Effect of stimulus level and frequency on ABR and MLR binaural interaction in human neonates

Binaural Integration of Spectrally Degraded Speech in Adult Cochlear Implant Recipients with Single-Sided Deafness

INTRODUCTION

Our purpose was to investigate binaural integration for spectrally degraded speech in normal-hearing (NH) subjects, single-sided deafness (SSD) cochlear implant (CI) recipients, and bilateral deaf bilateral CI recipients.

METHODS

We tested ten adult subjects in each group with a modified version of the binaural fusion test according to Matzker. Speech recognition was assessed for monotic listening with the better-hearing ear or CI, monotic listening with the poorer-hearing ear or CI, and dichotic listening. We employed two presentation modes: (1) low pass (LP)-filtered speech to the better ear or CI and high pass (HP)-filtered speech to the poorer ear or CI, and (2) LP-filtered speech to the poorer ear or CI and HP-filtered speech to the better ear or CI. Five magnitudes of LP and HP filtering, i.e., spectral degradation, for each presentation mode were applied yielding two spectrally overlapping and three nonoverlapping presentation conditions. Sentences from the Oldenburg Children's sentence test were applied to assess speech recognition.

RESULTS

NH subjects, SSD CI recipients, and bilateral CI recipients were able to understand spectrally degraded speech under both monotic and dichotic listening conditions for both the presentation modes. Speech performance decreased with the increasing loss of spectral information in all the three subject groups. In the NH subjects, speech recognition scores significantly improved for dichotic compared with each monotic listening type under two spectrally nonoverlapping conditions. The SSD CI subjects showed a significant improvement in speech scores for dichotic listening compared with monotic listening with the NH ear under one nonoverlapping condition, i.e., a dichotic benefit in speech recognition with CI. We saw a dichotic benefit in the bilateral CI recipients who achieved significantly better speech scores for dichotic compared with monotic listening with the better CI in three nonoverlapping conditions.

CONCLUSIONS

All the three groups (NH subjects, SSD CI recipients, and BiCI recipients) showed binaural integration for the side-separated presentation of spectrally degraded speech. Use of strictly side-separated dichotic stimulus presentation avoided the possible occurrence of physical effects such as the head shadow effect and therefore, confirmed the binaural benefit attributable to central binaural processing.

Unilateral Conductive Hearing Loss Disrupts the Developmental Refinement of Binaural Processing in the Rat Primary Auditory Cortex

Binaural hearing is critically important for the perception of sound spatial locations. The primary auditory cortex (AI) has been demonstrated to be necessary for sound localization. However, after hearing onset, how the processing of binaural cues by AI neurons develops, and how the binaural processing of AI neurons is affected by reversible unilateral conductive hearing loss (RUCHL), are not fully elucidated. Here, we determined the binaural processing of AI neurons in four groups of rats: postnatal day (P) 14–18 rats, P19–30 rats, P57–70 adult rats, and RUCHL rats (P57–70) with RUCHL during P14–30. We recorded the responses of AI neurons to both monaural and binaural stimuli with variations in interaural level differences (ILDs) and average binaural levels. We found that the monaural response types, the binaural interaction types, and the distributions of the best ILDs of AI neurons in P14–18 rats are already adult-like. However, after hearing onset, there exist developmental refinements in the binaural processing of AI neurons, which are exhibited by the increase in the degree of binaural interaction, and the increase in the sensitivity and selectivity to ILDs. RUCHL during early hearing development affects monaural response types, decreases the degree of binaural interactions, and decreases both the selectivity and sensitivity to ILDs of AI neurons in adulthood. These new evidences help us to understand the refinements and plasticity in the binaural processing of AI neurons during hearing development, and might enhance our understanding in the neuronal mechanism of developmental changes in auditory spatial perception.

Study of Binaural Auditory Cortical Response in Children with History of Recurrent Otitis

Introduction  Any type of sensory deprivation in childhood resulting from conductive hearing loss may impair the development of peripheral and central auditory pathway structures with negative consequences for binaural processing. Objective  To characterize and compare monoaural and binaural auditory responses in neonates and children without and with a history of recurrent otitis. Methods  The study included participants from 0 to 8 years and 11 months old, in good general health conditions, of both genders, divided into a control group, with no history of otitis, and a study group, with history of recurrent otitis. Cortical potential with speech stimulus /ba/-/da/ was used as collection procedure. The arithmetic calculation of the 512 points of the wave was performed to obtain the grand average of the waves of the subjects in both groups. The Shapiro-Wilk and mixed repeated measures analysis of covariance (ANCOVA) statistical tests were performed to analyze the group effect, the condition, and the interaction (group versus condition) controlling the effect of the age-sex covariable. Results  There was a statistically significant difference between the groups for all latency values; and for the P1, N1, P2, and N2 latencies, the differences between the groups occurred in the three analyzed conditions (right and left ears and binaural), revealing the influence of sensory deprivation. There were no significant differences in relation to wave amplitudes. Conclusion  There are differences in the cortical potential with speech stimuli and in the binaural interaction component of children with and without history of recurrent otitis.

Reinterpreting the human ABR binaural interaction component: isolating attention from stimulus effects

Subtracting the sum of left and right monaural auditory brainstem responses (ABRs) from the corresponding binaural ABR isolates the binaural interaction component (ABR-BIC). In a previous investigation (Ikeda, 2015), during auditory yet not visual tasks, tone-pips elicited a significant difference in amplitude between summed monaural and binaural ABRs. With click stimulation, this amplitude difference was task-independent. This self-critical reanalysis's purpose was to establish that a difference waveform (i.e., ABR-BIC DN1) reflected an auditory selective attention effect that was isolable from stimulus factors. Regardless of whether stimuli were tone-pips or clicks, effect sizes of the DN1 peak amplitudes relative to zero improved during auditory tasks over visual tasks. Auditory selective attention effects on the monaural and binaural ABR wave-V amplitudes were tone-pip specific. Those wave-V effects thus could not explain the stimulus-universal effect of auditory selective attention on DN1 detectability, which was thus entirely binaural. In a manner isolated from auditory selective attention, multiple mediation analyses indicated that the higher right monaural wave-V amplitudes mediated individual differences in how clicks, relative to tone-pips, augmented DN1 amplitudes. There are implications of these findings for advancing ABR-BIC measurement.

The Physiological Basis and Clinical Use of the Binaural Interaction Component of the Auditory Brainstem Response

The auditory brainstem response (ABR) is a sound-evoked noninvasively measured electrical potential representing the sum of neuronal activity in the auditory brainstem and midbrain. ABR peak amplitudes and latencies are widely used in human and animal auditory research and for clinical screening. The binaural interaction component (BIC) of the ABR stands for the difference between the sum of the monaural ABRs and the ABR obtained with binaural stimulation. The BIC comprises a series of distinct waves, the largest of which (DN1) has been used for evaluating binaural hearing in both normal hearing and hearing-impaired listeners. Based on data from animal and human studies, the authors discuss the possible anatomical and physiological bases of the BIC (DN1 in particular). The effects of electrode placement and stimulus characteristics on the binaurally evoked ABR are evaluated. The authors review how interaural time and intensity differences affect the BIC and, analyzing these dependencies, draw conclusion about the mechanism underlying the generation of the BIC. Finally, the utility of the BIC for clinical diagnoses are summarized.

Test-Retest Reliability of the Binaural Interaction Component of the Auditory Brainstem Response

OBJECTIVES

The binaural interaction component (BIC) is the residual auditory brainstem response (ABR) obtained after subtracting the sum of monaurally evoked from binaurally evoked ABRs. The DN1 peak-the first negative peak of the BIC-has been postulated to have diagnostic value as a biomarker for binaural hearing abilities. Indeed, not only do DN1 amplitudes depend systematically upon binaural cues to location (interaural time and level differences), but they are also predictive of central hearing deficits in humans. A prominent issue in using BIC measures as a diagnostic biomarker is that DN1 amplitudes not only exhibit considerable variability across subjects, but also within subjects across different measurement sessions.

DESIGN

In this study, the authors investigate the DN1 amplitude measurement reliability by conducting repeated measurements on different days in eight adult guinea pigs.

RESULTS

Despite consistent ABR thresholds, ABR and DN1 amplitudes varied between and within subjects across recording sessions. However, the study analysis reveals that DN1 amplitudes varied proportionally with parent monaural ABR amplitudes, suggesting that common experimental factors likely account for the variability in both waveforms. Despite this variability, the authors show that the shape of the dependence between DN1 amplitude and interaural time difference is preserved. The authors then provide a BIC normalization strategy using monaural ABR amplitude that reduces the variability of DN1 peak measurements. Finally, the authors evaluate this normalization strategy in the context of detecting changes of the DN1 amplitude-to-interaural time difference relationship.

CONCLUSIONS

The study results indicate that the BIC measurement variability can be reduced by a factor of two by performing a simple and objective normalization operation. The authors discuss the potential for this normalized BIC measure as a biomarker for binaural hearing.

Cortical Representation of Interaural Time Difference Is Impaired by Deafness in Development: Evidence from Children with Early Long-term Access to Sound through Bilateral Cochlear Implants Provided Simultaneously

Accurate use of interaural time differences (ITDs) for spatial hearing may require access to bilateral auditory input during sensitive periods in human development. Providing bilateral cochlear implants (CIs) simultaneously promotes symmetrical development of bilateral auditory pathways but does not support normal ITD sensitivity. Thus, although binaural interactions are established by bilateral CIs in the auditory brainstem, potential deficits in cortical processing of ITDs remain. Cortical ITD processing in children with simultaneous bilateral CIs and normal hearing with similar time-in-sound was explored in the present study. Cortical activity evoked by bilateral stimuli with varying ITDs (0, ±0.4, ±1 ms) was recorded using multichannel electroencephalography. Source analyses indicated dominant activity in the right auditory cortex in both groups but limited ITD processing in children with bilateral CIs. In normal-hearing children, adult-like processing patterns were found underlying the immature P1 (∼100 ms) response peak with reduced activity in the auditory cortex ipsilateral to the leading ITD. Further, the left cortex showed a stronger preference than the right cortex for stimuli leading from the contralateral hemifield. By contrast, children with CIs demonstrated reduced ITD-related changes in both auditory cortices. Decreased parieto-occipital activity, possibly involved in spatial processing, was also revealed in children with CIs. Thus, simultaneous bilateral implantation in young children maintains right cortical dominance during binaural processing but does not fully overcome effects of deafness using present CI devices. Protection of bilateral pathways through simultaneous implantation might be capitalized for ITD processing with signal processing advances, which more consistently represent binaural timing cues.SIGNIFICANCE STATEMENT Multichannel electroencephalography demonstrated impairment of binaural processing in children who are deaf despite early access to bilateral auditory input by first finding that foundations for binaural hearing are normally established during early stages of cortical development. Although 4- to 7-year-old children with normal hearing had immature cortical responses, adult patterns in cortical coding of binaural timing cues were measured. Second, children receiving two cochlear implants in the same surgery maintained normal-like input from both ears, but this did not support significant effects of binaural timing cues in either auditory cortex. Deficits in parieto-occiptal areas further suggested impairment in spatial processing. Results indicate that cochlear implants working independently in each ear do not fully overcome deafness-related binaural processing deficits, even after long-term experience.

A Comparison of Two Objective Measures of Binaural Processing: The Interaural Phase Modulation Following Response and the Binaural Interaction Component

There has been continued interest in clinical objective measures of binaural processing. One commonly proposed measure is the binaural interaction component (BIC), which is obtained typically by recording auditory brainstem responses (ABRs)—the BIC reflects the difference between the binaural ABR and the sum of the monaural ABRs (i.e., binaural − (left + right)). We have recently developed an alternative, direct measure of sensitivity to interaural time differences, namely, a following response to modulations in interaural phase difference (the interaural phase modulation following response; IPM-FR). To obtain this measure, an ongoing diotically amplitude-modulated signal is presented, and the interaural phase difference of the carrier is switched periodically at minima in the modulation cycle. Such periodic modulations to interaural phase difference can evoke a steady state following response. BIC and IPM-FR measurements were compared from 10 normal-hearing subjects using a 16-channel electroencephalographic system. Both ABRs and IPM-FRs were observed most clearly from similar electrode locations—differential recordings taken from electrodes near the ear (e.g., mastoid) in reference to a vertex electrode (Cz). Although all subjects displayed clear ABRs, the BIC was not reliably observed. In contrast, the IPM-FR typically elicited a robust and significant response. In addition, the IPM-FR measure required a considerably shorter recording session. As the IPM-FR magnitude varied with interaural phase difference modulation depth, it could potentially serve as a correlate of perceptual salience. Overall, the IPM-FR appears a more suitable clinical measure than the BIC.

Frequency Dependence of Binaural Interaction in the Auditory Brainstem and Middle Latency Responses

Purpose

The primary purpose of this investigation was to determine the relative frequency representation of binaural function in the brainstem and cortex of adults. The secondary purpose was to compare adult responses to previously reported infant responses.

Methods

Simultaneous auditory brainstem responses and auditory middle responses were recorded monaurally and binaurally in 20 young women. The binaural (BIN) response was subtracted from the summed monaural waves (L+R) to obtain the binaural interaction components (BIC) from waves V (peak A) and Pa (BIC-Pa). Amplitude ratios were calculated as BIC/L+R. Repeated-measures analyses of variance evaluated responses to frequency (500 Hz vs. 4000 Hz), wave condition (L+R vs. BIN), and wave class (auditory brainstem response vs. auditory middle response).

Results

Waveforms were present for all conditions. The L+R responses were larger than the BIN responses, 500 Hz produced larger amplitudes than 4000 Hz, and Pa was larger than wave V. The largest response, overall, was the Pa(L+R) response to 500 Hz. For amplitude ratios, BIC-Pa/Pa(L+R) was larger than Peak A/[V(L+R)].

Conclusion

More neural resources are devoted to binaural function in the cortex than in the brainstem, and more resources are devoted to lower frequencies than to higher frequencies. The adult data confirm that previously recorded infant data reveal binaural immaturity. Longitudinal data should characterize developmental characteristics of binaural function.

Effects of stimulation level and electrode pairing on the binaural interaction component of the electrically evoked auditory brain stem response

OBJECTIVES

The purpose of this study was to investigate the effects of stimulation level and electrode pairing on the binaural interaction component (BIC) of the electrically evoked auditory brain stem response (EABR) in Nucleus cochlear implant (CI) users.

DESIGN

Ten postlingually deafened adult CI users participated in this study. EABRs were measured using loudness balanced, biphasic current pulses presented in the left monaural, right monaural, and bilateral stimulation conditions. BICs were computed based on measures of the EABR obtained for each subject by pairing the electrode 12 (of 22 intracochlear electrodes) in the right ear with each of 11 electrodes spaced across the electrode array in the left ear. The effect of stimulation level on the amplitude of the BIC was investigated by measuring growth functions of the BIC from six subjects. The effect of electrode pairing on the amplitude of the BIC was studied at high stimulation levels in 10 subjects and at low stimulation levels in seven subjects. The high stimulation level was chosen as the 90% point of the subject's dynamic range (DR) or the highest stimulation level in which the electrophysiologic recordings were not contaminated by muscle artifacts. The low stimulation level was chosen as a level that was 10% point of subject's DR higher than the BIC threshold for six of these seven subjects. For one subject, BIC thresholds were not available and the low stimulation level was referred to the 70% point of subject's DR.

RESULTS

BICs were successfully recorded from all 11 interaural electrode pairs for a majority of subjects tested at both stimulation levels. BIC amplitudes increased with stimulation level. The effect of stimulation level on latencies of the BIC was less robust. At high stimulation levels, BIC amplitudes did not change significantly as the stimulating electrode used in the left ear was systematically varied. When low stimulation levels were used, BIC amplitude was maximal for interaural electrode pairs with similar intracochlear positions and decreased when the offset between interaural electrodes increased.

CONCLUSIONS

This study demonstrates that stimulation level affects amplitudes of the BIC response. It is possible to record the BIC of the EABR in bilateral CI users even from interaural electrode pairs that have large interaural offsets. This finding suggests that when high-level stimuli are used, there is a broad pattern of current spread within the two cochleae. At lower stimulation levels, the spread of excitation within the cochlea is reduced making the effect of electrode pairing on the amplitude of the BIC more pronounced.

Bilateral cochlear implants should be the standard for children with bilateral sensorineural deafness

PURPOSE OF REVIEW

Bilateral cochlear implants are provided to children in an attempt to establish binaural processing and allow hearing with greater ease. Arguments against implantation, which prevailed for many years, are countered by some of the findings reported over the past 1-2 years.

RECENT FINDINGS

Behavioral and electrophysiological outcomes in children receiving cochlear implants suggest that two issues are most important when considering bilateral cochlear implants for any child: the duration of deafness prior to the first implantation affecting development of oral speech and language skills and the inter-stage interval (between implantation of the first and second ears) likely affecting development of binaural processing.

SUMMARY

Based on the data reported to date, both the interval between onset of deafness and cochlear implantation and the interval between implantation of the first and second ears should be narrow in children. We recommend that simultaneous bilateral implantation be provided when possible and, if not, the inter-stage interval should be limited. We further recommend continued exploration of outcomes in children with longer inter-stage intervals with a view to defining a point at which bilateral cochlear implantation provides so little benefit that it is not cost-effective.

Auditory brainstem activity in children with 9–30 months of bilateral cochlear implant use

Bilateral cochlear implants aim to restore binaural processing along the auditory pathways in children with bilateral deafness. We assessed auditory brainstem activity evoked by single biphasic pulses delivered by an apical or basal electrode from the left, right and both cochlear implants in 13 children. Repeated measures were made over the first 9-30 months of bilateral implant use. In children with short or long periods of unilateral implant use prior to the second implantation, Wave eV of the auditory brainstem response was initially prolonged when evoked by the naïve versus experienced side. These differences tended to resolve in children first implanted <3 years of age but not in children implanted at older ages with long delays between implants. Latency differences were projected to persist for longer periods in children with long delays between implants compared with children with short delays. No differences in right versus left evoked eV latency were found in 2 children receiving bilateral implants simultaneously and their response latencies decreased over time. Binaural interaction responses showed effects of stimulating electrode position (responses were more detectable when evoked by an apical than basal pair of implant electrodes), and duration of delay between implants (measured by latency delays). The trends shown here suggest a negative impact of unilateral implant use on bilateral auditory brainstem plasticity.

Binaural processing in children using bilateral cochlear implants

Binaural auditory brainstem processing was examined using evoked potential measures in 40 children who were implanted early and received a second implant simultaneously or after long or short periods of unilateral implant use. Wave latencies were shorter when evoked by the experienced versus naïve implanted ear at initial bilateral activation. Binaural difference waves were detected in most children in response to apical but not basal electrode stimulation and were prolonged in latency in children implanted after long or short delays between implants. Timing differences between the implanted ears in children receiving sequential but not simultaneous bilateral implants reflect a relative immaturity of pathways innervating the second ear and results in abnormal timing of binaural processing at this initial implant stage.

Human auditory steady-state responses to changes in interaural correlation

Steady-state responses were evoked by noise stimuli that alternated between two levels of interaural correlation rho at a frequency fm. With rho alternating between +1 and 0, responses at fm dropped steeply above 4 Hz, but persisted up to 64 Hz. Two time constants of 47 and 4.4 ms with delays of 198 and 36 ms, respectively, were obtained by fitting responses to a transfer function based on symmetric exponential windows. The longer time constant, possibly reflecting cortical integration, is consistent with perceptual binaural "sluggishness". The shorter time constant may reflect running cross-correlation in the high brainstem or primary auditory cortex. Responses at 2fm peaked with an amplitude of 848+/-479 nV (fm=4 Hz). Investigation of this robust response revealed that: (1) changes in rho and lateralization evoked similar responses, suggesting a common neural origin, (2) response was most dependent on stimulus frequencies below 1000 Hz, but frequencies up to 4000 Hz also contributed, and (3) when rho alternated between [0.2-1] and 0, response amplitude varied linearly with rho, and the physiological response threshold was close to the average behavioral threshold (rho=0.31). This steady-state response may prove useful in the objective investigation of binaural hearing.

Prenatal alcohol and cocaine exposure: influences on cognition, speech, language, and hearing

This paper reviews research on the consequences of prenatal exposure to alcohol and cocaine on children's speech, language, hearing, and cognitive development. The review shows that cognitive impairment, learning disabilities, and behavioral disorders are the central nervous system manifestations of fetal alcohol syndrome (FAS), and cranio-facial abnormalities are also present. Delays in language acquisition, as well as receptive and expressive language deficits, are commonly reported. The cranio-facial abnormalities of FAS, which sometimes include cleft palate, make the child prone to otitis media with effusion and conductive hearing loss. The family environment in which one or both parents is a heavy alcohol user presents challenges to a child with normal intelligence, but may be especially deleterious to the child with mental retardation. Prenatal exposure to cocaine results in subtle cognitive disabilities when measured at 4 years of age. The cognitive effects may be ameliorated by a stimulating and sensitive care-giving environment. A small, deleterious "cocaine-effect" is also seen in speech and language development. The child with prenatal exposure to cocaine may be considered at increased risk for language delay or disorder. There is no evidence that prenatal cocaine exposure by itself is a risk factor for sensorineural hearing impairment, although auditory evoked potentials from the brainstem and cortex suggest some abnormalities in central auditory processing, at least during the newborn period. The strong effect of the home environment for ameliorating the effects of prenatal cocaine-exposure suggests that a family-focused approach for cognitive, language, and social-emotional habilitation would be beneficial to all.

LEARNING OUTCOMES

The learner will be able to describe the major features of fetal alcohol syndrome and how they relate to speech, language, hearing, and cognitive disorders. The learner will review the literature and determine research needs with respect to language, speech, and hearing among infants and children with fetal alcohol syndrome. Similarly, the learner will distinguish the outcomes of prenatal alcohol-exposure from those of prenatal cocaine-exposure. The learner will summarize the controversy regarding the possible stigmatization of cocaine-exposed infants. The learner will summarize the speech, language, and hearing effects of prenatal cocaine-exposure.

Interaural time coincidence detectors are present at birth: evidence from binaural interaction

Binaural processing of sounds in mammals is presumably initiated within the auditory nuclei of the caudal pons. The binaural difference waveform (BD) can be derived from the sum of the waveforms evoked by right monaural clicks plus left monaural clicks minus the waveform evoked by binaural clicks. In adults, the BD's first positive peak (beta) is large only for stimuli with interaural time differences (ITDs) that produce a fused acoustic percept. Humans at birth can localize and discriminate sound sources, but their head circumference is about two-thirds of an adult head. In order to test whether beta is related to head circumference, we recorded beta in human neonates as a function of ITD. Binaural clicks with ITDs ranging between 0 and 1000 micros were used to derive BD waveforms in 34 neonates. For ITD=0, beta was detectable in 56% of newborns. The incidence of beta detection then decreased as ITD increased. Only 9% of the babies had detectable beta for all ITDs. No correlation was found between the existence of beta and other properties of the monaural or binaural auditory brainstem response. The finding that for some infants beta was present for all ITDs up to 1.0 ms suggests that there is no recalibration of brainstem delay lines with head growth. Our data suggest that the brainstem auditory pathway for detecting interaural time differences in the adult is probably present at birth. Maturational factors such as increased myelination and greater firing synchrony probably improve the detectability of beta with age. The second peak in the BD waveform (delta) was highly correlated with the existence of wave VI in the binaural and monaural waveforms.

Dipole source analysis of auditory brain stem responses evoked by lateralized clicks

The objective of this paper was to elucidate the relation between psychophysical lateralization and the neural generators of the corresponding auditory evoked potentials. Auditory brain stem responses to binaural click stimuli with different interaural time- and level differences were obtained in 12 subjects by means of multi-channel EEG recording. Data were modeled by equivalent current dipoles representing the generating sources in the brain. A generalized maximum-likelihood method was used to solve the inverse problem, taking into account the noise covariance matrix of the data. The quality of the fit was assessed by computing the goodness-of-fit as the outcome of a chi 2-test. This measure was advantageous compared to the conventionally employed residual variance. At the latency of Jewett wave V, there was a systematic variation of the moment of a rotating dipole with the lateralization of the stimulus. Dipole moment trajectories of stimuli with similar lateralization were similar. A sign reversal of the interaural differences resulted in a mirrored trajectory. Centrally-perceived stimuli corresponded to dipoles with the largest vertical components. With increasing lateralization, the vertical component of the moment decreased, while the horizontal components increased. The similarity of trajectories inducted by the same lateralization show that interaural time- and level differences are not processed independently. The present data support the notion that directional information is already extracted and represented at the level of the brain stem.

Auditory brain stem responses evoked by lateralized clicks: is lateralization extracted in the human brain stem?

The dependence of binaurally evoked auditory brain stem responses and the binaural difference potential on simultaneously presented interaural time and level differences is investigated in order to assess the representation of stimulus lateralization in the brain stem. Auditory brain stem responses to binaural click stimuli with all combinations of three interaural time and three interaural level differences were recorded from 12 subjects and 4 channels. The latency of Jewett wave V is shortest for zero interaural time difference and longest for the trading stimuli. The amplitude of wave V is largest for centrally perceived stimuli, i.e., the diotic and trading stimuli, and smallest for the most laterally perceived stimuli. The latency of the most prominent peak of the binaural difference potential DN1 mainly depends on the interaural time difference. The amplitude of the components of the binaural difference potential, DP1-DN1, depends similarly on stimulus conditions as wave V amplitude in the case of the binaural stimuli: smallest amplitudes are found for the most lateral stimuli and largest amplitudes for central stimuli. The results demonstrate that interaural level and time differences are not processed independently. This supports the hypothesis that directional information in humans is already extracted and represented at the level of the brain stem.

Brain auditory activation measured by near-infrared spectroscopy (NIRS) in neonates

This study presents a new measure of the hemodynamic changes to an auditory stimulus in newborns. Nineteen newborns born at 28-41 wk and aged 1 to 49 d were studied in waking and/or sleeping state, for a median time of 4 min 40 s before, 2 min 40 s during, and 3 min 5 s after an acustic stimulus (tonal sweep of frequency increasing from 2 to 4 kHz, intensity 90 dB SPL) originating 5 cm from the external auditory meatus. The emitter and detector optodes were placed over the left or right temporal region, corresponding to T3 or T4 EEG electrodes. The concentration changes in cerebral chromophores Delta[HbO2], Delta[Hb] and Deltaoxidized-reduced cytochrome aa(3) were recorded every 5 s. Changes in cerebral blood volume were calculated from the changes in total Hb x 0.89/large vessel Hb concentration. Increased oxyhemoglobin, Delta[HbO2], total Hb, Delta[Hb (sum)], and cerebral blood volume, DeltaCBV, were found in 13/19 neonates, with the exception of a neonate who only had increased in Delta[Hb], Delta[Hb (sum)] and DeltaCBV. During the stimulation phase there was a significant increase in DeltaCBV (t test, p = 0.00006) in the responsive newborns from a mean value of 0.006 (+/-0.02) mL/100 g in the pretest phase to 0.09 (+/-0.06) mL/100 g during the auditory stimulus. After the test DeltaCBV decreased to 0.04 (+/-0.07) mL/100 g (t test, p = 0.01), so did Delta[Hb (sum)] (p = 0.02). Hemodynamic responses of the subjects who showed increases in Delta[Hb (sum)] and Delta[HbO(2)] were analyzed to study the Delta[Hb]. The responder subjects could be classified into two groups according to Delta[Hb] changes: 8/13 (61.5%) showed an increase of Delta[Hb] (pattern A), while 5/13 (38.4%) showed a decrease (pattern B) (t test, p = 0.03). These two patterns did not show differences related to Delta[HbO(2)] and Delta[Hb (sum)]. The DeltaCBV changes in nonresponders presented a decrease during the test phase (t test, p = 0.04). CBV did not return to pretest values, suggesting a fronto-temporal brain pathway for storing unusual sounds. The increase in CBV followed the local increase in oxyhemoglobin and total Hb concentrations due to a greater use of oxygen in the homolateral temporal cortex of the newborns.