise representation in the aging midbrain and cortex: Effects of hearing loss

Age-related deficits in speech-in-noise understanding pose a significant problem for older adults. Despite the vast number of studies conducted to investigate the neural mechanisms responsible for these communication difficulties, the role of central auditory deficits, beyond peripheral hearing loss, remains unclear. The current study builds upon our previous work that investigated the effect of aging on normal-hearing individuals and aims to estimate the effect of peripheral hearing loss on the representation of speech in noise in two critical regions of the aging auditory pathway: the midbrain and cortex. Data from 14 hearing-impaired older adults were added to a previously published dataset of 17 normal-hearing younger adults and 15 normal-hearing older adults. The midbrain response, measured by the frequency-following response (FFR), and the cortical response, measured with the magnetoencephalography (MEG) response, were recorded from subjects listening to speech in quiet and noise conditions at four signal-to-noise ratios (SNRs): +3, 0, -3, and -6 dB sound pressure level (SPL). Both groups of older listeners showed weaker midbrain response amplitudes and overrepresentation of cortical responses compared to younger listeners. No significant differences were found between the two older groups when the midbrain and cortical measurements were analyzed independently. However, significant differences between the older groups were found when investigating the midbrain-cortex relationships; that is, only hearing-impaired older adults showed significant correlations between midbrain and cortical measurements, suggesting that hearing loss may alter reciprocal connections between lower and higher levels of the auditory pathway. The overall paucity of differences in midbrain or cortical responses between the two older groups suggests that age-related temporal processing deficits may contribute to older adults' communication difficulties beyond what might be predicted from peripheral hearing loss alone; however, hearing loss does seem to alter the connectivity between midbrain and cortex. These results may have important ramifications for the field of audiology, as it indicates that algorithms in clinical devices, such as hearing aids, should consider age-related temporal processing deficits to maximize user benefit.

Masking Differentially Affects Envelope-following Responses in Young and Aged Animals

Age-related hearing decline typically includes threshold shifts as well as reduced wave I auditory brainstem response (ABR) amplitudes due to cochlear synaptopathy/neuropathy, which may compromise precise coding of suprathreshold speech envelopes. This is supported by findings with older listeners, who have difficulties in envelope and speech processing, especially in noise. However, separating the effects of threshold elevation, synaptopathy, and degradation by noise on physiological representations may be difficult. In the present study, the effects of notched, low- and high-pass noise on envelope-following responses (EFRs) in aging were compared when sound levels (aged: 85-dB SPL; young: 60- to 80-dB SPL) were matched between groups peripherally, by matching wave I ABR amplitudes, or centrally by matching EFR amplitudes. Low-level notched noise reduced EFRs to sinusoidally amplitude-modulated (SAM) tones in young animals for notch widths up to 2 octaves. High-pass noise above the carrier frequency reduced EFRs. Young animals showed EFR reductions at lower noise levels. Low-pass noise did not reduce EFRs in either young or aged animals. High-pass noise may affect EFR amplitudes in young animals more than aged by reducing the contributions of high-frequency-sensitive inputs. EFRs to SAM tones in modulated noise (NAM) suggest that neurons of young animals can synchronize to NAM at lower sound levels and maintain dual AM representations better than older animals. The overall results show that EFR amplitudes are strongly influenced by aging and the presence of a competing sound that likely reduces or shifts the pool of responsive neurons.

Individual Differences in Human Auditory Processing: Insights From Single-Trial Auditory Midbrain Activity in an Animal Model

Auditory-evoked potentials are classically defined as the summations of synchronous firing along the auditory neuraxis. Converging evidence supports a model whereby timing jitter in neural coding compromises listening and causes variable scalp-recorded potentials. Yet the intrinsic noise of human scalp recordings precludes a full understanding of the biological origins of individual differences in listening skills. To delineate the mechanisms contributing to these phenomena, in vivo extracellular activity was recorded from inferior colliculus in guinea pigs to speech in quiet and noise. Here we show that trial-by-trial timing jitter is a mechanism contributing to auditory response variability. Identical variability patterns were observed in scalp recordings in human children, implicating jittered timing as a factor underlying reduced coding of dynamic speech features and speech in noise. Moreover, intertrial variability in human listeners is tied to language development. Together, these findings suggest that variable timing in inferior colliculus blurs the neural coding of speech in noise, and propose a consequence of this timing jitter for human behavior. These results hint both at the mechanisms underlying speech processing in general, and at what may go awry in individuals with listening difficulties.

Developmental deprivation-induced perceptual and cortical processing deficits in awake-behaving animals

Sensory deprivation during development induces lifelong changes to central nervous system function that are associated with perceptual impairments. However, the relationship between neural and behavioral deficits is uncertain due to a lack of simultaneous measurements during task performance. Therefore, we telemetrically recorded from auditory cortex neurons in gerbils reared with developmental conductive hearing loss as they performed an auditory task in which rapid fluctuations in amplitude are detected. These data were compared to a measure of auditory brainstem temporal processing from each animal. We found that developmental HL diminished behavioral performance, but did not alter brainstem temporal processing. However, the simultaneous assessment of neural and behavioral processing revealed that perceptual deficits were associated with a degraded cortical population code that could be explained by greater trial-to-trial response variability. Our findings suggest that the perceptual limitations that attend early hearing loss are best explained by an encoding deficit in auditory cortex.

Age-related changes in envelope-following responses at equalized peripheral or central activation

Previous work has debated about the comparisons of hearing abilities faced with alterations in hearing thresholds and evoked potentials between groups following acoustic trauma- or age-related changes. This study compares envelope-following responses (EFRs) of young and aged rats when sound levels were matched according to (1) wave I amplitudes of auditory brainstem responses (ABRs) elicited by 8-kHz tones or (2) EFR amplitudes evoked by sinusoidally amplitude-modulated (SAM) tones at 100% depth. Matched wave I amplitudes across age corresponded to approximately 20-dB sound level differences. For matched wave I, no age-related differences were observed in wave V amplitudes. However, EFRs recorded in silence were enhanced with aging at 100% but not at 25% depth, consistent with enhanced central gain in aging. For matched EFRs, there were no age-related differences in EFRs of amplitude modulation (AM) depth and AM frequency processing. These results suggest novel, objective measures beyond threshold to compensate for differences in auditory nerve activation and to differentiate peripheral and central contributions of EFRs.

Differences in postinjury auditory system pathophysiology after mild blast and nonblast acute acoustic trauma

Hearing difficulties are the most commonly reported disabilities among veterans. Blast exposures during explosive events likely play a role, given their propensity to directly damage both peripheral (PAS) and central auditory system (CAS) components. Postblast PAS pathophysiology has been well documented in both clinical case reports and laboratory investigations. In contrast, blast-induced CAS dysfunction remains understudied but has been hypothesized to contribute to an array of common veteran behavioral complaints, including learning, memory, communication, and emotional regulation. This investigation compared the effects of acute blast and nonblast acoustic impulse trauma in adult male Sprague-Dawley rats. An array of audiometric tests were utilized, including distortion product otoacoustic emissions (DPOAE), auditory brain stem responses (ABR), middle latency responses (MLR), and envelope following responses (EFRs). Generally, more severe and persistent postinjury central auditory processing (CAP) deficits were observed in blast-exposed animals throughout the auditory neuraxis, spanning from the cochlea to the cortex. DPOAE and ABR results captured cochlear and auditory nerve/brain stem deficits, respectively. EFRs demonstrated temporal processing impairments suggestive of functional damage to regions in the auditory brain stem and the inferior colliculus. MLRs captured thalamocortical transmission and cortical activation impairments. Taken together, the results suggest blast-induced CAS dysfunction may play a complementary pathophysiological role to maladaptive neuroplasticity of PAS origin. Even mild blasts can produce lasting hearing impairments that can be assessed with noninvasive electrophysiology, allowing these measurements to serve as simple, effective diagnostics.NEW & NOTEWORTHY Blasts exposures often produce hearing difficulties. Although cochlear damage typically occurs, the downstream effects on central auditory processing are less clear. Moreover, outcomes were compared between individuals exposed to the blast pressure wave vs. those who experienced the blast noise without the pressure wave. It was found that a single blast exposure produced changes at all stages of the ascending auditory path at least 4 wk postblast, whereas blast noise alone produced largely transient changes.

Increasing GABA reverses age-related alterations in excitatory receptive fields and intensity coding of auditory midbrain neurons in aged mice

A key feature of age-related hearing loss is a reduction in the expression of inhibitory neurotransmitters in the central auditory system. This loss is partially responsible for changes in central auditory processing, as inhibitory receptive fields play a critical role in shaping neural responses to sound stimuli. Vigabatrin (VGB), an antiepileptic agent that irreversibly inhibits γ-amino butyric acid (GABA) transaminase, leads to increased availability of GABA throughout the brain. This study used multi-channel electrophysiology measurements to assess the excitatory frequency response areas in old CBA mice to which VGB had been administered. We found a significant post-VGB reduction in the proportion of V-type shapes, and an increase in primary-like excitatory frequency response areas. There was also a significant increase in the mean maximum driven spike rates across the tonotopic frequency range of all treated animals, consistent with observations that GABA buildup within the central auditory system increases spike counts of neural receptive fields. This increased spiking is also seen in the rate-level functions and seems to explain the improved low-frequency thresholds.

Ageing affects dual encoding of periodicity and envelope shape in rat inferior colliculus neurons

Extracting temporal periodicities and envelope shapes of sounds is important for listening within complex auditory scenes but declines behaviorally with age. Here, we recorded local field potentials (LFPs) and spikes to investigate how ageing affects the neural representations of different modulation rates and envelope shapes in the inferior colliculus of rats. We specifically aimed to explore the input-output (LFP-spike) response transformations of inferior colliculus neurons. Our results show that envelope shapes up to 256-Hz modulation rates are represented in the neural synchronisation phase lags in younger and older animals. Critically, ageing was associated with (i) an enhanced gain in onset response magnitude from LFPs to spikes; (ii) an enhanced gain in neural synchronisation strength from LFPs to spikes for a low modulation rate (45 Hz); (iii) a decrease in LFP synchronisation strength for higher modulation rates (128 and 256 Hz) and (iv) changes in neural synchronisation strength to different envelope shapes. The current age-related changes are discussed in the context of an altered excitation-inhibition balance accompanying ageing.

Age-Related Deterioration of Perineuronal Nets in the Primary Auditory Cortex of Mice

Age-related changes in inhibitory neurotransmission in sensory cortex may underlie deficits in sensory function. Perineuronal nets (PNNs) are extracellular matrix components that ensheath some inhibitory neurons, particularly parvalbumin positive (PV+) interneurons. PNNs may protect PV+ cells from oxidative stress and help establish their rapid spiking properties. Although PNN expression has been well characterized during development, possible changes in aging sensory cortex have not been investigated. Here we tested the hypothesis that PNN+, PV+ and PV/PNN co-localized cell densities decline with age in the primary auditory cortex (A1). This hypothesis was tested using immunohistochemistry in two strains of mice (C57BL/6 and CBA/CaJ) with different susceptibility to age-related hearing loss and at three different age ranges (1–3, 6–8 and 14–24 months old). We report that PNN+ and PV/PNN co-localized cell densities decline significantly with age in A1 in both mouse strains. In the PNN+ cells that remain in the old group, the intensity of PNN staining is reduced in the C57 strain, but not the CBA strain. PV+ cell density also declines only in the C57, but not the CBA, mouse suggesting a potential exacerbation of age-effects by hearing loss in the PV/PNN system. Taken together, these data suggest that PNN deterioration may be a key component of altered inhibition in the aging sensory cortex, that may lead to altered synaptic function, susceptibility to oxidative stress and processing deficits.

Effect of informational content of noise on speech representation in the aging midbrain and cortex

The ability to understand speech is significantly degraded by aging, particularly in noisy environments. One way that older adults cope with this hearing difficulty is through the use of contextual cues. Several behavioral studies have shown that older adults are better at following a conversation when the target speech signal has high contextual content or when the background distractor is not meaningful. Specifically, older adults gain significant benefit in focusing on and understanding speech if the background is spoken by a talker in a language that is not comprehensible to them (i.e., a foreign language). To understand better the neural mechanisms underlying this benefit in older adults, we investigated aging effects on midbrain and cortical encoding of speech when in the presence of a single competing talker speaking in a language that is meaningful or meaningless to the listener (i.e., English vs. Dutch). Our results suggest that neural processing is strongly affected by the informational content of noise. Specifically, older listeners' cortical responses to the attended speech signal are less deteriorated when the competing speech signal is an incomprehensible language rather than when it is their native language. Conversely, temporal processing in the midbrain is affected by different backgrounds only during rapid changes in speech and only in younger listeners. Additionally, we found that cognitive decline is associated with an increase in cortical envelope tracking, suggesting an age-related over (or inefficient) use of cognitive resources that may explain their difficulty in processing speech targets while trying to ignore interfering noise.

Evidence of degraded representation of speech in noise, in the aging midbrain and cortex

Humans have a remarkable ability to track and understand speech in unfavorable conditions, such as in background noise, but speech understanding in noise does deteriorate with age. Results from several studies have shown that in younger adults, low-frequency auditory cortical activity reliably synchronizes to the speech envelope, even when the background noise is considerably louder than the speech signal. However, cortical speech processing may be limited by age-related decreases in the precision of neural synchronization in the midbrain. To understand better the neural mechanisms contributing to impaired speech perception in older adults, we investigated how aging affects midbrain and cortical encoding of speech when presented in quiet and in the presence of a single-competing talker. Our results suggest that central auditory temporal processing deficits in older adults manifest in both the midbrain and in the cortex. Specifically, midbrain frequency following responses to a speech syllable are more degraded in noise in older adults than in younger adults. This suggests a failure of the midbrain auditory mechanisms needed to compensate for the presence of a competing talker. Similarly, in cortical responses, older adults show larger reductions than younger adults in their ability to encode the speech envelope when a competing talker is added. Interestingly, older adults showed an exaggerated cortical representation of speech in both quiet and noise conditions, suggesting a possible imbalance between inhibitory and excitatory processes, or diminished network connectivity that may impair their ability to encode speech efficiently.

Effects of Aging on the Encoding of Dynamic and Static Components of Speech

OBJECTIVES

The authors investigated aging effects on the envelope of the frequency following response to dynamic and static components of speech. Older adults frequently experience problems understanding speech, despite having clinically normal hearing. Improving audibility with hearing aids provides variable benefit, as amplification cannot restore the temporal precision degraded by aging. Previous studies have demonstrated age-related delays in subcortical timing specific to the dynamic, transition region of the stimulus. However, it is unknown whether this delay is mainly due to a failure to encode rapid changes in the formant transition because of central temporal processing deficits or as a result of cochlear damage that reduces audibility for the high-frequency components of the speech syllable. To investigate the nature of this delay, the authors compared subcortical responses in younger and older adults with normal hearing to the speech syllables /da/ and /a/, hypothesizing that the delays in peak timing observed in older adults are mainly caused by temporal processing deficits in the central auditory system.

DESIGN

The frequency following response was recorded to the speech syllables /da/ and /a/ from 15 younger and 15 older adults with normal hearing, normal IQ, and no history of neurological disorders. Both speech syllables were presented binaurally with alternating polarities at 80 dB SPL at a rate of 4.3 Hz through electromagnetically shielded insert earphones. A vertical montage of four Ag-AgCl electrodes (Cz, active, forehead ground, and earlobe references) was used.

RESULTS

The responses of older adults were significantly delayed with respect to younger adults for the transition and onset regions of the /da/ syllable and for the onset of the /a/ syllable. However, in contrast with the younger adults who had earlier latencies for /da/ than for /a/ (as was expected given the high-frequency energy in the /da/ stop consonant burst), latencies in older adults were not significantly different between the responses to /da/ and /a/. An unexpected finding was noted in the amplitude and phase dissimilarities between the two groups in the later part of the steady-state region, rather than in the transition region. This amplitude reduction may indicate prolonged neural recovery or response decay associated with a loss of auditory nerve fibers.

CONCLUSIONS

These results suggest that older adults' peak timing delays may arise from decreased synchronization to the onset of the stimulus due to reduced audibility, though the possible role of impaired central auditory processing cannot be ruled out. Conversely, a deterioration in temporal processing mechanisms in the auditory nerve, brainstem, or midbrain may be a factor in the sudden loss of synchronization in the later part of the steady-state response in older adults.

Age-Related Changes in Processing Simultaneous Amplitude Modulated Sounds Assessed Using Envelope Following Responses

Listening conditions in the real world involve segregating the stimuli of interest from competing auditory stimuli that differ in their sound level and spectral content. It is in these conditions of complex spectro-temporal processing that listeners with age-related hearing loss experience the most difficulties. Envelope following responses (EFRs) provide objective neurophysiological measures of auditory processing. EFRs were obtained to two simultaneous sinusoidally amplitude modulated (sAM) tones from young and aged Fischer-344 rats. One was held at a fixed suprathreshold sound level (sAM1FL) while the second varied in sound level (sAM2VL) and carrier frequency. EFR amplitudes to sAM1FL in the young decreased with signal-to-noise ratio (SNR), and this reduction was more pronounced when the sAM2VL carrier frequency was spectrally separated from sAM1FL. Aged animals showed similar trends, while having decreased overall response amplitudes compared to the young. These results were replicated using an established computational model of the auditory nerve. The trends observed in the EFRs were shown to be due to the contributions of the low-frequency tails of high-frequency neurons, rather than neurons tuned to the sAM1FL carrier frequency. Modeling changes in threshold and neural loss reproduced some of the changes seen with age, but accuracy improved when combined with an additional decrease representing synaptic loss of auditory nerve neurons. Sound segregation in this case derives primarily from peripheral processing, regardless of age. Contributions by more central neural mechanisms are likely to occur only at low SNRs.

Age-related shifts in distortion product otoacoustic emissions peak-ratios and amplitude modulation spectra

Amplitude modulation (AM) is an important temporal cue for precise speech and complex sound recognition. However, functional decline of the auditory periphery as well as degradation of central auditory processing due to aging can reduce the salience and resolution of temporal cues. Age-related deficits in central temporal processing have previously been observed at more rapid AM frequencies and various AM depths. These centrally observed changes result from cochlear changes compounded with changes along the ascending auditory pathway. In fact, a decrease in ability to detect temporally modulated sounds accurately could originate from changes in cochlear filtering properties and in cochlear mechanics due to aging. Nonetheless, few studies have examined cochlear mechanisms in AM detection. To assess integrity of the mechanical properties of the auditory periphery, distortion product otoacoustic emissions (DPOAEs) are a tool commonly used in clinics and in research. In this study, we measured DPOAEs to reveal age-related changes in peak f2/f1 ratio and degradation in AM detection by basilar membrane vibration. Two tones (f1 and f2, f2 > f1) at various f2/f1 ratios and simultaneous presentation of one AM and one pure tone were used as stimuli to evoke DPOAEs. In addition of observing reduced DPOAE amplitudes and steeper slopes in the input-output DPOAE functions, higher peak f2/f1 ratios and broader f2/f1 tuning were also observed in aged animals. Aged animals generally had lower distortion product (DP) and first sideband (SB 1) responses evoked by an f1 pure tone and an f2 AM tone, regardless of whether the AM frequency was 45 Hz or 128 Hz. SB 1 thresholds, which corresponds to the smallest stimulus AM depth that can induce cochlear vibrations at the DP generator locus, were higher in aged animals as well. The results suggest that age-related changes in peak f2/f1 ratio and AM detection by basilar membrane vibration are consistent with a reduction in endocochlear potential and reduced prestin activity but with preserved hair cell bundle function. SB 1 responses evoked by f2 AM/f1 pure tone with various AM depths could serve as an estimate for cochlear AM detection. The sidebands of DP could also serve as additional physiological cues for detection of AM in the presence of other tone(s), even at typical conversational levels in speech.

Reduced mate preference for dominant over subordinate males in old female Syrian hamsters (Mesocricetus auratus)

Why some females choose to mate with a 'preferred' male, whereas others choose to mate with an 'inferior' male is not always clear. Generally, the choosiness of females is thought to decline with advanced age, but relatively few studies have investigated this concept, and reports of this phenomenon in mammals are lacking. To address this deficiency, young and old female golden hamsters were evaluated for their preference for dominant vs. subordinate males. Females observed male dyads as a dominance relationship was established. Dominant and subordinate males were then placed within enclosures at the opposite ends of a Y-maze, and the first approach, scent marking, and time spent near each male were evaluated in young and old females during pro-oestrus-a time when females solicit visits by prospective mates by leaving vaginal and flank scent marks. Whereas the proportion of time spent near the dominant male was significantly greater than random for both young and old females, the proportions of vaginal and flank scent marks left for the dominant male were significantly greater than random for young females, but not for old females. Overall, these results are consistent with a decline in the preference for dominant males by old female hamsters.

Partial maintenance of auditory-based cognitive training benefits in older adults

The potential for short-term training to improve cognitive and sensory functions in older adults has captured the public's interest. Initial results have been promising. For example, eight weeks of auditory-based cognitive training decreases peak latencies and peak variability in neural responses to speech presented in a background of noise and instills gains in speed of processing, speech-in-noise recognition, and short-term memory in older adults. But while previous studies have demonstrated short-term plasticity in older adults, we must consider the long-term maintenance of training gains. To evaluate training maintenance, we invited participants from an earlier training study to return for follow-up testing six months after the completion of training. We found that improvements in response peak timing to speech in noise and speed of processing were maintained, but the participants did not maintain speech-in-noise recognition or memory gains. Future studies should consider factors that are important for training maintenance, including the nature of the training, compliance with the training schedule, and the need for booster sessions after the completion of primary training.

Age-related changes in the relationship between auditory brainstem responses and envelope-following responses

Hearing thresholds and wave amplitudes measured using auditory brainstem responses (ABRs) to brief sounds are the predominantly used clinical measures to objectively assess auditory function. However, frequency-following responses (FFRs) to tonal carriers and to the modulation envelope (envelope-following responses or EFRs) to longer and spectro-temporally modulated stimuli are rapidly gaining prominence as a measure of complex sound processing in the brainstem and midbrain. In spite of numerous studies reporting changes in hearing thresholds, ABR wave amplitudes, and the FFRs and EFRs under neurodegenerative conditions, including aging, the relationships between these metrics are not clearly understood. In this study, the relationships between ABR thresholds, ABR wave amplitudes, and EFRs are explored in a rodent model of aging. ABRs to broadband click stimuli and EFRs to sinusoidally amplitude-modulated noise carriers were measured in young (3-6 months) and aged (22-25 months) Fischer-344 rats. ABR thresholds and amplitudes of the different waves as well as phase-locking amplitudes of EFRs were calculated. Age-related differences were observed in all these measures, primarily as increases in ABR thresholds and decreases in ABR wave amplitudes and EFR phase-locking capacity. There were no observed correlations between the ABR thresholds and the ABR wave amplitudes. Significant correlations between the EFR amplitudes and ABR wave amplitudes were observed across a range of modulation frequencies in the young. However, no such significant correlations were found in the aged. The aged click ABR amplitudes were found to be lower than would be predicted using a linear regression model of the young, suggesting altered gain mechanisms in the relationship between ABRs and FFRs with age. These results suggest that ABR thresholds, ABR wave amplitudes, and EFRs measure complementary aspects of overlapping neurophysiological processes and the relationships between these measurements changes asymmetrically with age. Hence, measuring all three metrics provides a more complete assessment of auditory function, especially under pathological conditions like aging.

Insult-induced adaptive plasticity of the auditory system

The brain displays a remarkable capacity for both widespread and region-specific modifications in response to environmental challenges, with adaptive processes bringing about the reweighing of connections in neural networks putatively required for optimizing performance and behavior. As an avenue for investigation, studies centered around changes in the mammalian auditory system, extending from the brainstem to the cortex, have revealed a plethora of mechanisms that operate in the context of sensory disruption after insult, be it lesion-, noise trauma, drug-, or age-related. Of particular interest in recent work are those aspects of auditory processing which, after sensory disruption, change at multiple—if not all—levels of the auditory hierarchy. These include changes in excitatory, inhibitory and neuromodulatory networks, consistent with theories of homeostatic plasticity; functional alterations in gene expression and in protein levels; as well as broader network processing effects with cognitive and behavioral implications. Nevertheless, there abounds substantial debate regarding which of these processes may only be sequelae of the original insult, and which may, in fact, be maladaptively compelling further degradation of the organism's competence to cope with its disrupted sensory context. In this review, we aim to examine how the mammalian auditory system responds in the wake of particular insults, and to disambiguate how the changes that develop might underlie a correlated class of phantom disorders, including tinnitus and hyperacusis, which putatively are brought about through maladaptive neuroplastic disruptions to auditory networks governing the spatial and temporal processing of acoustic sensory information.

Age-related changes in the subcortical-cortical encoding and categorical perception of speech

Aging is associated with declines in auditory processing including speech comprehension abilities. Here, we evaluated both brainstem and cortical speech-evoked brain responses to elucidate how aging impacts the neural transcription and transfer of speech information between functional levels of the auditory nervous system. Behaviorally, older adults showed slower, more variable speech classification performance than younger listeners, which coincided with reduced brainstem amplitude and increased, but delayed, cortical speech-evoked responses. Mild age-related hearing loss showed differential correspondence with neurophysiological responses showing negative (brainstem) and positive (cortical) correlations with brain activity. Spontaneous brain activity, that is, "neural noise," did not differ between older and younger adults. Yet, mutual information and correlations computed between brainstem and cortex revealed higher redundancy (i.e., lower interdependence) in speech information transferred along the auditory pathway implying less neural flexibility in older adults. Results are consistent with the notion that weakened speech encoding in brainstem is overcompensated by increased cortical dysinhibition in the aging brain. Findings suggest aging negatively impacts speech listening abilities by distorting the hierarchy of speech representations, reducing neural flexibility through increased neural redundancy, and ultimately impairing the acoustic-phonetic mapping necessary for robust speech understanding.

Altered cortical spectrotemporal processing with age-related hearing loss

Presbycusis (age-related hearing loss) is a prevalent disability associated with aging that impairs spectrotemporal processing, but the mechanisms of such changes remain unclear. The goal of this study was to quantify cortical responses to frequency-modulated (FM) sweeps in a mouse model of presbycusis. Previous studies showed that cortical neurons in young mice are selective for the rate of frequency change in FM sweeps. Here single-unit data on cortical selectivity and response variability to FM sweeps of either direction and different rates (0.08–20 kHz/ms) were compared across young (1–3 mo), middle-aged (6–8 mo), and old (14–20 mo) groups. Three main findings are reported. First, there is a reduction in FM rate selectivity in the old group. Second, there is a slowing of the sweep rates at which neurons likely provide best detection and discrimination of sweep rates. Third, there is an increase in trial-to-trial variability in the magnitude and timing of spikes in response to sweeps. These changes were only observed in neurons that were selective for the fast or intermediate range of sweep rates and not in neurons that preferred slow sweeps or were nonselective. Increased variability of response magnitude, but not changes in temporal fidelity or selectivity, was seen even in the middle-aged group. The results show that spectrotemporal processing becomes slow and noisy with presbycusis in specific types of neurons, suggesting receptive field mechanisms that are altered. These data suggest neural correlates of presbycusis-related reduction in the ability of humans to process rapid spectrotemporal changes.

Postnatal development of auditory central evoked responses and thalamic cellular properties

During development, the sense of hearing changes rapidly with age, especially around hearing onset. During this period, auditory structures are highly sensitive to alterations of the acoustic environment, such as hearing loss or background noise. This sensitivity includes auditory temporal processing, which is important for processing complex sounds, and for acquiring reading and language skills. Developmental changes can be observed at multiple levels of brain organization-from behavioral responses to cellular responses, and at every auditory nucleus. Neuronal properties and sound processing change dramatically in auditory cortex neurons after hearing onset. However, development of its primary source, the auditory thalamus, or medial geniculate body (MGB), has not been well studied over this critical time window. Furthermore, to understand how temporal processing develops, it is important to determine the relative maturation of temporal processing not only in the MGB, but also in its inputs. Cellular properties of rat MGB neurons were studied using in vitro whole-cell patch-clamp recordings, at ages postnatal day (P) 7-9; P15-17, and P22-32. Auditory evoked potentials were measured in P14-17 and P22-32 rats. MGB action potentials became about five times faster, and the ability to generate spike trains increased with age, particularly at frequencies of 50 Hz and higher. Evoked potential responses, including auditory brainstem responses (ABR), middle latency responses (MLR), and amplitude modulation following responses, showed increased amplitudes with age, and ABRs and MLRs additionally showed decreased latencies with age. Overall, temporal processing at subthalamic nuclei is concurrently maturing with MGB cellular properties.