Characterizing spatial tuning functions of neurons in the auditory cortex of young and aged monkeys: a new perspective on old data

Age-related changes in excitatory and inhibitory intra-cortical circuits in auditory cortex of C57Bl/6 mice

A common impairment in aging is age-related hearing loss (presbycusis), which manifests as impaired spectrotemporal processing. Aging is accompanied by alteration in normal inhibitory (GABA) neurotransmission, and changes in excitatory (NMDA and AMPA) synapses in the auditory cortex (ACtx). However, the circuits affected by these synaptic changes remain unknown. Mice of the C57Bl/6J strain show premature age-related hearing loss and changes in functional responses in ACtx. We thus investigated how auditory cortical microcircuits change with age by comparing young (∼ 6 weeks) and aged (>1 year old) C57Bl/6J mice. We performed laser scanning photostimulation (LSPS) combined with whole-cell patch clamp recordings from Layer (L) 2/3 cells in primary auditory cortex (A1) of young adult and aged C57Bl/6J mice. We found that L2/3 cells in aged C57Bl/6J mice display functional hypoconnectivity of both excitatory and inhibitory circuits. Compared to cells from young C57Bl/6 mice, cells from aged C57Bl/6J mice have fewer excitatory connections with weaker connection strength. Whereas young adult and aged C57Bl/6J mice have similar amounts of inhibitory connections, the strength of local inhibition is weaker in the aged group. We confirmed these results by recording miniature excitatory (mEPSCs) and inhibitory synaptic currents (mIPSCs). Our results suggest a specific reduction in excitatory and inhibitory intralaminar cortical circuits in aged C57Bl/6J mice compared with young adult animals. We speculate that these unbalanced changes in cortical circuits contribute to the functional manifestations of age-related hearing loss.

Robustness of neuronal tuning to binaural sound localization cues against age-related loss of inhibitory synaptic inputs

Sound localization relies on minute differences in the timing and intensity of sound arriving at both ears. Neurons of the lateral superior olive (LSO) in the brainstem process these interaural disparities by precisely detecting excitatory and inhibitory synaptic inputs. Aging generally induces selective loss of inhibitory synaptic transmission along the entire auditory pathways, including the reduction of inhibitory afferents to LSO. Electrophysiological recordings in animals, however, reported only minor functional changes in aged LSO. The perplexing discrepancy between anatomical and physiological observations suggests a role for activity-dependent plasticity that would help neurons retain their binaural tuning function despite loss of inhibitory inputs. To explore this hypothesis, we use a computational model of LSO to investigate mechanisms underlying the observed functional robustness against age-related loss of inhibitory inputs. The LSO model is an integrate-and-fire type enhanced with a small amount of low-voltage activated potassium conductance and driven with (in)homogeneous Poissonian inputs. Without synaptic input loss, model spike rates varied smoothly with interaural time and level differences, replicating empirical tuning properties of LSO. By reducing the number of inhibitory afferents to mimic age-related loss of inhibition, overall spike rates increased, which negatively impacted binaural tuning performance, measured as modulation depth and neuronal discriminability. To simulate a recovery process compensating for the loss of inhibitory fibers, the strength of remaining inhibitory inputs was increased. By this modification, effects of inhibition loss on binaural tuning were considerably weakened, leading to an improvement of functional performance. These neuron-level observations were further confirmed by population modeling, in which binaural tuning properties of multiple LSO neurons were varied according to empirical measurements. These results demonstrate the plausibility that homeostatic plasticity could effectively counteract known age-dependent loss of inhibitory fibers in LSO and suggest that behavioral degradation of sound localization might originate from changes occurring more centrally.

A novel dynamic network imaging analysis method reveals aging-related fragmentation of cortical networks in mouse

Network analysis of large-scale neuroimaging data is a particularly challenging computational problem. Here, we adapt a novel analytical tool, the community dynamic inference method (CommDy), for brain imaging data from young and aged mice. CommDy, which was inspired by social network theory, has been successfully used in other domains in biology; this report represents its first use in neuroscience. We used CommDy to investigate aging-related changes in network metrics in the auditory and motor cortices by using flavoprotein autofluorescence imaging in brain slices and in vivo. We observed that auditory cortical networks in slices taken from aged brains were highly fragmented compared to networks observed in young animals. CommDy network metrics were then used to build a random-forests classifier based on NMDA receptor blockade data, which successfully reproduced the aging findings, suggesting that the excitatory cortical connections may be altered during aging. A similar aging-related decline in network connectivity was also observed in spontaneous activity in the awake motor cortex, suggesting that the findings in the auditory cortex reflect general mechanisms during aging. These data suggest that CommDy provides a new dynamic network analytical tool to study the brain and that aging is associated with fragmentation of intracortical networks.

Choice-related activity and neural encoding in primary auditory cortex and lateral belt during feature-selective attention

Selective attention is necessary to sift through, form a coherent percept of, and make behavioral decisions on the vast amount of information present in most sensory environments. How and where selective attention is employed in cortex and how this perceptual information then informs the relevant behavioral decisions is still not well understood. Studies probing selective attention and decision-making in visual cortex have been enlightening as to how sensory attention might work in that modality; whether or not similar mechanisms are employed in auditory attention is not yet clear. Therefore, we trained rhesus macaques on a feature-selective attention task, where they switched between reporting changes in temporal (amplitude modulation, AM) and spectral (carrier bandwidth) features of a broadband noise stimulus. We investigated how the encoding of these features by single neurons in primary (A1) and secondary (middle lateral belt, ML) auditory cortex was affected by the different attention conditions. We found that neurons in A1 and ML showed mixed selectivity to the sound and task features. We found no difference in AM encoding between the attention conditions. We found that choice-related activity in both A1 and ML neurons shifts between attentional conditions. This finding suggests that choice-related activity in auditory cortex does not simply reflect motor preparation or action and supports the relationship between reported choice-related activity and the decision and perceptual process.NEW & NOTEWORTHY We recorded from primary and secondary auditory cortex while monkeys performed a nonspatial feature attention task. Both areas exhibited rate-based choice-related activity. The manifestation of choice-related activity was attention dependent, suggesting that choice-related activity in auditory cortex does not simply reflect arousal or motor influences but relates to the specific perceptual choice.

Impaired Binaural Hearing in Adults: A Selected Review of the Literature

Despite over 100 years of study, there are still many fundamental questions about binaural hearing that remain unanswered, including how impairments of binaural function are related to the mechanisms of binaural hearing. This review focuses on a number of studies that are fundamental to understanding what is known about the effects of peripheral hearing loss, aging, traumatic brain injury, strokes, brain tumors, and multiple sclerosis (MS) on binaural function. The literature reviewed makes clear that while each of these conditions has the potential to impair the binaural system, the specific abilities of a given patient cannot be known without performing multiple behavioral and/or neurophysiological measurements of binaural sensitivity. Future work in this area has the potential to bring awareness of binaural dysfunction to patients and clinicians as well as a deeper understanding of the mechanisms of binaural hearing, but it will require the integration of clinical research with animal and computational modeling approaches.

Exaggerated cortical representation of speech in older listeners: mutual information analysis

Aging is associated with an exaggerated representation of the speech envelope in auditory cortex. The relationship between this age-related exaggerated response and a listener's ability to understand speech in noise remains an open question. Here, information-theory-based analysis methods are applied to magnetoencephalography recordings of human listeners, investigating their cortical responses to continuous speech, using the novel nonlinear measure of phase-locked mutual information between the speech stimuli and cortical responses. The cortex of older listeners shows an exaggerated level of mutual information, compared with younger listeners, for both attended and unattended speakers. The mutual information peaks for several distinct latencies: early (∼50 ms), middle (∼100 ms), and late (∼200 ms). For the late component, the neural enhancement of attended over unattended speech is affected by stimulus signal-to-noise ratio, but the direction of this dependency is reversed by aging. Critically, in older listeners and for the same late component, greater cortical exaggeration is correlated with decreased behavioral inhibitory control. This negative correlation also carries over to speech intelligibility in noise, where greater cortical exaggeration in older listeners is correlated with worse speech intelligibility scores. Finally, an age-related lateralization difference is also seen for the ∼100 ms latency peaks, where older listeners show a bilateral response compared with younger listeners' right lateralization. Thus, this information-theory-based analysis provides new, and less coarse-grained, results regarding age-related change in auditory cortical speech processing, and its correlation with cognitive measures, compared with related linear measures.NEW & NOTEWORTHY Cortical representations of natural speech are investigated using a novel nonlinear approach based on mutual information. Cortical responses, phase-locked to the speech envelope, show an exaggerated level of mutual information associated with aging, appearing at several distinct latencies (∼50, ∼100, and ∼200 ms). Critically, for older listeners only, the ∼200 ms latency response components are correlated with specific behavioral measures, including behavioral inhibition and speech comprehension.

Reversal of Age-Related Changes in Cortical Sound-Azimuth Selectivity with Training

The compromised abilities to understand speech and localize sounds are two hallmark deficits in aged individuals. Earlier studies have shown that age-related deficits in cortical neural timing, which is clearly associated with speech perception, can be partially reversed with auditory training. However, whether training can reverse aged-related cortical changes in the domain of spatial processing has never been studied. In this study, we examined cortical spatial processing in ~21-month-old rats that were trained on a sound-azimuth discrimination task. We found that animals that experienced 1 month of training displayed sharper cortical sound-azimuth tuning when compared to the age-matched untrained controls. This training-induced remodeling in spatial tuning was paralleled by increases of cortical parvalbumin-labeled inhibitory interneurons. However, no measurable changes in cortical spatial processing were recorded in age-matched animals that were passively exposed to training sounds with no task demands. These results that demonstrate the effects of training on cortical spatial domain processing in the rodent model further support the notion that age-related changes in central neural process are, due to their plastic nature, reversible. Moreover, the results offer the encouraging possibility that behavioral training might be used to attenuate declines in auditory perception, which are commonly observed in older individuals.

Age-related changes in sound onset and offset intensity coding in auditory cortical fields A1 and CL of rhesus macaques

Inhibition plays a key role in shaping sensory processing in the central auditory system and has been implicated in sculpting receptive field properties such as sound intensity coding and also in shaping temporal patterns of neuronal firing such as onset- or offset-evoked responses. There is substantial evidence supporting a decrease in inhibition throughout the ascending auditory pathway in geriatric animals. We therefore examined intensity coding of onset (ON) and offset (OFF) responses in auditory cortex of aged and young monkeys. A large proportion of cells in the primary auditory cortex (A1) and the caudolateral field (CL) displayed nonmonotonic rate-level functions for OFF responses in addition to nonmonotonic coding of ON responses. Aging differentially affected ON and OFF responses; the magnitude of effects was generally greater for ON responses. In addition to higher firing rates, neurons in old monkeys exhibited a significant increase in the proportion of monotonic rate-level functions and had higher best intensities than those in young monkeys. OFF responses in young monkeys displayed a range of intensity coding relationships with ON responses of the same cells, ranging from highly similar to highly dissimilar. Dissimilarity in ON/OFF coding was greater in CL and was reduced with aging, which was largely explained by a preferential decrease in the percentage of cells with nonmonotonic coding of ON and OFF responses. The changes we observed are consistent with previously demonstrated alterations in inhibition in the ascending auditory pathway of primates and could be involved in age-related deficits in the temporal processing of sounds.NEW & NOTEWORTHY Aging has a major impact on intensity coding of neurons in auditory cortex of rhesus macaques. Neural responses to sound onset and offset were affected to different extents, and their rate-level functions became more mutually similar, which could be accounted for by the loss of nonmonotonic intensity coding in geriatric monkeys. These findings were consistent with weakened inhibition in the central auditory system and could contribute to auditory processing deficits in elderly subjects.

Effects of Ketamine Administration on Auditory Information Processing in the Neocortex of Nonhuman Primates

Ketamine, an N-methyl-D-aspartate (NMDA) receptor antagonist, exerts broad effects on consciousness and perception. Since NMDA receptor antagonists induce cognitive impairments, ketamine has been used for translational research on several psychiatric diseases, such as schizophrenia. Whereas the effects of ketamine on cognitive functions have been extensively studied, studies on the effects of ketamine on simple sensory information processing remain limited. In this study, we investigated the cortex-wide effects of ketamine administration on auditory information processing in nonhuman primates using whole-cortical electrocorticography (ECoG). We first recorded ECoG from awake monkeys on presenting auditory stimuli of different frequencies or different durations. We observed auditory evoked responses (AERs) across the cortex, including in frontal, parietal, and temporal areas, while feature-specific responses were obtained around the temporal sulcus. Next, we examined the effects of ketamine on cortical auditory information processing. We conducted ECoG recordings from monkeys that had been administered anesthetic doses of ketamine from 10 to 180 min following administration. We observed significant changes in stimulus feature-specific responses. Electrodes showing a frequency preference or offset responses were altered following ketamine administration, while those of the AERs were not strongly influenced. However, the frequency preference of a selected electrode was not significantly altered by ketamine administration over time following administration, while the imbalances in the onset and offset persisted over the course of 150 min following ketamine administration in all three monkeys. These results suggest that ketamine affects the ability to distinguish between sound frequency and duration in different ways. In conclusion, future research on the NMDA sensitivity of cortical wide sensory information processing may provide a new perspective into the development of nonhuman primate models of psychiatric disorders.

Age-Related Changes in Temporal Processing of Rapidly-Presented Sound Sequences in the Macaque Auditory Cortex

The mammalian auditory cortex is necessary to resolve temporal features in rapidly-changing sound streams. This capability is crucial for speech comprehension in humans and declines with normal aging. Nonhuman primate studies have revealed detrimental effects of normal aging on the auditory nervous system, and yet the underlying influence on temporal processing remains less well-defined. Therefore, we recorded from the core and lateral belt areas of auditory cortex when awake young and old monkeys listened to tone-pip and noise-burst sound sequences. Elevated spontaneous and stimulus-driven activity were the hallmark characteristics in old monkeys. These old neurons showed isomorphic-like discharge patterns to stimulus envelopes, though their phase-locking was less precise. Functional preference in temporal coding between the core and belt existed in the young monkeys but was mostly absent in the old monkeys, in which old belt neurons showed core-like response profiles. Finally, the analysis of population activity patterns indicated that the aged auditory cortex demonstrated a homogenous, distributed coding strategy, compared to the selective, sparse coding strategy observed in the young monkeys. Degraded temporal fidelity and highly-responsive, broadly-tuned cortical responses could underlie how aged humans have difficulties to resolve and track dynamic sounds leading to speech processing deficits.

Neural Dedifferentiation in the Aging Brain

Many cognitive abilities decline with age even in the absence of detectable pathology. Recent evidence indicates that age-related neural dedifferentiation, operationalized in terms of neural selectivity, may contribute to this decline. We review here work exploring the relationship between neural dedifferentiation, cognition, and age. Compelling evidence for age effects on neural selectivity comes from both non-human animal and human research. However, current data suggest that age does not moderate the observed relationships between neural dedifferentiation and cognitive performance. We propose that functionally significant variance in measures of neural dedifferentiation reflects both age-dependent and age-independent factors. We further propose that the effects of age on neural dedifferentiation do not exclusively reflect detrimental consequences of aging.

The effects of aging on auditory cortical function

Age-related hearing loss is a prominent deficit, afflicting approximately half of the geriatric population. In many cases, the person may have no deficits in detecting sounds, but nonetheless suffers from a reduced ability to understand speech, particularly in a noisy environment. While rodent models have shown that there are a variety of age-related changes throughout the auditory neuraxis, far fewer studies have investigated the effects at the cortical level. Here I review recent evidence from a non-human primate model of age-related hearing loss at the level of the core (primary auditory cortex, A1) and belt (caudolateral field, CL) in young and aged animals with normal detection thresholds. The findings are that there is an increase in both the spontaneous and driven activity, an increase in spatial tuning, and a reduction in the temporal fidelity of the response in aged animals. These results are consistent with an age-related imbalance of excitation and inhibition in the auditory cortex. These spatial and temporal processing deficits could underlie the major complaint of geriatrics, that it is difficult to understand speech in noise.

Robust Neuronal Discrimination in Primary Auditory Cortex Despite Degradations of Spectro-temporal Acoustic Details: Comparison Between Guinea Pigs with Normal Hearing and Mild Age-Related Hearing Loss

This study investigated to which extent the primary auditory cortex of young normal-hearing and mild hearing-impaired aged animals is able to maintain invariant representation of critical temporal-modulation features when sounds are submitted to degradations of fine spectro-temporal acoustic details. This was achieved by recording ensemble of cortical responses to conspecific vocalizations in guinea pigs with either normal hearing or mild age-related sensorineural hearing loss. The vocalizations were degraded using a tone vocoder. The neuronal responses and their discrimination capacities (estimated by mutual information) were analyzed at single recording and population levels. For normal-hearing animals, the neuronal responses decreased as a function of the number of the vocoder frequency bands, so did their discriminative capacities at the single recording level. However, small neuronal populations were found to be robust to the degradations induced by the vocoder. Similar robustness was obtained when broadband noise was added to exacerbate further the spectro-temporal distortions produced by the vocoder. A comparable pattern of robustness to degradations in fine spectro-temporal details was found for hearing-impaired animals. However, the latter showed an overall decrease in neuronal discrimination capacities between vocalizations in noisy conditions. Consistent with previous studies, these results demonstrate that the primary auditory cortex maintains robust neural representation of temporal envelope features for communication sounds under a large range of spectro-temporal degradations.

Crossmodal Connections of Primary Sensory Cortices Largely Vanish During Normal Aging

During aging, human response times (RTs) to unisensory and crossmodal stimuli decrease. However, the elderly benefit more from crossmodal stimulus representations than younger people. The underlying short-latency multisensory integration process is mediated by direct crossmodal connections at the level of primary sensory cortices. We investigate the age-related changes of these connections using a rodent model (Mongolian gerbil), retrograde tracer injections into the primary auditory (A1), somatosensory (S1), and visual cortex (V1), and immunohistochemistry for markers of apoptosis (Caspase-3), axonal plasticity (Growth associated protein 43, GAP 43), and a calcium-binding protein (Parvalbumin, PV). In adult animals, primary sensory cortices receive a substantial number of direct thalamic inputs from nuclei of their matched, but also from nuclei of non-matched sensory modalities. There are also direct intracortical connections among primary sensory cortices and connections with secondary sensory cortices of other modalities. In very old animals, the crossmodal connections strongly decrease in number or vanish entirely. This is likely due to a retraction of the projection neuron axonal branches rather than ongoing programmed cell death. The loss of crossmodal connections is also accompanied by changes in anatomical correlates of inhibition and excitation in the sensory thalamus and cortex. Together, the loss and restructuring of crossmodal connections during aging suggest a shift of multisensory processing from primary cortices towards other sensory brain areas in elderly individuals.

Positive impacts of early auditory training on cortical processing at an older age

Progressive negative behavioral changes in normal aging are paralleled by a complex series of physical and functional declines expressed in the cerebral cortex. In studies conducted in the auditory domain, these degrading physical and functional cortical changes have been shown to be broadly reversed by intensive progressive training that improves the spectral and temporal resolution of acoustic inputs and suppresses behavioral distractors. Here we found older rats that were intensively trained on an attentionally demanding modulation-rate recognition task in young adulthood substantially retained training-driven improvements in temporal rate discrimination abilities over a subsequent 18-mo epoch-that is, forward into their older age. In parallel, this young-adult auditory training enduringly enhanced temporal and spectral information processing in their primary auditory cortices (A1). Substantially greater numbers of parvalbumin- and somatostatin-labeled inhibitory neurons (closer to the numbers recorded in young vigorous adults) were recorded in the A1 and hippocampus in old trained versus untrained age-matched rats. These results show that a simple form of training in young adulthood in this rat model enduringly delays the otherwise expected deterioration of the physical status and functional operations of the auditory nervous system, with evident training impacts generalized to the hippocampus.

Spectral and spatial tuning of onset and offset response functions in auditory cortical fields A1 and CL of rhesus macaques

The mammalian auditory cortex is necessary for spectral and spatial processing of acoustic stimuli. Most physiological studies of single neurons in the auditory cortex have focused on the onset and sustained portions of evoked responses, but there have been far fewer studies on the relationship between onset and offset responses. In the current study, we compared spectral and spatial tuning of onset and offset responses of neurons in primary auditory cortex (A1) and the caudolateral (CL) belt area of awake macaque monkeys. Several different metrics were used to determine the relationship between onset and offset response profiles in both frequency and space domains. In the frequency domain, a substantial proportion of neurons in A1 and CL displayed highly dissimilar best stimuli for onset- and offset-evoked responses, although even for these neurons, there was usually a large overlap in the range of frequencies that elicited onset, and offset responses and distributions of tuning overlap metrics were mostly unimodal. In the spatial domain, the vast majority of neurons displayed very similar best locations for onset- and offset-evoked responses, along with unimodal distributions of all tuning overlap metrics considered. Finally, for both spectral and spatial tuning, a slightly larger fraction of neurons in A1 displayed nonoverlapping onset and offset response profiles, relative to CL, which supports hierarchical differences in the processing of sounds in the two areas. However, these differences are small compared with differences in proportions of simple cells (low overlap) and complex cells (high overlap) in primary and secondary visual areas.NEW & NOTEWORTHY In the current study, we examine the relationship between the tuning of neural responses evoked by the onset and offset of acoustic stimuli in the primary auditory cortex, as well as a higher-order auditory area-the caudolateral belt field-in awake rhesus macaques. In these areas, the relationship between onset and offset response profiles in frequency and space domains formed a continuum, ranging from highly overlapping to highly nonoverlapping.

Effects of aging on the response of single neurons to amplitude-modulated noise in primary auditory cortex of rhesus macaque

Temporal envelope processing is critical for speech comprehension, which is known to be affected by normal aging. Whereas the macaque is an excellent animal model for human cerebral cortical function, few studies have investigated neural processing in the auditory cortex of aged, nonhuman primates. Therefore, we investigated age-related changes in the spiking activity of neurons in primary auditory cortex (A1) of two aged macaque monkeys using amplitude-modulated (AM) noise and compared these responses with data from a similar study in young monkeys (Yin P, Johnson JS, O'Connor KN, Sutter ML. J Neurophysiol 105: 582-600, 2011). For each neuron, we calculated firing rate (rate code) and phase-locking using phase-projected vector strength (temporal code). We made several key findings where neurons in old monkeys differed from those in young monkeys. Old monkeys had higher spontaneous and driven firing rates, fewer neurons that synchronized with the AM stimulus, and fewer neurons that had differential responses to AM stimuli with both a rate and temporal code. Finally, whereas rate and temporal tuning functions were positively correlated in young monkeys, this relationship was lost in older monkeys at both the population and single neuron levels. These results are consistent with considerable evidence from rodents and primates of an age-related decrease in inhibition throughout the auditory pathway. Furthermore, this dual coding in A1 is thought to underlie the capacity to encode multiple features of an acoustic stimulus. The apparent loss of ability to encode AM with both rate and temporal codes may have consequences for stream segregation and effective speech comprehension in complex listening environments.

Aging alters visual processing of objects and shapes in inferotemporal cortex in monkeys

Visual perception declines with age. Perceptual deficits may originate not only in the optical system serving vision but also in the neural machinery processing visual information. Since homologies between monkey and human vision permit extrapolation from monkeys to humans, data from young, middle aged and old monkeys were analyzed to show age-related changes in the neuronal activity in the inferotemporal cortex, which is critical for object and shape vision. We found an increased neuronal response latency, and a decrease in the stimulus selectivity in the older animals and suggest that these changes may underlie the perceptual uncertainties found frequently in the elderly.

Regional and age-related differences in GAD67 expression of parvalbumin- and calbindin-expressing neurons in the rhesus macaque auditory midbrain and brainstem

Neurons expressing the calcium binding proteins (CaBPs) parvalbumin (PV) and calbindin (CB) have shown age-related density changes throughout the ascending auditory system of both rodents and macaque monkeys. In the cerebral cortex, neurons expressing these CaBPs express markers of γ-aminobutyric acidergic neurotransmission, such as GAD67, and have well-understood physiological response properties. Recent evidence suggests that, in the rodent auditory brainstem, CaBP-containing cells do not express GAD67. It is unknown whether PV- and CB-containing cells in subcortical auditory structures of macaques similarly do not express GAD67, and a better understanding of the neurotransmission of neurons expressing these proteins is necessary for understanding the age-related changes in their density throughout the macaque auditory system. This was investigated with immunofluorescent double-labeling techniques to coregister PV- and CB-expressing neurons with GAD67 in the superior olivary complex and the inferior colliculus of young and aged rhesus macaques. The proportions of GAD67-expressing PV- and CB-positive neurons were computed with unbiased sampling techniques. Our results indicate that between 42% and 62% of PV- and CB-positive neurons in the auditory brainstem and midbrain express GAD67, which is significantly less than in the cerebrum. In general, fewer PV(+) neurons and more CB(+) neurons expressed GAD67 as a function of age. These results demonstrate that the inhibitory molecular profile of PV- and CB-expressing neurons can change with age in subcortical auditory structures and that these neurons are distinct from the well-described inhibitory interneurons that express these proteins in the cerebral cortex.

Age-related neurochemical changes in the rhesus macaque cochlear nucleus

Neurochemical changes in the expression of various proteins within the central auditory system have been associated with natural aging. These changes may compensate in part for the loss of auditory sensitivity arising from two phenomena of the aging auditory system: cochlear histopathologies and increased excitability of central auditory neurons. Recent studies in the macaque monkey have revealed age-related changes in the density of nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase (NADPHd) and parvalbumin (PV)-positive cells within the inferior colliculus and superior olivary complex. The cochlear nucleus (CN), which is the first central auditory nucleus, remains unstudied. Since the CN participates in the generation of the auditory brainstem response (ABR) and receives direct innervation from the cochlea, it serves as an ideal nucleus to compare the relationship between these neurochemical changes and the physiological and peripheral changes of the aging auditory system. We used stereological sampling to calculate the densities of NADPHd and PV reactive neurons within the three subdivisions of the CN in middle-aged and aged rhesus macaques. Regression analyses of these values with ABR properties and cochlear histopathologies revealed relationships between these cell types and the changing characteristics of the aging auditory system. Our results indicate that NADPHd expression does change with age in a specific subdivision of the CN, but PV does not. Conversely, PV expression correlated with ABR amplitudes and outer hair cell loss in the cochlea, but NADPHd did not. These results indicate that NADPHd and PV may take part in distinct compensatory efforts of the aging auditory system.

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 neurochemical changes in the rhesus macaque inferior colliculus

Age-related hearing loss (ARHL) is marked by audiometric hearing deficits that propagate along the auditory pathway. Neurochemical changes as a function of aging have also been identified in neurons along the auditory pathway in both rodents and carnivores, however, very little is known about how these neurochemicals change in the non-human primate. To examine how these compensatory neurochemical changes relate to normal aging and audiometric sensitivity along the auditory pathway, we collected auditory brainstem responses (ABRs) and brain specimens from seven rhesus monkeys spanning in age from 15 to 35 years old, and examined the relationship between click evoked ABR thresholds and the ABR evoked pure tone average (PTA) and changes in the number of parvalbumin and NADPH-diaphorase positive cells in the auditory midbrain. We found that the number of parvalbumin positive cells in the central nucleus and the surrounding cortex regions of the inferior colliculus were strongly correlated with advancing age and ABR PTA. We also found that the numbers of NADPHd positive cells in these same regions were not associated with normal aging or changes in the ABR thresholds. These findings suggest that the auditory midbrain undergoes an up-regulation of parvalbumin expressing neurons with aging that is related to changes in the processing of frequencies across the audiometric range.

Age-related decrease in the mitochondrial sirtuin deacetylase Sirt3 expression associated with ROS accumulation in the auditory cortex of the mimetic aging rat model

Age-related dysfunction of the central auditory system, also known as central presbycusis, can affect speech perception and sound localization. Understanding the pathogenesis of central presbycusis will help to develop novel approaches to prevent or treat this disease. In this study, the mechanisms of central presbycusis were investigated using a mimetic aging rat model induced by chronic injection of D-galactose (D-Gal). We showed that malondialdehyde (MDA) levels were increased and manganese superoxide dismutase (SOD2) activity was reduced in the auditory cortex in natural aging and D-Gal-induced mimetic aging rats. Furthermore, mitochondrial DNA (mtDNA) 4834 bp deletion, abnormal ultrastructure and cell apoptosis in the auditory cortex were also found in natural aging and D-Gal mimetic aging rats. Sirt3, a mitochondrial NAD+-dependent deacetylase, has been shown to play a crucial role in controlling cellular reactive oxygen species (ROS) homeostasis. However, the role of Sirt3 in the pathogenesis of age-related central auditory cortex deterioration is still unclear. Here, we showed that decreased Sirt3 expression might be associated with increased SOD2 acetylation, which negatively regulates SOD2 activity. Oxidative stress accumulation was likely the result of low SOD2 activity and a decline in ROS clearance. Our findings indicate that Sirt3 might play an essential role, via the mediation of SOD2, in central presbycusis and that manipulation of Sirt3 expression might provide a new approach to combat aging and oxidative stress-related diseases.

Age-related neurochemical changes in the rhesus macaque superior olivary complex

Positive immunoreactivity to the calcium-binding protein parvalbumin (PV) and nitric oxide synthase NADPH diaphorase (NADPHd) is well documented within neurons of the central auditory system of both rodents and primates. These proteins are thought to play roles in the regulation of auditory processing. Studies examining the age-related changes in expression of these proteins have been conducted primarily in rodents but are sparse in primate models. In the brainstem, the superior olivary complex (SOC) is crucial for the computation of sound source localization in azimuth, and one hallmark of age-related hearing deficits is a reduced ability to localize sounds. To investigate how these histochemical markers change as a function of age and hearing loss, we studied eight rhesus macaques ranging in age from 12 to 35 years. Auditory brainstem responses (ABRs) were obtained in anesthetized animals for click and tone stimuli. The brainstems of the sesame animals were then stained for PV and NADPHd reactivity. Reactive neurons in the three nuclei of the SOC were counted, and the densities of each cell type were calculated. We found that PV and NADPHd expression increased with both age and ABR thresholds in the medial superior olive but not in either the medial nucleus of the trapezoid body or the lateral superior olive. Together these results suggest that the changes in protein expression employed by the SOC may compensate for the loss of efficacy of auditory sensitivity in the aged primate.

Age-related deterioration of the representation of space in human auditory cortex

One of the principal auditory disabilities associated with older age is difficulty in locating and tracking sources of sound. This study investigated whether these difficulties are associated with deterioration in the representation of space in the auditory cortex. In psychophysical tests, half of a group of older (>60 years) adults displayed spatial acuity similar to that of young adults throughout the frontal horizontal plane. The remaining half had considerably poorer spatial acuity at the more peripheral regions of frontal space. Computational modeling of electroencephalographic responses to abrupt location shifts demonstrated marked differences in the spatial tuning of populations of cortical neurons between the older adults with poor spatial acuity on the one hand, and those with good spatial acuity, as well as young adults, on the other hand. In those with poor spatial acuity, cortical responses contained little information with which to distinguish peripheral locations. We demonstrate a clear link between neural responses and spatial acuity measured behaviorally, and provide evidence for age-related changes in the coding of horizontal space.