Central Auditory System Function Research Articles

Comprehensive behavioral and physiologic assessment of peripheral and central auditory function in individuals with mild traumatic brain injury

Auditory complaints are frequently reported by individuals with mild traumatic brain injury (mTBI) yet remain difficult to detect in the absence of clinically significant hearing loss. This highlights a growing need to identify sensitive indices of auditory-related mTBI pathophysiology beyond pure-tone thresholds for improved hearing healthcare diagnosis and treatment. Given the heterogeneity of mTBI etiology and the diverse peripheral and central processes required for normal auditory function, the present study sought to determine the audiologic assessments sensitive to mTBI pathophysiology at the group level using a well-rounded test battery of both peripheral and central auditory system function. This test battery included pure-tone detection thresholds, word understanding in quiet, sentence understanding in noise, distortion product otoacoustic emissions (DPOAEs), middle-ear muscle reflexes (MEMRs), and auditory evoked potentials (AEPs), including auditory brainstem responses (ABRs), middle latency responses (MLRs), and late latency responses (LLRs). Each participant also received magnetic resonance imaging (MRI). Compared to the control group, we found that individuals with mTBI had reduced DPOAE amplitudes that revealed a compound effect of age, elevated MEMR thresholds for an ipsilateral broadband noise elicitor, longer ABR Wave I latencies for click and 4 kHz tone burst elicitors, longer ABR Wave III latencies for 4 kHz tone bursts, larger MLR Na and Nb amplitudes, smaller MLR Pb amplitudes, longer MLR Pa latencies, and smaller LLR N1 amplitudes for older individuals with mTBI. Further, mTBI individuals with combined hearing difficulty and noise sensitivity had a greater number of deficits on thalamic and cortical AEP measures compared to those with only one/no self-reported auditory symptoms. This finding was corroborated with MRI, which revealed significant structural differences in the auditory cortical areas of mTBI participants who reported combined hearing difficulty and noise sensitivity, including an enlargement of left transverse temporal gyrus (TTG) and bilateral planum polare (PP). These findings highlight the need for continued investigations toward identifying individualized audiologic assessments and treatments that are sensitive to mTBI pathophysiology.

Strategies of hearing aids fitting for improving speech recognition in noisy environments

To assess speech recognition in users of mono- and binaural hearing aids (HA) with dynamic environment control system (DECS). 25 HA users with symmetrical bilateral moderate to severe sensorineural hearing loss and 10 individuals with normal hearing underwent speech audiometry in a free sound field by listening to an adaptive Russian matrix sentence test with a background noise. The testing was conducted for hearing impaired patients with one and two HA. The Speech Recognition Thresholds in noise (SRTN) using HA with DESC and using HA without DESC were compared. The dichotic digits test and the rapidly alternating speech perception test were performed to evaluate the central auditory system function. The SRTN in normal hearing listeners was -16.4±1.9 dB SNR. In the case of monaural using HA without DESC the SRTN was -2.5±4.4 dB SNR, using HA with DESC it was -5.8±4.0 dB SNR; in the case of using binaural HAs it was -6.3±3.8 dB SNR and -9.9±3.1 dB SNR accordingly. HAs with Dynamic Environment Control System are highly effective for speech perception in noisy environments. Binaural HA use is more effective than monaural HA use regardless of HA model. Speech recognition in noisy environments in cases of binaural HA use correlates significantly with results of tests assessing the central auditory system function.

Functional brain alterations following mild-to-moderate sensorineural hearing loss in children.

Auditory deprivation in the form of deafness during development leads to lasting changes in central auditory system function. However, less is known about the effects of mild-to-moderate sensorineural hearing loss (MMHL) during development. Here, we used a longitudinal design to examine late auditory evoked responses and mismatch responses to nonspeech and speech sounds for children with MMHL. At Time 1, younger children with MMHL (8-12 years; n = 23) showed age-appropriate mismatch negativities (MMNs) to sounds, but older children (12-16 years; n = 23) did not. Six years later, we re-tested a subset of the younger (now older) children with MMHL (n = 13). Children who had shown significant MMNs at Time 1 showed MMNs that were reduced and, for nonspeech, absent at Time 2. Our findings demonstrate that even a mild-to-moderate hearing loss during early-to-mid childhood can lead to changes in the neural processing of sounds in late childhood/adolescence.

Reduction of sound-evoked midbrain responses observed by functional magnetic resonance imaging following acute acoustic noise exposure.

Short duration and high intensity acoustic exposures can lead to temporary hearing loss and auditory nerve degeneration. This study investigates central auditory system function following such acute exposures after hearing loss recedes. Adult rats were exposed to 100 dB sound pressure level noise for 15 min. Auditory brainstem responses (ABRs) were recorded with click sounds to check hearing thresholds. Functional magnetic resonance imaging (fMRI) was performed with tonal stimulation at 12 and 20 kHz to investigate central auditory changes. Measurements were performed before exposure (0D), 7 days after (7D), and 14 days after (14D). ABRs show an ∼6 dB threshold shift shortly after exposure, but no significant threshold differences between 0D, 7D, and 14D. fMRI responses are observed in the lateral lemniscus (LL) and inferior colliculus (IC) of the midbrain. In the IC, responses to 12 kHz are 3.1 ± 0.3% (0D), 1.9 ± 0.3% (7D), and 2.9 ± 0.3% (14D) above the baseline magnetic resonance imaging signal. Responses to 20 kHz are 2.0 ± 0.2% (0D), 1.4 ± 0.2% (7D), and 2.1 ± 0.2% (14D). For both tones, responses at 7D are less than those at 0D (p < 0.01) and 14D (p < 0.05). In the LL, similar trends are observed. Acute exposure leads to functional changes in the auditory midbrain with timescale of weeks.

Assessing the applications of cortical auditory evoked potentials as a biomarker in children with hearing aids

Abstract Cortical auditory evoked potentials (CAEPs) are noninvasive measures used to quantify central auditory system function in humans. More specifically, the P1-N1-P2 CAEP has a unique role in identifying a central auditory system that has benefited from amplification or implantation. P1 reflects the maturation of the auditory system in general as it has developed over time. The objective of this study was to assess the CAEP in children with hearing aids versus age-matched controls, and to compare the pattern of P1 CAEP in patients with hearing aids versus those with cochlear implants. Twenty hearing-impaired children (using their own binaural digital hearing aids) were compared with 20 age-matched and sex-matched children with normal hearing. In both groups, P1 CAEP latency and waveform morphology were recorded by free-field auditory stimulation using tone bursts at 500 and 2000 Hz at 100 dB sound pressure level. Finally, P1 CAEP was compared between patients using hearing aids and 20 children with cochlear implants. We have assessed the use of P1 latency and CAEP waveform morphology in a total of 20 children with hearing aids as a biomarker for the development of the central auditory pathway in patients with hearing loss. Children using hearing aids exhibited an exponential decay in P1 latencies, indicating an overall delay in maturation when compared with that in children with normal hearing. There was no statistically significant difference as regards P1 latencies and amplitudes between cochlear implant and hearing aid users, who showed statistically significantly higher mean values compared with the normal group. Children’s auditory systems develop comparatively as long as they are receiving appropriate amplification, whether this is through a cochlear implant or through the use of hearing aids. The P1 CAEP test can be applied as a tool in the diagnosis of central processing disorders in children with hearing impairments fitted with cochlear implants or hearing aids. This information will be useful when monitoring a child’s progress with his hearing device and in auditory training.

Effects of chronic cochlear electrical stimulation after an extended period of profound deafness on primary auditory cortex organization in cats

Extended periods of deafness have profound effects on central auditory system function and organization. Neonatal deafening results in loss of the normal cochleotopic organization of the primary auditory cortex (AI), but environmentally-derived intracochlear electrical stimulation, via a cochlear implant, initiated shortly after deafening, can prevent this loss. We investigated whether such stimulation initiated after an extended period of deafness can restore cochleotopy. In two groups of neonatally-deafened cats, a multi-channel intracochlear electrode array was implanted at 8 weeks of age. One group received only minimal stimulation, associated with brief recordings at 4-6-week intervals, over the following 6 months to check the efficacy of the implant. In the other group, this 6-month period was followed by 6 months of near-continuous intracochlear electrical stimulation from a modified clinical cochlear implant system. We recorded multi-unit clusters in the auditory cortex and used two different methods to define the region of interest in the putative AI. There was no evidence of cochleotopy in any of the minimally stimulated animals, confirming our earlier finding. In three of six chronically stimulated cats there was clear evidence of AI cochleotopy, and in a fourth cat in which the majority of penetrations were in the anterior auditory field there was clear evidence of cochleotopy in that field. The finding that chronic intracochlear electrical stimulation after an extended period of deafness is able to restore cochleotopy in some (but not all) cases has implications for the performance of patients implanted after an extended period of deafness.

Assessing the applications of cortical auditory evoked potentials as a biomarker in children with cochlear implants

EnAbstract Background Cortical auditory evoked potentials (CAEPs) are noninvasive measures used to quantify central auditory system function in humans. More specifically, the P1–N1–P2 cortical auditory evoked potential has a unique role in identifying the central auditory system that has benefited from amplification or implantation. P1 reflects the maturation of the auditory system in general as it has developed over time. Objective The aims of this study were to assess the CAEP in children with cochlear implants compared with age-matched controls, to study the different variables affecting the results, and to compare the pattern of P1 CAEP in cochlear implant patients compared with that in those with hearing aids. Methodology Thirty-five hearing-impaired children (using cochlear implants) were compared with 20 age-matched and sex-matched children with normal hearing. In both groups, P1 CAEP latency and waveform morphology were recorded using free-field auditory stimulation with tone bursts at 500 and 2000 Hz at 100 dB sound pressure level in two sessions that were 6 months apart. Results Children using cochlear implants exhibited prolongation of P1 latencies, indicating an overall delay in maturation when compared with that in children who could hear normally. P1 CAEP latency and amplitude improved significantly after 6 months of device use. Conclusion Standardized age-appropriate normative data on P1 CAEPs in the pediatric Egyptian population could be used to determine implantation or amplification results.

Speech Evoked Auditory Brainstem Responses: A New Tool to Study Brainstem Encoding of Speech Sounds

The neural encoding of speech sound begins in the auditory nerve and travels to the auditory brainstem. Non speech stimuli such as click or tone bursts stimulus are used to check the auditory neural integrity routinely. Recently Speech evoked Auditory Brainstem measures (ABR) are being used as a tool to study the brainstem processing of Speech sounds. The aim of the study was to study the Speech evoked ABR to a consonant vowel (CV) stimulus. 30 subjects with normal hearing participated for the study. Speech evoked ABR were measured to a CV stimulus in all the participants. The speech stimulus used was a 40ms synthesized/da/sound. The consonant and vowel portion was analysed separately. Speech evoked ABR was present in all the normal hearing subjects. The consonant portion of the stimulus elicited peak V in response waveform. Response to the vowel portion elicited a frequency following response (FFR). The FFR further showed a coding of the fundamental frequency (F0) and the first formant frequency (F1). The results of the present study throw light on the processing of speech in brainstem. The understanding of speech evoked ABR has other applications both in research as well as in clinical purposes. Such understanding is specially important if one is interested in studying the central auditory system function.

Temporal processing in inferior colliculus and auditory cortex affected by high doses of salicylate

Speech recognition and language learning can be affected by both peripheral and central auditory system impairment. However, whether sensorineural hearing loss would affect central auditory processing is not clear. Recent studies found that salicylate not only affects outer hair cell motility in the cochlea, but also blocks GABAergic neuron activities in central nervous systems. This provides a good animal model to evaluate the role of sensorineural hearing loss and central inhibition in auditory temporal processing. In this study, gap prepulse inhibition (gap-PPI) of the acoustic startle reflex was used to measure effects of salicylate on gap detection acuity. Salicylate administration (250 mg/kg) resulted in a significant reduction in gap-PPI amplitude and an increased gap detection threshold at 50 dB SPL, but not at 60 or 80 dB SPL. To identify the physiological effects of salicylate on central auditory system function, the inferior colliculus (IC) and auditory cortex (AC) responses were measured from conscious rats with chronically implanted electrodes. Salicylate induced a significant increase of the gap-detection threshold in AC-evoked potentials, but not in the IC-evoked potentials. The AC gap-detection threshold shift was diminished measured at an equal sensational level. These results suggest that salicylate-induced temporal processing deficits may be due to peripheral hearing loss, not central disinhibition.

GAD levels and muscimol binding in rat inferior colliculus following acoustic trauma

Pharmacological studies of the inferior colliculus (IC) suggest that the inhibitory amino acid neurotransmitter γ-aminobutyric acid (GABA) plays an important role in shaping responses to simple and complex acoustic stimuli. Several models of auditory dysfunction, including age-related hearing loss, tinnitus, and peripheral deafferentation, suggest an alteration of normal GABA neurotransmission in central auditory pathways. The present study attempts to further characterize noise-induced changes in GABA markers in the IC. Four groups (unexposed control, 0 h post-exposure, 42 h post-exposure, and 30 days post-exposure) of 3-month-old male Fischer 344 rats were exposed to a high intensity sound (12 kHz, 106 dB) for 10 h. Observed hair cell damage was primarily confined to the basal half of the cochlea. There was a significant decrease in glutamic acid decarboxylase (GAD 65) immunoreactivity in the IC membrane fraction compared to controls ( P<0.05) at 0 h (−41%) and 42 h (−28%) post-exposure, with complete recovery by 30 days post-exposure ( P>0.98). Observed decreases in cytosolic levels of GAD 65 were not significant. Quantitative muscimol receptor binding revealed a significant increase (+20%) in IC 30 days after sound exposure ( P<0.05). These data suggest that changes in GABA neurotransmission occur in the IC of animals exposed to intense sound. Additional studies are needed to determine whether these changes are a result of protective/compensatory mechanisms or merely peripheral differentiation, as well as whether these changes preserve or diminish central auditory system function.

Sound Lateralization and Speech Discrimination in Patients with Sensorineural Hearing Loss

Sound lateralization and speech discrimination abilities are both closely related to central auditory system function. In this study, we investigated the relationship between sound lateralization and speech discrimination in patients with sensorineural hearing loss (SNHL) of unknown aetiology or presbycusis. Interaural intensity difference (IID) and interaural time difference (ITD) discriminations were measured using a self-recording apparatus for dichotic sound presentation, and the means of bilateral maximum scores in speech discrimination tests were calculated. Subjects with normal sound lateralization had good speech discrimination scores above 70%, while those with abnormal sound lateralization had poor scores below 70%. Some patients, however, had good speech discrimination but abnormal sound lateralization. These findings were obtained in both IID and ITD discrimination tests. Furthermore, some subjects could not discriminate ITD, although all could discriminate IID. These findings suggest that sound lateralization ability may be affected more than speech discrimination in SNHL of unknown aetiology or in presbycusis.

Development of the middle latency response in an animal model and its relation to the human response.

Although the clinical use of the middle latency response (MLR) in adults is fairly straightforward, its use is complicated by maturational changes that continue throughout the first decade of life. In order to telescope the time period of this long developmental course, we have approached the study of MLR maturation using the gerbil as an animal model. The course of MLR obtained over the temporal lobe development was characterized in the Mongolian gerbil ranging in age from 10 days to 3 months of life. The adult gerbil MLR consists of two positive peaks (A and C) at 11 and 25 ms, respectively, and a negative component (B) at 16 ms. These components emerge in a systematic fashion as a function of age. The present work supports a strong age effect of increased MLR detectability in the gerbil, similar to findings reported for humans. Wave A was infrequently detected in young animals, but when present, it occurred at adult latencies. The latency of waves B and C decreased systematically with age. The amplitude of all components increased with age, similar to findings in humans. The fact that adult-like thresholds were obtained shortly after birth indicates that when present, MLRs may be a good index of hearing threshold. Effects of stimulating across a wide range of intensities were described. The gerbil model appears appropriate for the study of development of the central auditory system function.