Cortical Auditory Evoked Response Research Articles

The Maturation of the Acoustic Change Complex in Response to Iterated Ripple Noise in 'Normal'-Hearing Infants, Toddlers, and Adults.

Infants and toddlers are still being evaluated for their hearing sensitivity but not their auditory-processing skills. Iterated rippled noise (IRN) stimuli require the auditory system to utilize the temporal periodicity and autocorrelate the iterations to perceive pitch. This study investigated the acoustic change complex (ACC) elicited by IRN in "normal"-hearing infants, toddlers, and adults to determine the maturation of cortical processing of IRN stimuli. Cortical responses to filtered white noise (onset) concatenated with IRN stimuli (d = 10 milliseconds, gain = 0.7 dB: 4-32 iterations) were recorded in quiet, alert participants. Participants included 25 infants (2.5-15 months), 27 toddlers (22-59 months), and 8 adults (19-25 years) with "normal" hearing sensitivity. Cortical auditory-evoked responses were recorded for each participant, including the onset response to the noise and an ACC to the transition from noise to IRN. Group differences were assessed using repeated-measures analyses of variance. Most infants had a replicable onset (P) response, while only about half had a measurable ACC (PACC) response to the high-saliency IRN condition. Most toddlers had onset responses and showed a P-NACC response to the IRN16 and IRN32 conditions. Most of the toddler group had responses present to the onset and showed a P-NACC response to all IRN conditions. Toddlers and adults showed similar P-NACC amplitudes; however, adults showed an increase in N1ACC amplitude with increase in IRN iterations (i.e., increased salience). While cortical responses to the percept of sound as determined by the onset response (P) to a stimulus are present in most infants, ACC responses to IRN stimuli are not mature in infancy. Most toddlers as young as 22 months, however, exhibited ACC responses to the IRN stimuli even when the pitch saliency was low (e.g., IRN4). The findings of the current study have implications for future research when investigating maturational effects on ACC and the optimal choice of stimuli.

Fully objective hearing threshold estimation in cochlear implant users using phase-locking value growth functions

Cochlear implant users require fitting of electrical threshold and comfort levels for optimal access to sound. In this study, we used single-channel cortical auditory evoked responses (CAEPs) obtained from 20 participants using a Nucleus device. A fully objective method to estimate threshold levels was developed, using growth function fitting and the peak phase-locking value feature. Results demonstrated that growth function fitting is a viable method for estimating threshold levels in cochlear implant users, with a strong correlation (r = 0.979, p < 0.001) with behavioral thresholds. Additionally, we compared the threshold estimates using CAEPs acquired from a standard montage (Cz to mastoid) against using a montage of recording channels near the cochlear implant, simulating recording from the device itself. The correlation between estimated and behavioural thresholds remained strong (r = 0.966, p < 0.001), however the recording time needed to be increased to produce a similar estimate accuracy. Finally, a method for estimating comfort levels was investigated, and showed that the comfort level estimates were mildly correlated with behavioral comfort levels (r = 0.50, p = 0.024).

Sensory and movement evoked EEG patterns in the preterm neonate

Due to the similarities between EEG activity of humans during second trimester of gestation and the activity patterns of neonatal rodents, the latter became valuable model to study the mechanisms by which early patterns of activity may act as template for later-emerging functional maps [2] . In rodents, early EEG activity (Spindles Bursts [SB]) are generated within a complex neural network including the peripheral sensory organs, subcortical nuclei as well as cortical areas. In the barrel cortex, sensory evoked SB contain early gamma oscillations (EGO) that are primarily driven by rhythmic gamma excitation from the thalamus and enable precise temporal binding of the topographically aligned thalamic and cortical neurons. EGO support long-term dependent potentiation in the thalamo-cortical synapses [1] . Characteristic preterm EEG patterns of “Delta-brushes” (DBs) are spontaneous EEG activities and have been reported in the sensory cortices following sensory stimuli and spontaneous movements. Spatiotemporal analyses of the cortical auditory-evoked responses (AERs) using 32-electrode EEG recordings in healthy neonates aged 30 to 38 post-menstrual weeks (PMW) showed that AERs are characterized by a power spectrum increase at frequency bands ranging from delta to gamma on the electrodes overlaying temporal regions. Time-frequency wavelet analysis also showed that fast (alpha-gamma) oscillations co-occurred with the auditory-evoked delta wave from around 650 ms after the stimulus and lasted for nearly 700 ms. Stimulus locked averaging disclosed cortical auditory-evoked potentials (CAEP) with negative high amplitude delta waves peaking successively over middle and posterior temporal regions. Thus, auditory-evoked DBs in preterm neonates are slow delta waves nesting fast oscillations in the temporal regions and correspond to the late part of CAEPs, which is lost when using classical CAEP processing [3] . Decreasing from 30 to 38 PMW, this EEG pattern is thus associated with the prenatal development of sensory processing circuits and likely participate to the cortical maps maturation.

Cortical Auditory-Evoked Responses in Preterm Neonates: Revisited by Spectral and Temporal Analyses.

Characteristic preterm EEG patterns of "Delta-brushes" (DBs) have been reported in the temporal cortex following auditory stimuli, but their spatio-temporal dynamics remains elusive. Using 32-electrode EEG recordings and co-registration of electrodes' position to 3D-MRI of age-matched neonates, we explored the cortical auditory-evoked responses (AERs) after 'click' stimuli in 30 healthy neonates aged 30-38 post-menstrual weeks (PMW). (1) We visually identified auditory-evoked DBs within AERs in all the babies between 30 and 33 PMW and a decreasing response rate afterwards. (2) The AERs showed an increase in EEG power from delta to gamma frequency bands over the middle and posterior temporal regions with higher values in quiet sleep and on the right. (3) Time-frequency and averaging analyses showed that the delta component of DBs, which negatively peaked around 550 and 750 ms over the middle and posterior temporal regions, respectively, was superimposed with fast (alpha-gamma) oscillations and corresponded to the late part of the cortical auditory-evoked potential (CAEP), a feature missed when using classical CAEP processing. As evoked DBs rate and AERs delta to alpha frequency power decreased until full term, auditory-evoked DBs are thus associated with the prenatal development of auditory processing and may suggest an early emerging hemispheric specialization.

Temporal Encoding of Voice Onset Time in Young Children

Objective: Voice onset time (VOT) is an important parameter of speech that denotes the time interval between consonant onset and the onset of low frequency periodicity generated. In this study we examined the temporal encoding of voice onset time in the cortical auditory evoked response (CAEP) of young children. Methods: Scalp recoded CAEP were measured in 18 children aged from 5-8 participated (n=18). The N2 latency was evoked using differences in voice-onset-times (VOTs) using stop consonant-vowel syllables /ga/-/ka/. Results: A significant and systematic shift in the N2 latency was observed for differences in VOT. Conclusions: Our results demonstrate that temporal encoding of VOT exists in the developing cortical evoked response.

Can You Hear Me Now? Musical Training Shapes Functional Brain Networks for Selective Auditory Attention and Hearing Speech in Noise

Even in the quietest of rooms, our senses are perpetually inundated by a barrage of sounds, requiring the auditory system to adapt to a variety of listening conditions in order to extract signals of interest (e.g., one speaker's voice amidst others). Brain networks that promote selective attention are thought to sharpen the neural encoding of a target signal, suppressing competing sounds and enhancing perceptual performance. Here, we ask: does musical training benefit cortical mechanisms that underlie selective attention to speech? To answer this question, we assessed the impact of selective auditory attention on cortical auditory-evoked response variability in musicians and non-musicians. Outcomes indicate strengthened brain networks for selective auditory attention in musicians in that musicians but not non-musicians demonstrate decreased prefrontal response variability with auditory attention. Results are interpreted in the context of previous work documenting perceptual and subcortical advantages in musicians for the hearing and neural encoding of speech in background noise. Musicians’ neural proficiency for selectively engaging and sustaining auditory attention to language indicates a potential benefit of music for auditory training. Given the importance of auditory attention for the development and maintenance of language-related skills, musical training may aid in the prevention, habilitation, and remediation of individuals with a wide range of attention-based language, listening and learning impairments.

Concurrent Sound Segregation Across the Adult Life-Span

Abstract To be a successful participant in a single conversation during difficult listening conditions, the listener has to segregate incoming sounds into separate conversations and focus on the co...

Aging in Binaural Hearing Begins in Mid-Life: Evidence from Cortical Auditory-Evoked Responses to Changes in Interaural Phase

Older adults often have difficulty understanding speech in a noisy environment or with multiple speakers. In such situations, binaural hearing improves the signal-to-noise ratio. How does this binaural advantage change with increasing age? Using magnetoencephalography, we recorded cortical activity evoked by changes in interaural phase differences of amplitude-modulated tones. These responses occurred for frequencies up to 1225 Hz in young subjects but only up to 940 Hz in middle-aged and 760 Hz in older adults. Behavioral thresholds also decreased with increasing age but were more variable, likely because some older adults make effective use of compensatory mechanisms. The reduced frequency range for binaural hearing became significant in middle age, before decline in hearing sensation and the morphology of cortical responses, which became apparent only in the older subjects. This study provides evidence from human physiological data for the early onset of biological aging in binaural hearing.

Preservation of brainstem neurophysiological function in hydranencephaly

The preservation of central neurophysiological function was assessed in a 32-year-old woman with hydranencephaly using brainstem auditory evoked responses (BAER), auditory middle latency responses (MLR), cortical auditory evoked responses (CER), strobe electroretinograms (ERG), strobe-flash visual evoked responses (VER) and median and tibial nerve somatosensory evoked responses (SER). The BAER to the right ear stimulation revealed wave peaks I through VII with normal thresholds, morphology and latencies, while the BAER in the left ear was abnormal. The auditory MLR and CER were absent. Grossly normal strobe ERGs were acquired bilaterally with peak waves at 20 and 50 ms. Strobe VERs were poorly defined and abnormal bilaterally. Left and right median nerve SER revealed significant conduction defects in the large fiber sensory system caudal to the thalamus, above the lower pontine level. Bilateral tibial nerve stimulation revealed normal knee popliteal fossa potentials, but distinct conduction defects in the large fiber sensory system rostral to the lower spinal cord. Brainstem electrophysiological measures revealed functional auditory afferent tracts and nuclei, in the absence of cortical influence, suggesting intact unilateral auditory function, which would support clinical observations of behavioral auditory responses in hydranencephaly.

I've heard it all before: Perceptual invariance represented by early cortical auditory-evoked responses

Sensitivity to acoustic invariance is critical for establishing stable representations in a shifting world of sound. By recording early auditory cortical responses to complex sounds in human listeners and categorising these responses according to the maintenance or change of stimulus attributes across consecutive presentations, we show that repetition within a constantly varying acoustic context produces enhanced neural responding in auditory cortices.

Electrophysiological measures of binaural interaction in cochlear implantees

Electrophysiological measures that can be used to objectively evaluate binaural interaction in cochlear implantees include the binaural interaction component (BIC) and an evoked potential analog of the ‘masking-level difference’ (MLD). Results from normal listeners show that cortical auditory-evoked responses obtained using these methods can be used to measure binaural processing. Preliminary data from three sequentially implanted adults are presented.

Effect of bolus doses of alfentanil on the arousal response to intubation, as assessed by the auditory evoked response

We have studied the effect of bolus doses of alfentanil on the arousal response to intubation, using the auditory evoked response (AER) of the electroencephalogram (EEG) in 45 anaesthetized patients. After induction of anaesthesia with propofol and 20 min of positive pressure ventilation via a laryngeal mask airway with 0.6 MAC of isoflurane and 50% nitrous-oxide in oxygen, patients received saline (control), or alfentanil 15 or 50 micrograms kg-1 before intubation. The early cortical AER before and after intubation in each group was measured and compared. After intubation, Pa amplitude increased in the saline group by 60% (95% CI +10 to +130), decreased in the low-dose alfentanil group by 11% (-38 to +29) and decreased further in the high-dose alfentanil group by 26% (-49 to +7). There were significant (P < 0.005) linear trends in the three group means for Pa amplitude. Similarly, Nb latency increased in the saline group by 30% (-2 to +73), decreased in the low-dose alfentanil group by 10% (-32 to +19) and decreased further in the high-dose alfentanil group by 19% (-39 to +7). There were significant (P = 0.02) linear trends in the three group means for Nb amplitude. Systolic and diastolic arterial pressures increased after intubation in the control group, and to a lesser extent in the low-dose alfentanil group, but decreased after high-dose alfentanil. Heart rate increased after intubation in the control group but decreased in both alfentanil groups, decreasing to a greater degree in the high-dose group. There were significant (P < 0.001) linear trends for all three haemodynamic variables in the three groups. We conclude that the increase in Pa amplitude after intubation was blocked by bolus administration of alfentanil and that this effect was dose dependent.

Auditory evoked response, median frequency and 95% spectral edge during anaesthesia with desflurane and nitrous oxide.

We have studied in 12 patients the effect of desflurane in nitrous oxide on the electroencephalogram (EEG) and the early cortical auditory evoked response (AER). After induction with desflurane, patients' lungs were ventilated to maintain three different end-expiratory concentrations of desflurane (1.5, 3 and 6%) during four consecutive 10-min periods before surgery. As the end-expiratory concentration of desflurane was increased, Pa and Nb (AER) amplitudes decreased and their latencies increased, and spontaneous EEG showed an increase in amplitude and a slowing of frequency. A linear relationship was demonstrated between log10 concentration of desflurane and all variables (P = 0.001). Pa amplitude showed the greatest linearity followed by the derived variable F95 of the EEG. From regression slopes, mean percentage changes of each variable were calculated for a 1 MAC change in desflurane concentration, Pa amplitude showed the largest change (mean 49% (95% confidence interval 40-56%) decrease for a 1 MAC increase). This was greater than that of F95 for a similar confidence interval, indicating better resolution. This study confirms that the early cortical AER is affected by desflurane in a similar manner to that of other anaesthetic agents and as such remains the most promising EEG derived measure of depth of anaesthesia.

Anaesthetic depth and auditory evoked potentials

We have investigated the auditory evoked response (AER) in a series of experimental and clinical studies to assess its usefulness as a measure of depth of anaesthesia. Three questions were posed. Does the AER show: –l) graded changes with increasing concentrations of general anaesthetics which are similar for different anaesthetic drugs 2) appropriate changes with surgical stimulation 3) changes with loss of consciousness?The AER was found to satisfy these criteria in that six general anaesthetics, belonging to different chemical groups, showed similar graded reductions in the amplitudes and increases in the latencies of the AER waves originating from the primary cortical section of the auditory nerve pathway. These changes were reversed by surgical stimulation indicating that the AER does not simply act as a concentration monitor but reflects true depth of anaesthesia. In addition there were changes in these early cortical AER waves which indicate potential awareness.The clinical investigations also suggested that the AER is a valid measure of depth of anaesthesia. A numerical index derived from the AER decreased when the anaesthetic blood concentration increased and increased when surgical stimulation increased in severity. The numerical index gave the same value at the same depth of anaesthesia in different patients. This means that a monitoring device based on the AER would not have to be calibrated separately for each patient and further that it could be attached to the patient when they are already anaesthetised. Further work is in progress on the display of information for the anaesthetist and in reducing the time taken to produce a reliable response.

AUDITORY EVOKED RESPONSE AND AWARENESS: A STUDY IN VOLUNTEERS AT SUB-MAC CONCENTRATIONS OF ISOFLURANE

We have investigated the relationship between the auditory evoked response (AER) and simple tests of conscious awareness at four end-expiratory concentrations (0.0, 0.1, 0.2 and 0.4 MAC) of isoflurane in oxygen in each of eight anaesthetist volunteers, in random order, at least 1 week apart. The early cortical AER was recorded from electrodes at the vertex and inion. Amplitudes of the waves Pa, Nb and Pc and latencies of the waves Na, Pa, Nb, Pb and Nc were measured. All the AER variables were highly significantly related to end-expiratory anaesthetic concentration. Amplitudes decreased and latencies increased progressively with increasing anaesthetic concentration. The AER variables were also highly significantly related to the level of response. Amplitudes were greatest and the latencies shortest when there was full response to command. (Nb latency increased from 47.5 to 54.5 ms between partial and no response.) The close correlation between the effects of concentration and level of response, and between concentration and the AER implied that it was difficult to demonstrate those changes in the AER which specifically relate to changes in response. At 0.2 MAC, however, which was the concentration at which all subjects showed some deficit, the response to a shock word was distinguished clearly by Nb latency. In eight of 24 possible comparisons (eight AER variables and three types of psychological test) the AER fitted the response more closely than concentration.

Short and Long Latency Auditory Evoked Potentials in Traumatic Brain Injury Patients

Short latency auditory nerve and brainstem evoked responses (BAERs) and long latency cortical auditory evoked responses (CAERs) in 75 long-term traumatic brain injury (TBI) cases were compared. CAERs were found to be significantly correlated with clinical disability as measured by the Disability Rating Scale, while BAERs were not. Also, BAER patterns were consistently and significantly less abnormal and less sensitive to overall dysfunction than CAER patterns. Findings support previous observations that BAERs have relatively little utility for evaluating in surviving TBI patients the degree of overall brain impairment. In general, long latency AEP patterns are better able to reflect the extent and severity of brain dysfunction and overall clinical condition than are short latency AEP patterns in long-term severe TBI patients, and these patterns should be obtained routinely in the evaluation of such patients.

THE AUDITORY EVOKED RESPONSE AS AN INDICATOR OF AWARENESS

The latency of the early cortical wave Nb of the auditory evoked response (AER) was compared with responses to Tunstall's isolated forearm test, while the concentration of nitrous oxide was progressively reduced during light anaesthesia in seven patients. A threshold Nb latency of 44.5 ms was chosen to discriminate between an early cortical AER containing three waves and that with two waves of longer latency. When Nb latency decreased below this threshold, four of the patients has positive responses, indicating awareness. The addition of a volatile anaesthetic abolished any response, and increased Nb latency to more than 44.5 ms. The three wave AER pattern, therefore, is associated with a depth of anaesthesia at which awareness occurs.

Electroencephalographic correlates of electroconvulsive therapy.

The aim of this investigation was to study the detailed profile and persistence of electroconvulsive therapy (ECT)-induced EEG correlates. Subjects were submitted to a detailed clinical evaluation as well as automated, computerized EEG recordings. Analysis was performed of different frequency bands, mean frequency and total power as well as a detailed study of cortical auditory-evoked responses. Age and sex-matched control subjects comprised a group of comparable patients, none of whom had received ECT, a second control group of healthy volunteers and a third group of amitriptyline-treated patients. No statistically significant EEG changes were found among patients who had been treated with ECT from several months to several years prior to the investigation. During ECT, highly significant cumulative effects were observed in most of the EEG measures, particularly increased slow-wave activity and a corresponding reduction in fast-wave activity. Predictably, significant changes also occurred in mean EEG frequency and total power. No significant changes were observed in the cortical-evoked responses. EEG abnormalities were still significant one week following the completion of a course of ECT, but reverted to baseline levels within four weeks.

Otoneurological findings in Friedreich's ataxia and other inherited neuropathies.

Fifteen cases of Friedreich's ataxia (FA) were examined using an otoneurological test battery that included tone and speech audiometry, the synthetic sentence identification (SSI) test, impedance audiometry, cortical auditory-evoked response (CAER), brainstem auditory-evoked response (ABR) and electronystagmography. We also obtained ABR and CAER findings in 2 cases of familial spastic paraplegia, in 5 cases of Charcot-Marie-Tooth disease and 6 in cases of atypical FA of uncertain classification. The results of puretone and impedance audiometry were normal in all cases. ABR could not be elicited in 11 FA patients and were abnormal at higher intensity levels in the remaining 4 patients. In these 4 cases, however, the latencies were normal. ABR did not show any marked abnormalities in patients with familial spastic paraplegia or Charcot-Marie-Tooth disease. CAERs were normal in all 28 patients. ABRs tended to be absent with the progression of FA. ABR thresholds were correlated with the Inherited Ataxias Clinical Rating Scale score, which is an index of the severity of the illness. ABRs contributed to the diagnosis or to excluding FA in patients with an atypical clinical picture. The absence of ABRs and the normal latencies of the waves, when evoked, agree with the pathological finding of a reduction of fibers in the spinal root ganglion. SSI abnormalities and vestibular findings agree with this hypothesis.

Effets of Intensity Variations on Auditory Processing in Aphasia II. Different Intensities at Each Ear

A neurologic extinction model was applied to the auditory-processing disorders evidenced by 10 persons with aphasia. This model suggests that messages travel faster to the intact hemisphere, where they are more differentiated and articulated, than to the affected hemisphere. This leads to extinction and interference of the message. To overcome this extinction, the stimulus intensity was raised by 15 or 30 dB to one ear at a time. The stimuli were a cortical auditory-evoked response (AER) measure, a nonverbal intensity sequencing test (NVIST), a minimally varied phoneme-in-word discrimination and sequencing test (MVPT). and a semantic-syntactic level test (RTT). The results suggest that intensity can be traded for time in quantities large enough to overcome the extinction interference of auditory stimuli. Although some statistically significant results and meaningful trends toward improved performance were evident on the NVIST and the MVPT, a unilateral increase of stimulus intensity did not prove to be a potent mechanism for improving auditory comprehension. Sentence length material was not affected in either direction by selective amplification. The role of the left ear/right hemisphere as a facilitator of processing for linguistic and nonlinguistic material was suggested by the results of this study.