Dynamic effects of habituation and novelty detection on newborn event-related potentials

Newborns habituate to repeated auditory stimuli, and discriminate syllables, generating opportunities for early language learning. This study investigated trial-by-trial changes in newborn electrophysiological responses to auditory speech syllables as an index of habituation and novelty detection. Auditory event-related potentials (ERPs) were recorded from 16 term newborn infants, aged 1-3 days, in response to monosyllabic speech syllables presented during habituation and novelty detection tasks. Multilevel models demonstrated that newborns habituated to repeated auditory syllables, as ERP amplitude attenuated for a late-latency component over successive trials. Subsequently, during the novelty detection task, early- and late-latency component amplitudes decreased over successive trials for novel syllables only, indicating encoding of the novel speech syllable. We conclude that newborns dynamically encoded novel syllables over relatively short time periods, as indicated by a systematic change in response patterns with increased exposure. These results have important implications for understanding early precursors of learning and memory in newborns.

Differences in Neural Correlates of Speech Perception in 3 Month Olds at High and Low Risk for Autism Spectrum Disorder

In this study, we investigated neural precursors of language acquisition as potential endophenotypes of autism spectrum disorder (ASD) in 3-month-old infants at high and low familial ASD risk. Infants were imaged using functional near-infrared spectroscopy while they listened to auditory stimuli containing syllable repetitions; their neural responses were analyzed over left and right temporal regions. While female low risk infants showed initial neural activation that decreased over exposure to repetition-based stimuli, potentially indicating a habituation response to repetition in speech, female high risk infants showed no changes in neural activity over exposure. This finding may indicate a potential neural endophenotype of language development or ASD specific to females at risk for the disorder.

Multiple Biomarkers of Maternal Iron Predict Infant Cognitive Outcomes

This study examined relations between multiple measures of maternal iron status assessed 3 months post-partum, and infant processing speed (longest look during visual habituation), memory (novelty preference), attention (heart rate changes), and neural response variability (in auditory event-related potentials) at 3 and 9 months. Plasma iron was associated with 9-month novelty preference and longest look, and developmental changes in longest look. Hemoglobin predicted sustained attention, and both plasma iron and soluble transferrin receptors predicted neural response variability at 9 months. Improved maternal iron appears to have a positive impact on infant cognitive development even in a well-nourished, low-risk sample.

Auditory ERP response to successive stimuli in infancy

Background. Auditory Event-Related Potentials (ERPs) are useful for understanding early auditory development among infants, as it allows the collection of a relatively large amount of data in a short time. So far, studies that have investigated development in auditory ERPs in infancy have mainly used single sounds as stimuli. Yet in real life, infants must decode successive rather than single acoustic events. In the present study, we tested 4-, 8-, and 12-month-old infants' auditory ERPs to musical melodies comprising three piano notes, and examined ERPs to each individual note in the melody. Methods. Infants were presented with 360 repetitions of a three-note melody while EEG was recorded from 128 channels on the scalp through a Geodesic Sensor Net. For each infant, both latency and amplitude of auditory components P1 and N2 were measured from averaged ERPs for each individual note. Results. Analysis was restricted to response collected at frontal central site. For all three notes, there was an overall reduction in latency for both P1 and N2 over age. For P1, latency reduction was significant from 4 to 8 months, but not from 8 to 12 months. N2 latency, on the other hand, decreased significantly from 4 to 8 to 12 months. With regard to amplitude, no significant change was found for either P1 or N2. Nevertheless, the waveforms of the three age groups were qualitatively different: for the 4-month-olds, the P1-N2 deflection was attenuated for the second and the third notes; for the 8-month-olds, such attenuation was observed only for the middle note; for the 12-month-olds, the P1 and N2 peaks show relatively equivalent amplitude and peak width across all three notes. Conclusion. Our findings indicate that the infant brain is able to register successive acoustic events in a stream, and ERPs become better time-locked to each composite event over age. Younger infants may have difficulties in responding to late occurring events in a stream, and the onset response to the late events may overlap with the incomplete response to preceding events.

Elicitation of the Acoustic Change Complex to Long-Duration Speech Stimuli in Four-Month-Old Infants

The acoustic change complex (ACC) is an auditory-evoked potential elicited to changes within an ongoing stimulus that indicates discrimination at the level of the auditory cortex. Only a few studies to date have attempted to record ACCs in young infants. The purpose of the present study was to investigate the elicitation of ACCs to long-duration speech stimuli in English-learning 4-month-old infants. ACCs were elicited to consonant contrasts made up of two concatenated speech tokens. The stimuli included native dental-dental /dada/ and dental-labial /daba/ contrasts and a nonnative Hindi dental-retroflex /daDa/ contrast. Each consonant-vowel speech token was 410 ms in duration. Slow cortical responses were recorded to the onset of the stimulus and to the acoustic change from /da/ to either /ba/ or /Da/ within the stimulus with significantly prolonged latencies compared with adults. ACCs were reliably elicited for all stimulus conditions with more robust morphology compared with our previous findings using stimuli that were shorter in duration. The P1 amplitudes elicited to the acoustic change in /daba/ and /daDa/ were significantly larger compared to /dada/ supporting that the brain discriminated between the speech tokens. These findings provide further evidence for the use of ACCs as an index of discrimination ability.

From early markers to neuro-developmental mechanisms of autism

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Studies of infants at-risk could reveal the developmental origin of autism.

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Behavioral and brain markers differentiate infants that develop autism symptoms from controls, during the first year of life.

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Little evidence for decreased social orienting or social motivation.

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Some evidence for multiple developmental pathways to autism.

A fast growing field, the study of infants at risk because of having an older sibling with autism (i.e. infant sibs) aims to identify the earliest signs of this disorder, which would allow for earlier diagnosis and intervention. More importantly, we argue, these studies offer the opportunity to validate existing neuro-developmental models of autism against experimental evidence. Although autism is mainly seen as a disorder of social interaction and communication, emerging early markers do not exclusively reflect impairments of the “social brain”. Evidence for atypical development of sensory and attentional systems highlight the need to move away from localized deficits to models suggesting brain-wide involvement in autism pathology. We discuss the implications infant sibs findings have for future work into the biology of autism and the development of interventions.

Development of auditory event-related potentials in infants prenatally exposed to methadone

Developmental features of the P2 auditory ERP in a change detection paradigm were examined in infants prenatally exposed to methadone. Opiate dependent pregnant women maintained on methadone replacement therapy were recruited during pregnancy (N = 60). Current and historical alcohol and substance use, SES, and psychiatric status were assessed with a maternal interview during the third trimester. Medical records were used to collect information regarding maternal medications, monthly urinalysis, and breathalyzer to confirm comorbid drug and alcohol exposures. Between birth and 4 months infant ERP change detection performance was evaluated on one occasion with the oddball paradigm (.2 probability oddball) using pure-tone stimuli (standard = 1 kHz and oddball = 2 kHz frequency) at midline electrode sites, Fz, Cz, Pz. Infant groups were examined in the following developmental windows: 4-15, 16-32, or 33-120 days PNA. Older groups showed increased P2 amplitude at Fz and effective change detection performance at P2 not seen in the newborn group. Developmental maturation of amplitude and stimulus discrimination for P2 has been reported in developing infants at all of the ages tested and data reported here in the older infants are consistent with typical development. However, it has been previously reported that the P2 amplitude difference is detectable in neonates; therefore, absence of a difference in P2 amplitude between stimuli in the 4-15 days group may represent impaired ERP performance by neonatal abstinence syndrome or prenatal methadone exposure.

Dynamics of infant cortical auditory evoked potentials (CAEPs) for tone and speech tokens

OBJECTIVES

Cortical auditory evoked potentials (CAEPs) to tones and speech sounds were obtained in infants to: (1) further knowledge of auditory development above the level of the brainstem during the first year of life; (2) establish CAEP input-output functions for tonal and speech stimuli as a function of stimulus level and (3) elaborate the data-base that establishes CAEP in infants tested while awake using clinically relevant stimuli, thus providing methodology that would have translation to pediatric audiological assessment. Hypotheses concerning CAEP development were that the latency and amplitude input-output functions would reflect immaturity in encoding stimulus level. In a second experiment, infants were tested with the same stimuli used to evoke the CAEPs. Thresholds for these stimuli were determined using observer-based psychophysical techniques. The hypothesis was that the behavioral thresholds would be correlated with CAEP input-output functions because of shared cortical response areas known to be active in sound detection.

DESIGN

36 infants, between the ages of 4 and 12 months (mean=8 months, s.d.=1.8 months) and 9 young adults (mean age 21 years) with normal hearing were tested. First, CAEPs amplitude and latency input-output functions were obtained for 4 tone bursts and 7 speech tokens. The tone bursts stimuli were 50 ms tokens of pure tones at 0.5, 1.0, 2.0 and 4.0 kHz. The speech sound tokens, /a/, /i/, /o/, /u/, /m/, /s/, and /∫/, were created from natural speech samples and were also 50 ms in duration. CAEPs were obtained for tone burst and speech token stimuli at 10 dB level decrements in descending order from 70 dB SPL. All CAEP tests were completed while the infants were awake and engaged in quiet play. For the second experiment, observer-based psychophysical methods were used to establish perceptual threshold for the same speech sound and tone tokens.

RESULTS

Infant CAEP component latencies were prolonged by 100-150 ms in comparison to adults. CAEP latency-intensity input output functions were steeper in infants compared to adults. CAEP amplitude growth functions with respect to stimulus SPL are adult-like at this age, particularly for the earliest component, P1-N1. Infant perceptual thresholds were elevated with respect to those found in adults. Furthermore, perceptual thresholds were higher, on average, than levels at which CAEPs could be obtained. When CAEP amplitudes were plotted with respect to perceptual threshold (dB SL), the infant CAEP amplitude growth slopes were steeper than in adults.

CONCLUSIONS

Although CAEP latencies indicate immaturity in neural transmission at the level of the cortex, amplitude growth with respect to stimulus SPL is adult-like at this age, particularly for the earliest component, P1-N1. The latency and amplitude input-output functions may provide additional information as to how infants perceive stimulus level. The reasons for the discrepancy between electrophysiologic and perceptual threshold may be due to immaturity in perceptual temporal resolution abilities and the broad-band listening strategy employed by infants. The findings from the current study can be translated to the clinical setting. It is possible to use tonal or speech sound tokens to evoke CAEPs in an awake, passively alert infant, and thus determine whether these sounds activate the auditory cortex. This could be beneficial in the verification of hearing aid or cochlear implant benefit.

Maturation of the cortical auditory evoked potential in infants and young children

The aim of this study was to evaluate the maturation of the cortical auditory evoked potential (CAEP) in humans. The participants in this experiment were 10 newborns (<7 days), 19 toddlers (13-41 months), 20 children (4-6 years) and 9 adults (18-45 years). CAEPs were obtained in response to low (400 Hz) and high (3000 Hz) tones and to the word token /baed/, all presented at 60 dB HL, at a rate of 0.22 Hz. Latency and amplitude measures were made for CAEP components P1, N1, P2 and N2 as a function of participant age, stimulus type and electrode montage. CAEP component latencies were relatively stable from birth to 6 years, but adults demonstrated significantly shorter latencies compared to infants and children. Components P1 and N2 decreased in amplitude, while components N1 and P2 increased in amplitude from birth to adulthood. Words evoked significantly larger CAEPs in newborns compared to responses evoked by tones, but in other age groups the effects of stimulus type on component amplitudes and latencies were less consistent. There was evidence of immature tonotopic organisation of the generators of N1 when responses from infants and young children were compared to those of adults. The scalp distribution of components N1 and P2 was clearly different in newborns and toddlers compared to children and adults. In the younger groups, both N1 and P2 were uniformly distributed across the scalp but in children and adults these components showed more focal distributions, with evidence of response laterality increasing with maturity. The results of the present study describe, for the first time, CAEPs recorded from multiple scalp electrodes, for tones and speech stimuli, in infants and children from birth to 6 years of age. Frequency-related differences in component amplitude were apparent at all ages reflecting development of tonotopic organisation of the CAEP neural generators.

Maturation of CAEP in infants and children: A review

This paper reviews our current understanding of the development of the obligatory cortical auditory evoked potential (CAEP) components P1, N1, P2, and N2. Firstly, the adult CAEP is briefly reviewed with respect to its morphology, neural generators and stimulus-dependence. Secondly, age-related changes occurring from the newborn period through childhood and adolescence are reviewed. The focus is on the maturation of CAEP morphology, changes in the scalp topography of the various components, changes in their amplitude and latency and in their stimulus-dependence. This review identifies periods of development in which we have only limited understanding of cortical auditory processing, as revealed by evoked potentials.

The Influence of Unequal Numbers of Trials on Comparisons of Average Event-Related Potentials

Four studies examined (a) how event-related potentials (ERPs) change as the number of trials averaged increases and (b) the statistical implications of comparing ERPs composed of different numbers of trials. Experiment 1 utilized data from 7-year-old children performing an oddball task. The other three experiments used simulated data with different distributions of P3 peak latency. In all 4 experiments, peak amplitude decreased and the mean amplitude of the 300 to 900 msec interval remained stable as the number of trials averaged increased. The standard deviations of both measures decreased. These data show that the decrease in peak amplitude with increasing numbers of trials that has been found in other studies is not solely due to the elimination of residual noise but is likely to also involve a fundamental aspect of signal averaging and the algorithm used to select peaks. Furthermore, these experiments expose the possibility of statistical errors when investigators compare average ERPs composed of small versus large numbers of trials as is often done when the oddball paradigm is used.

Fetal brain activity and hemodynamic response to a vibroacoustic stimulus

Previous studies have demonstrated the practicality of using functional magnetic resonance imaging (fMRI) techniques to assess fetal brain activity. The purpose of this study was to compare the fetal hemodynamic response to that of the adult. Seventeen pregnant subjects, all of whom were at more than 36 weeks gestation were scanned while the fetus was exposed to a vibroacoustic stimulus. Thirteen adult subjects were scanned with an equivalent acoustic stimulus. Of the fetal subjects, two could not be analyzed due to technical problems, eight did not show significant activation, and seven showed significant activation. In all cases, activation was localized within the temporal region. Measures of fetal hemodynamic responses revealed an average time to peak (ttp) of 7.36 +/- 0.94 sec and an average percentage change of 2.67 +/- 0.93%. In contrast, activation was detected in 5 of 13 adults with an average ttp of 6.54 +/- 0.54 sec and an average percentage change of 1.02 +/- 0.40%. The measurement of changes in the fetal hemodynamic response may be important in assessing compromised pregnancies.

Measuring temporal resolution in infants using mismatch negativity

We show that the mismatch negativity (MMN) component of the event-related potential can be used to measure auditory temporal resolution in human infants. Infrequent stimuli with silent gaps of 4, 8, or 12 ms modulated the P2 component, generated MMN, and produced a P3a-like positivity. The data indicate that within-channel gap detection thresholds at 6 months are essentially at adult levels under conditions of little adaptation. Since MMN is elicited without attention and does not require a behavioural response, it can be measured similarly across the lifespan. We are now in a position to study the development of cross-channel temporal resolution and adaptation effects in infancy, and to examine how these abilities in infancy relate to later language acquisition.