Processing Of Acoustic Information Research Articles

On-chip topological phononic crystal acoustic waveguide based on lithium niobate thin films

This Letter introduces an on-chip topological phononic crystal (PnC) acoustic waveguide employing lithium niobate thin films. Utilizing a C3v symmetry-breaking mechanism, the topological PnC acoustic waveguide is achieved, operating at frequencies exceeding 430 MHz. A SH0 mode acoustic transceiver is designed, enabling highly efficient on-chip acoustic wave transmission and reception. The fabricated topological PnC waveguide demonstrates a propagation loss of 11.3 dB/mm at the edge mode. An acoustic beam splitter utilizing topological edge mode has been demonstrated, showing the configurability of the topological PnC acoustic waveguide. These results offer significant promise for advancing the development of hybrid phononic circuits tailored for high-frequency acoustic information processing, sensing, and manipulation.

Frequency effects can modulate the neural correlates of prosodic processing in Mandarin.

In tonal languages, tone perception involves the processing of both acoustic and phonological information conveyed by tonal signals. In Mandarin, in addition to four canonical full tones, there exists a group of weak syllables known as neutral tones. This study aims to investigate the impact of lexical frequency effects and prosodic information associated with neutral tones on the auditory representation of Mandarin compounds. We initially selected disyllabic compounds as targets, manipulating their lexical frequencies and prosodic structures. Subsequently, these target compounds were embedded into selected sentences and auditorily presented to native speakers. During the experiments, participants engaged in lexical decision tasks while their event-related potentials were recorded. The results showed that the auditory lexical representation of disyllabic compounds was modulated by lexical frequency effects. Rare compounds and compounds with rare first constituents elicited larger N400 effects compared to frequent compounds. Furthermore, neutral tones were found to play a role in the processing, resulting in larger N400 effects. Our findings showed significantly increased amplitudes of the N400 component, suggesting that the processing of rare compounds and compounds with neutral tones may require more cognitive resources. Additionally, we observed an interaction effect between lexical frequency and neutral tones, indicating that they could serve as determining cues in the auditory processing of disyllabic compounds.

Combined multi-session transcranial alternating current stimulation (tACS) and language skills training improves individual gamma band activity and literacy skills in developmental dyslexia

Developmental dyslexia is characterized by the pathologically diminished ability to acquire reading and spelling skills. Accurate processing of acoustic information at the phonemic scale is crucial for successful sound-to-letter-mapping which, in turn, is elemental in reading and spelling. Altered activation patterns in the auditory cortex are thought to provide the neurophysiological basis for the inaccurate phonemic perception. Recently, transcranial electrical stimulation has been shown to be an effective method to ameliorate cortical activation patterns in the auditory cortex. In a sample of children and adolescents with dyslexia, we investigated the effect of multi-session transcranial alternating current stimulation delivered concurrently with a phonological training and in combination with a behavioral literacy skills training. Over a 5-week period the participants received 10 training sessions while gamma-tACS was administered over bilateral auditory cortex. We found that gamma-tACS shifted the peak frequency of auditory gamma oscillations reflecting a more fine-grained processing of time-critical acoustic information. This amelioration was accompanied by increased phonemic processing skills. Moreover, individuals who received gamma-tACS showed significant improvements in their spelling skills four months after the intervention. Our results demonstrate that multi-session gamma-tACS enhances the effects of a behavioral intervention and induces long-term improvement on literacy skills in dyslexia.

Spatial separation between two sounds affects the timing of action potentials elicited by the sounds in the rat's auditory midbrain neurons

Timing of action potentials (i.e., spikes) elicited by sounds is used by auditory neurons to encode and process acoustic information. In the presence of multiple sounds, the timing of sound-driven spikes is dependent on the temporal, spectral, and spatial relationships among the sounds. We used two tone bursts with different frequencies to form a train of stimuli that were presented at a random odor and a constant rate. Such a train was used to mimic two competing sounds that occurred at the same (50%) probability or a novel sound (i.e., a low probability oddball sound) that was interleaved with a frequently occurring background sound (i.e., a high probability standard sound). We used the rat as an animal model to study how the spatial relationship between two sounds affected the timing of spikes elicited by the sounds in individual neurons in the auditory midbrain. Results indicate that a lower probability of sound presentation led to a higher temporal precision of the timing of the first spike elicited by the sound and the timing could be affected by a spatial separation between two sounds. These results are important for understanding neural mechanisms responsible for hearing in a natural acoustic environment.

Processing of acoustic and phonological information of lexical tones at pre-attentive and attentive stages

ABSTRACT While Mismatch Negativity (MMN) and P300 have been found to correlate with the processing of acoustic and phonological information involved in speech perception, there is controversy surrounding how these two components index acoustic and/or phonological processing at pre-attentive and attentive stages. The current study employed both passive and active oddball paradigms to examine neural responses to lexical tones at the two stages in Cantonese speakers, using the paradigm of categorical perception (CP) where the between- and within-category deviants share the same acoustic distance from the standard but differ in the involvement of phonological information. We failed to observe a CP effect in P300, which might indicate that this component doesn’t necessarily index phonological processing, while MMN does, as reflected by the finding of a greater MMN amplitude elicited from the between-category than within-category deviant. Nevertheless, phonological processing might be overridden by acoustic processing among participants who were sensitive to pitch.

Click-train evoked steady state harmonic response as a novel pharmacodynamic biomarker of cortical oscillatory synchrony

Sensory networks naturally entrain to rhythmic stimuli like a click train delivered at a particular frequency. Such synchronization is integral to information processing, can be measured by electroencephalography (EEG) and is an accessible index of neural network function. Click trains evoke neural entrainment not only at the driving frequency (F), referred to as the auditory steady state response (ASSR), but also at its higher multiples called the steady state harmonic response (SSHR). Since harmonics play an important and non-redundant role in acoustic information processing, we hypothesized that SSHR may differ from ASSR in presentation and pharmacological sensitivity. In female SD rats, a 2 s-long train stimulus was used to evoke ASSR at 20 Hz and its SSHR at 40, 60 and 80 Hz, recorded from a prefrontal epidural electrode. Narrow band evoked responses were evident at all frequencies; signal power was strongest at 20 Hz while phase synchrony was strongest at 80 Hz. SSHR at 40 Hz took the longest time (∼180 ms from stimulus onset) to establish synchrony. The NMDA antagonist MK801 (0.025–0.1 mg/kg) did not consistently affect 20 Hz ASSR phase synchrony but robustly and dose-dependently attenuated synchrony of all SSHR. Evoked power was attenuated by MK801 at 20 Hz ASSR and 40 Hz SSHR only. Thus, presentation as well as pharmacological sensitivity distinguished SSHR from ASSR, making them non-redundant markers of cortical network function. SSHR is a novel and promising translational biomarker of cortical oscillatory dynamics that may have important applications in CNS drug development and personalized medicine.

The nuclei of the lateral lemniscus: unexpected players in the descending auditory pathway.

In the mammalian auditory pathway, the nuclei of the lateral lemniscus (NLL) are thought to be exclusively involved in the bottom-up transmission of auditory information. However, our repeated observation of numerous NLL neurons labeled after injection of retrograde tracers into the superior olivary complex (SOC) led us to systematically investigate with retrograde tracers the descending projections from the NLL to the SOC of the rat. We performed large injections of FluoroGold into the SOC to determine NLL contributions to descending projections, and focal injections of biotinylated dextran amine (BDA) to pinpoint the specific nuclei of the SOC innervated by each NLL. The SOC is innervated by thousands of neurons distributed across four nuclei or regions associated with the lateral lemniscus: the ipsilateral ventral and intermediate nuclei of the lateral lemniscus (VNLL and INLL); the medial paralemniscal region (PL) of both sides; and the ipsilateral semilunar nucleus (SLN), a previously unrecognized nucleus that wraps around the INLL dorsally, medially, and caudally and consists of small, flat neurons. In some experiments, at least 30% of neurons in the VNLL and INLL were retrogradely labeled. All nuclei of the SOC, except the medial and lateral superior olives, are innervated by abundant lemniscal neurons, and each SOC nucleus receives a unique combination of lemniscal inputs. The primary target of the projections from the VNLL is the ventral nucleus of the trapezoid body (VNTB), followed by the superior paraolivary nucleus (SPON), and the medial nucleus of the trapezoid body (MNTB). The INLL selectively innervates the VNTB. The PL innervates dorsal periolivary regions bilaterally. The SLN preferentially innervates the MNTB and may provide the first identified non-calyceal excitatory input to MNTB neurons. Our novel findings have strong implications for understanding acoustic information processing in the initial stages of the auditory pathway. Based on the proportion of lemniscal neurons involved in all the projections described, the NLL should be considered major players in the descending auditory pathway.

The Occipital Place Area Is Recruited for Echo-Acoustically Guided Navigation in Blind Human Echolocators.

In the investigation of the brain areas involved in human spatial navigation, the traditional focus has been on visually guided navigation in sighted people. Consequently, it is unclear whether the involved areas also support navigational abilities in other modalities. We explored this possibility by testing whether the occipital place area (OPA), a region associated with visual boundary-based navigation in sighted people, has a similar role in echo-acoustically guided navigation in blind human echolocators. We used fMRI to measure brain activity in 6 blind echolocation experts (EEs; five males, one female), 12 blind controls (BCs; six males, six females), and 14 sighted controls (SCs; eight males, six females) as they listened to prerecorded echolocation sounds that conveyed either a route taken through one of three maze environments, a scrambled (i.e., spatiotemporally incoherent) control sound, or a no-echo control sound. We found significantly greater activity in the OPA of EEs, but not the control groups, when they listened to the coherent route sounds relative to the scrambled sounds. This provides evidence that the OPA of the human navigation brain network is not strictly tied to the visual modality but can be recruited for nonvisual navigation. We also found that EEs, but not BCs or SCs, recruited early visual cortex for processing of echo acoustic information. This is consistent with the recent notion that the human brain is organized flexibly by task rather than by specific modalities.SIGNIFICANCE STATEMENT There has been much research on the brain areas involved in visually guided navigation, but we do not know whether the same or different brain regions are involved when blind people use a sense other than vision to navigate. In this study, we show that one part of the brain (occipital place area) known to play a specific role in visually guided navigation is also active in blind human echolocators when they use reflected sound to navigate their environment. This finding opens up new ways of understanding how people navigate, and informs our ability to provide rehabilitative support to people with vision loss.

Impurity-induced multi-bit acoustic topological system

Acoustic topological insulators (ATIs) are known to own topologically protected properties similar to their electronic and optical counterparts. However, for the topological insulators (TIs) with intact time-reversal symmetry (TRS), some severe defects can also induce strong backscattering. Many works try to avoid the effects of defects on topological wave transport, merely works explore the functions of the defects. Here, based on the interactions between the impurity and topological edge states, we investigate and experimentally demonstrate the capability of impurity to control and switch the valley Hall edge modes in acoustic systems. The results show the acoustic transmission spectrum is sensitive to the rotation angle of the impurity, which can be regarded as a potential platform for switchable acoustic devices or filters. Based on these discoveries, a multi-bit acoustic transmission system is designed by introducing multiple impurities into a multichannel structure, where the acoustic flows to a designated channel can be controlled by rotating impurities. Our experimental measurements match well with simulation results. The results demonstrated in this paper add a new degree of freedom in controlling ATIs and will expedite the progress of programmable acoustic information processing technology. The method of controlling topological wave transmission by impurity operations can also be applied to other systems.

Layer-specific correlates of detected and undetected auditory targets during attention.

In everyday life, the processing of acoustic information allows us to react to subtle changes in the auditory scene. Yet even when closely attending to sounds in the context of a task, we occasionally miss task-relevant features. The neural computations that underlie our ability to detect behavioral relevant sound changes are thought to be grounded in both feedforward and feedback processes within the auditory hierarchy. Here, we assessed the role of feedforward and feedback contributions in primary and non-primary auditory areas during behavioral detection of target sounds using submillimeter spatial resolution functional magnetic resonance imaging (fMRI) at high-fields (7T) in humans. We demonstrate that the successful detection of subtle temporal shifts in target sounds leads to a selective increase of activation in superficial layers of primary auditory cortex (PAC). These results indicate that feedback signals reaching as far back as PAC may be relevant to the detection of targets in the auditory scene.

Broadband acoustic triggers based on multiport waveguide structures

The study of acoustic information processing has attracted great attention owing to its advantages of anti-electromagnetic interference and low energy consumption. Acoustic logic device, as a fundamental component, plays an important role in designing integrated acoustic systems. In the past few years, with the rapid development of sonic crystals, acoustic metamaterials and metasurfaces, researchers have demonstrated a variety of acoustic logic gates based on different mechanisms, and have devoted their efforts to the promotion of the practical applications. The more complex acoustic triggers with broad bandwidth and subwavelength size are very important for developing integrated sound devices, but it is difficult to realize them. In this work, we design two types of acoustic triggers based on the mechanisms of linear interference and phase modulation. The acoustic trigger with a width of 0.32<i>λ</i> and length of 0.82<i>λ</i> is composed of phased unit cells and multi-port waveguide structures, showing a subwavelength structure. Based on the phase modulation of the phased unit cells and the mechanism of linear interferences, the acoustic T-type trigger and D-type trigger with the same threshold are designed and demonstrated experimentally. The corresponding working bands of the T-type and D-type triggers are 3.293–4.069 kHz and 3.400–4.138 kHz, and their fractional bandwidths (the ratio of the bandwidth to the center frequency) can reach about 0.23 and 0.22, respectively, showing a broadband characteristic of both triggers. The mechanism of the T-type trigger is attributed to the linear interference caused by two phased unit cells with a phase difference of π. However, the realization of the D-type trigger is closely related to the incident sound energy and the phase modulation caused by the phased unit cell in the control port. The measured results and simulated results agree well with each other. Compared with other types of acoustic logic devices, the designed acoustic triggers have the advantages of broad bandwidth, subwavelength size, same threshold, and passive structure, as well as being easy to integrate, thus providing great potential applications in acoustic computing, acoustic communication, acoustic information processing and integrated acoustics. Our experimental demonstration of acoustic triggers can further promote the theoretical and experimental investigations of basic acoustic components.

Decoding syntactic class from EEG during spoken word recognition

A fundamental issue in spoken language comprehension involves understanding the interaction of linguistic representations across different levels of organization (e.g., phonological, lexical, syntactic, and semantic). In particular, there is debate about when different levels are accessed during spoken word recognition. Under serial processing models, comprehension is sequential. In contrast, under parallel processing models, simultaneous activation of representations at multiple levels can occur. The current study investigates this issue by isolating neural responses to syntactic class distinctions from acoustic and phonological responses. EEG data were collected in an event-related potential (ERP) experiment in which participants (N = 26) listened to words varying in syntactic class (nouns versus adjectives) that were controlled for low-level acoustic differences via cross-splicing. Machine learning techniques were used to decode syntactic class from ERP responses over time. Results showed that syntactic class is decodable approximately 160–190 ms after the average syntactic point of disambiguation in the words, during which listeners are still processing acoustic information. This supports the prediction that different levels of representation have overlapping timecourses. Overall, these results are consistent with a parallel, interactive processing model of spoken word recognition, in which higher-level information—such as syntactic class—is accessed while acoustic analysis is still occurring.

Strong tunable phonon-phonon interactions induced by silicon-vacancy centers in one-dimensional chiral phononic waveguides

In solid quantum systems, strong phonon-phonon repulsive or attractive interactions are difficult to achieve. The highly adjustable phonon-phonon interactions created by silicon-vacancy (SiV) centers in a one-dimensional chiral phononic waveguide are investigated in this paper. Under specific situations, phonon-phonon attractive or repulsive interactions between the chiral transport phonons are created when they are scattered by two-level or three-level structural SiV centers. Furthermore, by modulating the laser Rabi frequencies and detunings, the resultant attractive or repulsive interactions between phonons can be adjusted over a large frequency range. Single-phonon creation and phonon manipulation in solid acoustic quantum information processing could benefit from the tunable phonon-phonon interactions.

Adiabatic conversion between gigahertz quasi-Rayleigh and quasi-Love modes for phononic integrated circuits

Unsuspended phononic integrated circuits have been proposed for on-chip acoustic information processing. Limited by the operation mechanism of a conventional interdigital transducer, the excitation of the quasi-Love mode in GaN-on-sapphire is inefficient, and thus, a high-efficiency Rayleigh-to-Love mode converter is of great significance for future integrated phononic devices. Here, we propose a high-efficiency and robust phononic mode converter based on an adiabatic conversion mechanism. Utilizing the anisotropic elastic property of the substrate, the adiabatic mode converter is realized by a simple tapered phononic waveguide. A conversion efficiency exceeding 96% with a 3 dB bandwidth of 1.44 GHz can be realized for phononic waveguides working at GHz frequency band, and excellent tolerance to the fabrication errors is also numerically validated. The device that we proposed can be useful in both classical and quantum phononic information processing, and the adiabatic mechanism could be generalized to other phononic device designs.

Local targets of T-stellate cells in the ventral cochlear nucleus.

T‐stellate cells in the ventral cochlear nucleus (VCN) are known to have local axon collaterals that terminate in the vicinity of their dendrites and cell bodies within the same isofrequency lamina in parallel with the auditory nerve fibers that innervate them. Excitatory synaptic connections between stellate cells within an isofrequency lamina are hypothesized to be involved in the nitric oxide‐mediated upregulation of T‐stellate responses to their auditory input. This could serve as a mechanism of variable gain control in the enhancement of responses to vowel spectral peaks. Previous studies have provided indirect evidence for these possible synaptic interconnections between T‐stellate cells, but unequivocal identification has yet to be established. Here, we used retrograde neuronal tracing with adeno‐associated viral vector or biotinylated dextran amine injected into the inferior colliculus (IC) to detect the postsynaptic target of T‐stellate cells within the VCN. We show that backfilled T‐stellate cell axons make monosynapatic connections on the labeled cell bodies and dendrites of other labeled T‐stellate cells within an isofrequency lamina. Electron microscopy revealed that T‐stellate terminals can also make synapses on structures not retrogradely labeled from the IC. Glycine antibodies combined with the viral labeling indicated that these nonbackfilled structures that the labeled T‐stellate terminals were synapsing on are most likely the cell bodies and dendrites of two size categories of glycinergic VCN cells, whose sizes and relative numbers indicated they are the D‐ and L‐stellate cells. These cells are known to provide inhibitory inputs back onto T‐stellate cells. Our data indicate that, in addition to their auditory nerve input, T‐stellate cells provide a second modulatable excitatory input to both inhibitory and excitatory cells in a VCN isofrequency lamina and may play a significant role in acoustic information processing.

Auditory Spectro-Temporal Modulation Training in Children With Auditory Processing Disorder

Objectives: Auditory processing disorder (APD) is due to the deficits in perceptual processing of acoustic information in the auditory system, characterized by poor speech perception of noise, regardless of normal hearing. The variability in speech function of APD children can be partly explained by changes in the encoding of spectro-temporal modulations (STMs) which have been overlooked, despite their significance. Given that enhancing STM sensitivity and its processing can be an appropriate way to improve the listener’s ability to retrieve and integrate speech segments covered by noise, we decided to evaluate the effects of STM-based auditory training on speech perception in noise and the reliability of this training in children with APD. Methods: Thirty-five children with APD (8-12 years old) were randomly divided into the training (n=17) and control groups (n=18) to evaluate the effectiveness and reliability of STM training on speech in noise perception. The intervention group was trained to detect STM by 120 trials every day for ten days. The STM detection thresholds and speech perception in noise were evaluated before and immediately after the finalization of formal training sessions in both groups. To address the training reliability, the tests were repeated one month after practice in the training group. Results: Following the completion of STM auditory training, the trained APD children improved notably in STM detection tasks and speech in noise tests (P<0.05). The post-training progress of STM detection thresholds and consonant-vowel in the noise test was preserved for one month after training (P>0.05), but the word in the noise test, especially in the right ear, was not retained (P<0.05). Discussion: Auditory spectro-temporal modulation training can lead to better processing of STM modulation. Its effects can be generalized to higher-order processing, such as speech perception in noise. Auditory training based on STM processing enhancement can play an essential role in improving speech comprehension in the noise abilities of children with APD.

ESTIMATION OF RELATIONSHIP BETWEEN INFORMATION COMPONENTS AND NOISE OF ACOUSTIC EMISSION SIGNALS

In this article, a method for processing acoustic information is presented to assess the correlation relationship of information components and noise of acoustic emission (AE) signals. The method is based on a polynomial approximation of bidirectional Butterworth high and low pass filters. The operability of the processing method on full-scale samples of the noisy AE signal is analyzed and the evaluation of the received processing is carried out on the basis of quantitative indicators. Bidirectional implementation of high-pass filters improves the quality of processing when compared with a low-pass filter. To assess the correlation relationship using the considered processing method, fragments of the information component and noise are isolated from the noisy signal. Based on the selected components, a high correlation relationship between AE information signals and noise has been established.

Ear Asymmetry and Contextual Influences on Speech Perception in Hearing-Impaired Patients

The left hemisphere preference for verbal stimuli is well known, with a right ear (RE) advantage obtained when competing verbal stimuli are presented simultaneously, at comfortable intensities, to both ears. Speech perception involves not only the processing of acoustic peripheral information but also top–down contextual influences, filling the gaps in the incoming information that is particularly degraded in hearing-impaired individuals. This study aimed to analyze the potential asymmetry of those contextual influences on a simple speech perception task in hearing-impaired patients in light of hemispheric asymmetry. Contextual influences on disyllabic word perception scores of 60 hearing-impaired patients were compared between left ear (LE) and RE, in a balanced design, involving two repetitions of the same task. Results showed a significantly greater contextual influence on the RE versus the LE and, for the second repetition versus the first one, without any interaction between the two. Furthermore, the difference in contextual influences between RE and LE increased significantly with the RE advantage measured by a dichotic listening test in the absence of any significant correlation with hearing threshold asymmetry. Lastly, the contextual influence asymmetry decreased significantly as age increased, which was mainly due to a greater increase, with age, of contextual influences on the LE versus the RE. Those results agree with the literature reporting a relative right-shift of hemispheric asymmetry observed with age in speech in noise perception tasks in normal hearing subjects and the clinical reports of generally better audiometric speech scores obtained in RE versus LE.

Effects of M currents on the persistent activity of pyramidal neurons in mouse primary auditory cortex.

Neuronal persistent activity (PA) is a common phenomenon observed in many types of neurons. PA can be induced in neurons in the mouse auditory nucleus by activating cholinergic receptors with carbachol (CCh), a dual muscarinic and nicotinic receptor agonist. PA is presumed to be associated with learning-related auditory plasticity at the cellular level. However, the mechanism is not clearly understood. Many studies have reported that muscarinic receptor agonists inhibit muscarinic-sensitive potassium channels (M channels). Potassium efflux through M channels produces potassium currents, called M currents, that play an essential role in regulating neural excitability and synaptic plasticity. Further study is needed to determine whether M currents affect the PA of auditory central neurons and provide additional analysis of the variations in electrophysiological properties. We used in vitro whole cell patch-clamp recordings in isolated mouse brain slices to investigate the effects of M currents on the PA in pyramidal neurons in layer V of the primary auditory cortex (AI-L5). We found that blocking M currents with XE991 depolarized the AI-L5 pyramidal neurons, which significantly increased the input resistance. The active threshold and threshold intensity were significantly reduced, indicating that intrinsic excitability was enhanced. Our results also showed that blocking M currents with XE991 switched the neuronal firing patterns in the AI-L5 pyramidal neurons from regular spiking to intrinsic bursting. Blocking M currents facilitated PA by increasing the plateau potential and enhancing intrinsic excitability. Our results suggested that blocking M currents might facilitate the PA in AI-L5 pyramidal neurons, which underlies auditory plasticity.NEW & NOTEWORTHY Persistent activity (PA) in AI-L5 pyramidal neurons plays an essential role in acoustic information processing. We used in vitro whole cell patch-clamp recordings to investigate the effects of M currents on the PA in AI-L5 pyramidal neurons. Blocking M currents with XE991 facilitated PA by increasing the plateau potential and enhancing intrinsic excitability, causing the firing patterns of AI-L5 pyramidal neurons to become more bursting. These results provide new insight into our understanding of the cellular mechanisms that govern learning-induced auditory plasticity.

The frequency-following response to assess the neural representation of spectral speech cues in older adults

Older adults often present difficulties understanding speech that cannot be explained by age-related changes in sound audibility. Psychoacoustic and electrophysiologic studies have linked these suprathreshold difficulties to age-related deficits in the auditory processing of temporal and spectral sound information. These studies suggest the existence of an age-related temporal processing deficit in the central auditory system, but the existence of such deficit in the spectral domain remains understudied. The FFR is an electrophysiological evoked response that assesses the ability of the neural auditory system to reproduce the spectral and temporal patterns of a sound. The main goal of this short review is to investigate if the FFR can identify and measure spectral processing deficits in the elderly compared to younger adults (for both, without hearing loss or competing noise). Furthermore, we want to determine what stimuli and analyses have been used in the literature to assess the neural encoding of spectral cues in older adults. Almost all reviewed articles showed an age-related decline in the auditory processing of spectral acoustic information. Even when using different speech and non-speech stimuli, studies reported an age-related decline at the fundamental frequency, at the first formant, and at other harmonic components using different metrics, such as the response's amplitude, inter-trial phase coherence, signal-to-response correlation, and signal-to-noise ratio. These results suggest that older adults may present age-related spectral processing difficulties, but further FFR studies are needed to clarify the effect of advancing age on the neural encoding of spectral speech cues. Spectral processing research on aging would benefit from using a broader variety of stimuli and from rigorously controlling for hearing thresholds even in the absence of disabling hearing loss. Advances in the understanding of the effect of age on FFR measures of spectral encoding could lead to the development of new clinical tools, with possible applications in the field of hearing aid fitting.