Positive auditory cortical responses in patients with absent brainstem response

ObjectiveTo compare the detectability of the different auditory evoked responses in patients with retrocochlear lesion.MethodsThe 40-Hz auditory steady state response (ASSR) and the N1m auditory cortical response were examined by magnetoencephalography in 4 patients with vestibular schwannoma, in whom the auditory brainstem response (ABR) was absent.ResultsApparent N1m responses were observed despite total absence of the ABR or absence except for small wave I in all patients, although the latency of N1m was delayed in most patients. On the other hand, clear ASSFs could be observed only in one patient. Very small 40-Hz ASSFs could be detected in 2 patients (amplitude less than 1 fT), but no apparent ASSFs were observed in one patient, in whom maximum speech intelligibility was extremely low and the latency of N1m was most prolonged.ConclusionThe N1m response and 40-Hz ASSR could be detected in patients with absent ABR, but the N1m response appeared to be more detectable than the 40-Hz ASSR.SignificanceCombined assessment with several different evoked responses may be useful to evaluate the disease conditions of patients with retrocochlear lesions.     

Effect of auditory input on activations in infant diverse cortical regions during audiovisual processing

A fundamental question with regard to perceptual development is how multisensory information is processed in the brain during the early stages of development. Although a growing body of evidence has shown the early emergence of modality‐specific functional differentiation of the cortical regions, the interplay between sensory inputs from different modalities in the developing brain is not well understood. To study the effects of auditory input during audio‐visual processing in 3‐month‐old infants, we evaluated the spatiotemporal cortical hemodynamic responses of 50 infants while they perceived visual objects with or without accompanying sounds. The responses were measured using 94‐channel near‐infrared spectroscopy over the occipital, temporal, and frontal cortices. The effects of sound manipulation were pervasive throughout the diverse cortical regions and were specific to each cortical region. Visual stimuli co‐occurring with sound induced the early‐onset activation of the early auditory region, followed by activation of the other regions. Removal of the sound stimulus resulted in focal deactivation in the auditory regions and reduced activation in the early visual region, the association region of the temporal and parietal cortices, and the anterior prefrontal regions, suggesting multisensory interplay. In contrast, equivalent activations were observed in the lateral occipital and lateral prefrontal regions, regardless of sound manipulation. Our findings indicate that auditory input did not generally enhance overall activation in relation to visual perception, but rather induced specific changes in each cortical region. The present study implies that 3‐month‐old infants may perceive audio‐visual multisensory inputs by using the global network of functionally differentiated cortical regions. Hum Brain Mapp, 2013. © 2011 Wiley Periodicals, Inc.     

Reliability of early cortical auditory gamma-band responses

ObjectiveTo evaluate the test–retest reliability of event-related power changes in the 30–150 Hz gamma frequency range occurring in the first 150 ms after presentation of an auditory stimulus.MethodsRepeat intracranial electrocorticographic (ECoG) recordings were performed with 12 epilepsy patients, at ?1-day intervals, using a passive odd-ball paradigm with steady-state tones. Time–frequency matching pursuit analysis was used to quantify changes in gamma-band power relative to pre-stimulus baseline. Test–retest reliability was estimated based on within-subject comparisons (paired t-test, McNemar’s test) and correlations (Spearman rank correlations, intra-class correlations) across sessions, adjusting for within-session variability. Reliability estimates of gamma-band response robustness, spatial concordance, and reproducibility were compared with corresponding measurements from concurrent auditory evoked N1 responses.ResultsAll patients showed increases in gamma-band power, 50–120 ms post-stimulus onset, that were highly robust across recordings, comparable to the evoked N1 responses. Gamma-band responses occurred regardless of patients’ performance on behavioral tests of auditory processing, medication changes, seizure focus, or duration of test–retest interval. Test–retest reproducibility was greatest for the timing of peak power changes in the high-gamma range (65–150 Hz). Reliability of low-gamma responses and evoked N1 responses improved at higher signal-to-noise levels.ConclusionsEarly cortical auditory gamma-band responses are robust, spatially concordant, and reproducible over time.SignificanceThese test–retest ECoG results confirm the reliability of auditory gamma-band responses, supporting their utility as objective measures of cortical processing in clinical and research studies.     

Human cortical processing of auditory events over time

An essential feature of the human brain is the ability to extract information from temporally distributed events in the auditory environment. We addressed this temporal encoding ability by modelling how stimulus rate is represented in the auditory cortex. We propose that a cortical representation of stimulus rate can be achieved through the oscillatory properties of nerve cells. Using non-invasive brain measures, we tested the prediction of the model that multiple rebound responses (RRs) occur after the ending of stimulation. MEG recordings revealed successive RRs which originated in the same areas as the N1s elicited by the auditory stimuli at latencies predicted by the model. Our modelling and experimental results therefore provide evidence that the temporal structure of the auditory environment is decomposed in human auditory cortex by cells performing bandpass filtering on periodic input.     

Rapid acoustic processing in the auditory brainstem is not related to cortical asymmetry for the syllable rate of speech

ObjectiveTemporal acuity in the auditory brainstem is correlated with left-dominant patterns of cortical asymmetry for processing rapid speech-sound stimuli. Here we investigate whether a similar relationship exists between brainstem processing of rapid speech components and cortical processing of syllable patterns in speech.MethodsWe measured brainstem and cortical evoked potentials in response to speech tokens in 23 children. We used established measures of auditory brainstem and cortical activity to examine functional relationships between these structures.ResultsWe found no relationship between brainstem responses to fast acoustic elements of speech and right-dominant cortical processing of syllable patterns.ConclusionsBrainstem processing of rapid elements in speech is not functionally related to rightward cortical asymmetry associated with the processing of syllable-rate features in speech. Viewed together with previous evidence linking brainstem timing with leftward cortical asymmetry for faster acoustic features, findings support the existence of distinct mechanisms for encoding rapid vs. slow elements of speech.SignificanceResults provide a fundamental advance in our knowledge of the segregation of sub-cortical input associated with cortical asymmetries for acoustic rate processing in the human auditory system. Implications of these findings for auditory perception, reading ability and development are discussed.     

Deafferentation induces novel axonal projections in the auditory brainstem after hearing onset

Deafferentation of neural tissue can result in cell death, morphological changes, and/or alterations in sources of innervation. These changes often occur during a limited period of development. In the auditory brainstem, the ventral cochlear nucleus (VCN) projects to the contralateral but not ipsilateral medial nucleus of the trapezoid body (MNTB). This pathway is part of a circuit that computes interaural intensity differences used in sound localization. Previous studies have shown that, after the cochlea is removed early in postnatal development, cells in the VCN on the deafferented side die, and the intact VCN innervates MNTB on both sides of the brain. These changes after cochlea removal are limited to an early postnatal period that preceeds hearing onset. In this study, we lesioned the VCN directly to evaluate plasticity in axonal pathways after hearing onset. We found that novel projections from the intact VCN to ipsilateral MNTB emerge after lesions performed as late as postnatal day 25. The morphological sequence of events is similar to that seen during the initial development of this pathway. These data suggest that plasticity in the auditory brainstem is possible when pathways are challenged with denervation of target nuclei. The results show that the opportunity for plasticity in auditory brainstem circuitry is more prolonged than previously thought and that novel pathways can form after the normal pathways are fully mature and functional. Moreover, sensitive periods for changes in individual pathways are independently regulated.     

Dose-dependent effects of atropine sulfate on the brainstem and cortical auditory evoked potentials in the rat

Because brainstem auditory evoked potentials (BAEPs) are frequently recorded in anesthetized animals and humans, it is important to become familiar with the effects on the BAEP of drugs used during anesthesia, including pre-anesthetics. The dose-dependent and stimulus intensity-dependent effects on the BAEP of a pre-anesthetic, atropine sulfate, were studied in the unanesthetized rat. The animal subjects were 11 adult female Long-Evans rats. BAEPs in response to 0.1 ms clicks (12.5/s) were recorded from skull screw electrodes during a baseline period, as well as after saline and atropine treatments. Atropine sulfate was given i.p. in doses ranging from 0.250 to 40 mg/kg. Contrary to a prior report, doses in the standard pre-anesthetic range (i.e. 0.250-1.000 mg/kg) did not convincingly influence the BAEP. Only the highest dose (40 mg/kg) produced a significant and noteworthy change in the BAEP. This effect was characterized by significant amplitude increases in the P1, P2 and P3 components, but not in the P4, P5 and P6 components. This selective effect occurred at the highest stimulus intensity of 110 dB peak equivalent sound pressure level, but not at lower intensities. There were no convincing atropine-induced changes in BAEP latencies. Atropine-induced changes in the cortical auditory evoked potential (CAEP) were characterized by amplitude decrements. Thus, atropine seemed to have an excitatory effect on the BAEP and an inhibitory or depressive effect on the CAEP.     

Abnormal auditory cortical activation in dyslexia 100 msec after speech onset

Reading difficulties are associated with problems in processing and manipulating speech sounds. Dyslexic individuals seem to have, for instance, difficulties in perceiving the length and identity of consonants. Using magnetoencephalography (MEG), we characterized the spatio-temporal pattern of auditory cortical activation in dyslexia evoked by three types of natural bisyllabic pseudowords (/ata/, /atta/, and /a a/), complex nonspeech sound pairs (corresponding to /atta/ and /a a/) and simple 1-kHz tones. The most robust difference between dyslexic and non-reading-impaired adults was seen in the left supratemporal auditory cortex 100 msec after the onset of the vowel /a/. This N100m response was abnormally strong in dyslexic individuals. For the complex nonspeech sounds and tone, the N100m response amplitudes were similar in dyslexic and nonimpaired individuals. The responses evoked by syllable /ta/ of the pseudoword /atta/ also showed modest latency differences between the two subject groups. The responses evoked by the corresponding nonspeech sounds did not differ between the two subject groups. Further, when the initial formant transition, that is, the consonant, was removed from the syllable /ta/, the N100m latency was normal in dyslexic individuals. Thus, it appears that dyslexia is reflected as abnormal activation of the auditory cortex already 100 msec after speech onset, manifested as abnormal response strengths for natural speech and as delays for speech sounds containing rapid frequency transition. These differences between the dyslexic and nonimpaired individuals also imply that the N100m response codes stimulus-specific features likely to be critical for speech perception. Which features of speech (or nonspeech stimuli) are critical in eliciting the abnormally strong N100m response in dyslexic individuals should be resolved in future studies.     

Effects of laserneedle stimulation in the external auditory meatus on brainstem and very early auditory evoked potentials in humans

OBJECTIVES: For the first time, brainstem auditory evoked potentials (BAEP) and very early auditory evoked potentials (EAEP) were investigated in 23 volunteers (mean age: 26.5 +/- 3.6 years) under resting conditions and during continuous and frequency modulated (2 Hz) laser stimulation in the external auditory meatus. METHODS: Using a new ear adapter, the outer region of the auditory canal was stimulated with laser light (685 nm, 4 x 30-40 mW, duration: 10 minutes). RESULTS: The EAEPs were significantly changed during continuous (p=0.019), as well as frequency modulated (p=0.014) laser stimulation compared with control measurements. DISCUSSION: Physiologic alterations of inner ear mechanism such as extra-cerebral changes in conductance or stimulation-dependent depolarization processes in extra-cerebral regions of the auditory system could be possible explanations for the significant difference in measurement parameters.     

Hypothyroidism impairs chloride homeostasis and onset of inhibitory neurotransmission in developing auditory brainstem and hippocampal neurons

Thyroid hormone (TH) deficiency during perinatal life causes a multitude of functional and morphological deficits in the brain. In rats and mice, TH dependency of neural maturation is particularly evident during the first 1–2 weeks of postnatal development. During the same period, synaptic transmission via the inhibitory transmitters glycine and GABA changes from excitatory depolarizing effects to inhibitory hyperpolarizing ones in most neurons [deploarizing–hyperpolarizing (D/H) shift]. The D/H shift is caused by the activation of the K⁺–Cl⁻ co-transporter KCC2 which extrudes Cl⁻ from the cytosol, thus generating an inward-directed electrochemical Cl⁻ gradient. Here we analyzed whether the D/H shift and, consequently, the onset of inhibitory neurotransmission are influenced by TH. Gramicidin perforated-patch recordings from auditory brainstem neurons of experimentally hypothyroid rats revealed depolarizing glycine effects until postnatal day (P)11, i.e. almost 1 week longer than in control rats, in which the D/H shift occurred at ∼P5–6. Likewise, until P12–13 the equilibrium potential E_Gly in hypothyroids was more positive than the membrane resting potential. Normal E_Gly could be restored upon TH substitution in P11–12 hypothyroids. These data demonstrate a disturbed Cl⁻ homeostasis following TH deficiency and point to a delayed onset of synaptic inhibition. Interestingly, immunohistochemistry demonstrated an unchanged KCC2 distribution in hypothyroids, implying that TH deficiency did not affect KCC2 gene expression but may have impaired the functional status of KCC2. Hippocampal neurons of hypothyroid P16–17 rats also demonstrated an impaired Cl⁻ homeostasis, indicating that TH may have promoted the D/H shift and maturation of synaptic inhibition throughout the brain.     

Effect of auditory motion velocity on reaction time and cortical processes

The study investigated the processing of sound motion, employing a psychophysical motion discrimination task in combination with electroencephalography. Following stationary auditory stimulation from a central space position, the onset of left- and rightward motion elicited a specific cortical response that was lateralized to the hemisphere contralateral to the direction of motion. The contralaterality of the motion onset response decreased when the velocity was reduced. Higher motion velocity was associated with larger and earlier cortical responses and with shorter reaction times to motion onset. The results indicate a close correspondence of brain activity and behavioral performance in auditory motion detection.     

Visual, cortical somatosensory and brainstem auditory evoked potentials following incidental irradiation of the rhombencephalon.

Visual, cortical somatosensory and brainstem auditory evoked potentials were recorded before incidental irradiation of the rhombencephalon and at 11 weeks and eight months after completion of treatment. No patient experienced neurological symptoms during this period. No consistent changes in evoked potentials were found. The failure to demonstrate subclinical radiation-induced demyelination suggests either that the syndrome of early-delayed radiation rhombencephalopathy occurs in an idiosyncratic manner, or that any subclinical lesions are not detectable by serial evoked potential recordings.     

Air pollution is associated with brainstem auditory nuclei pathology and delayed brainstem auditory evoked potentials

We assessed brainstem inflammation in children exposed to air pollutants by comparing brainstem auditory evoked potentials (BAEPs) and blood inflammatory markers in children age 96.3 ± 8.5 months from highly polluted (n = 34) versus a low polluted city (n = 17). The brainstems of nine children with accidental deaths were also examined. Children from the highly polluted environment had significant delays in wave III (t(50) = 17.038; p < 0.0001) and wave V (t(50) = 19.730; p < 0.0001) but no delay in wave I (p = 0.548). They also had significantly longer latencies than controls for interwave intervals I-III, III-V, and I-V (all t(50) > 7.501; p < 0.0001), consisting with delayed central conduction time of brainstem neural transmission. Highly exposed children showed significant evidence of inflammatory markers and their auditory and vestibular nuclei accumulated α synuclein and/or β amyloid_1-42. Medial superior olive neurons, critically involved in BAEPs, displayed significant pathology. Children's exposure to urban air pollution increases their risk for auditory and vestibular impairment.     

A comparison of the three-dimensional auditory brainstem response and the conventional auditory brainstem response in children

For measurement of neural activity in the brainstem auditory pathway, the conventional two-dimensional (2D) auditory brainstem response (ABR) does not provide a true response, because the equivalent dipoles originate from the stereoregularity pathway. It is thus necessary to use three-dimensional (3D) ABR to estimate the true response of the brainstem. We recorded 3D ABR in a group of children and adults, and compared the results with those of the conventional 2D ABR.

The subjects were 22 children (age range 3–10 years) and 10 adults with no neurological disorders, and three patients: a boy and a girl who had experienced sudden brainstem dysfunction, and a girl who had sudden deafness. 3D ABR was recorded for all subjects, and the results were displayed on a computer screen for off-line analysis using an original 3D ABR analysis program.

Four leaf-like vector segments of 3D ABR existed during the first 8 ms after stimulation. Each vector segment corresponded to a peak of the conventional ABR, and showed the original directivity. The amplitudes of waves II and IV of the 3D ABR were significantly larger than those of the conventional ABR. 3D ABR was shown to be superior to the conventional ABR in obtaining absolute amplitude. We were able to clarify the development of brainstem function using 3D ABR. In one patient in whom only one wave was obtained, 3D ABR was able to identify the wave as wave V. These results indicate that ABR is useful both for identifying the kind of wave produced and for suggesting the wave origin.     

Rapid development of cortical auditory evoked potentials after early cochlear implantation

The aim of our research was to estimate the time course of development and plasticity of the human central auditory pathways following cochlear implantation. We recorded cortical auditory-evoked potentials in 3-year-old congenitally deaf children after they were fitted with cochlear implants. Immediately after implantation cortical response latencies resembled those of normal-hearing newborns. Over the next few months, the cortical evoked responses showed rapid changes in morphology and latency that resulted in age-appropriate latencies by 8 months after implantation. Overall, the development of cortical response latencies for the implanted children was more rapid than for their normal-hearing age-matched peers. Our results demonstrate a high degree of central auditory system plasticity during early human development.     

No evidence for dependence of early cortical auditory processing on dopamine D(2)-receptor modulation: a whole-head magnetoencephalographic study

Magnetoencephalography (MEG) was used to determine the effect of neuroleptic challenge on brain responses in healthy subjects. In a double-blind, randomized, placebo-controlled, cross-over design study, the dopamine D(2) receptor antagonist haloperidol (2 mg) was given orally to 12 healthy volunteers. The middle-latency auditory evoked magnetic fields (MAEF) were recorded 3 h after administration of haloperidol or placebo with a whole-head 122-channel MEG. Haloperidol did not significantly affect MAEF responses. The dipole moments and source locations of the responses were not significantly influenced by haloperidol. These results suggest that dopamine D(2) receptors are not involved in the early phases of auditory cortical processing.     

Effect of stimulus intensity variation on brainstem auditory evoked potentials. Maturational changes

The effect of stimulus intensity variation on brainstem auditory evoked potentials in premature and full-term neonates has been studied in two experiments. They show that the effects are different according to age and to waves. The variation of latencies is greater the younger the child and it is clearer with wave I than waves III and V. The findings are discussed in terms of the maturation of the basal part of the cochlea.