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Speech scientists have long proposed that formant exaggeration in infant-directed speech plays an important role in language acquisition. This event-related potential (ERP) study investigated neural coding of formant-exaggerated speech in 6-12-month-old infants. Two synthetic /i/ vowels were presented in alternating blocks to test the effects of formant exaggeration. ERP waveform analysis showed significantly enhanced N250 for formant exaggeration, which was more prominent in the right hemisphere than the left. Time-frequency analysis indicated increased neural synchronization for processing formant-exaggerated speech in the delta band at frontal-central-parietal electrode sites as well as in the theta band at frontal-central sites. Minimum norm estimates further revealed a bilateral temporal-parietal-frontal neural network in the infant brain sensitive to formant exaggeration. Collectively, these results provide the first evidence that formant expansion in infant-directed speech enhances neural activities for phonetic encoding and language learning.
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PAPER
Neural coding of formant-exaggerated speech in the infant brain
Yang Zhang,
1,2
Tess Koerner,
1
Sharon Miller,
1
Zach Grice-Patil,
3
Adam
Svec,
1
David Akbari,
1
Liz Tusler
1
and Edward Carney
1
1. Department of Speech-Language-Hearing Sciences, University of Minnesota, USA
2. Center for Neurobehavioral Development, University of Minnesota, USA
3. Department of Psychology, University of Minnesota, USA
Abstract
Speech scientists have long proposed that formant exaggeration in infant-directed speech plays an important role in language
acquisition. This event-related potential (ERP) study investigated neural coding of formant-exaggerated speech in 6–12-month-
old infants. Two synthetic ivowels were presented in alternating blocks to test the effects of formant exaggeration. ERP
waveform analysis showed significantly enhanced N250 for formant exaggeration, which was more prominent in the right
hemisphere than the left. Time-frequency analysis indicated increased neural synchronization for processing formant-
exaggerated speech in the delta band at frontal-central-parietal electrode sites as well as in the theta band at frontal-central
sites. Minimum norm estimates further revealed a bilateral temporal-parietal-frontal neural network in the infant brain sensitive
to formant exaggeration. Collectively, these results provide the first evidence that formant expansion in infant-directed speech
enhances neural activities for phonetic encoding and language learning.
Introduction
Language input is assigned roles of varying importance
in acquisition models and theories. The poverty-
of-stimulusargument asserts that language is unlearn-
able from the impoverished input data available to
children (Chomsky, 1980). In contrast, speech research
over the past five decades has established that enriched
exposure adaptively guides language acquisition early in
life (Hçhle, 2009; Kuhl, Conboy, Coffey-Corina, Padden,
Rivera-Gaxiola & Nelson, 2008). For example, when
talking to infants, people across cultures tend to use
exaggerated pitch, elongated words, and expanded vowel
space with stretched formant frequencies (Ferguson,
1964; Fernald, 1992; Kuhl, Andruski, Chistovich,
Chistovich, Kozhevnikova, Ryskina, Stolyarova, Sund-
berg & Lacerda, 1997). This special speech style under-
goes important age-related changes to accommodate the
communicative capacity of the developing mind (Amano,
Nakatani & Kondo, 2006; Fernald & Morikawa, 1993;
Kitamura, Thanavishuth, Burnham & Luksaneeyana-
win, 2001; Liu, Tsao & Kuhl, 2009).
The acoustic alterations of infant-directed speech
(IDS) purportedly serve vital social and linguistic func-
tions in early learning. Prosodic exaggeration is thought
to direct infantsattention, modulate arousal and affect
initially, and later fulfill more specific linguistic purposes
such as lexical segmentation (Cooper & Aslin, 1994;
Fernald, 1992). Formant exaggeration is phonetically
associated with hyperarticulation (Johnson, Flemming &
Wright, 1993), which may facilitate language learning by
making the critical acoustic distinctions more salient and
the phonetic categories more discriminable (Kuhl et al.,
1997). Supporting evidence indicates that vowel space in
maternal speech is positively correlated with infants
speech perception mothers who tended to stretch out
their vowels had better-performing babies in phonetic
discrimination (Liu, Kuhl & Tsao, 2003). Furthermore,
computer models have demonstrated robust unsuper-
vised learning of speech sounds based on IDS input (de
Boer & Kuhl, 2003; Kirchhoff & Schimmel, 2005; Val-
labha, McClelland, Pons, Werker & Amano, 2007).
However, little is known about the neurobiological
mechanisms that promote learning by exploiting the
physical properties of IDS.
Brain research studies offer new insights into
speech processing and language acquisition (Dehaene-
Lambertz & Gliga, 2004; Kuhl et al., 2008). Several
Addressfor correspondence: YangZhang, Department ofSpeech-Language-Hearing Sciences, 164 Pillsbury Dr. SE, University ofMinnesota, Minneapolis,
MN 55455, USA; e-mail: zhang470@umn.edu
2010 Blackwell Publishing Ltd, 9600 Garsington Road, Oxford OX4 2DQ, UK and 350 Main Street, Malden, MA 02148, USA.
Developmental Science 14:3 (2011), pp 566–581 DOI: 10.1111/j.1467-7687.2010.01004.x
neurophysiological indices have been shown to be
associated with IDS compared to adult-directed speech
(ADS), including increased frontal cerebral blood flow
(Saito, Aoyama, Kondo, Fukumoto, Konishi, Nakam-
ura, Kobayashi & Toshima, 2007), increased frontal
electroencephalography (EEG) power (Santesso,
Schmidt & Trainor, 2007), and enhanced event-related
potentials (ERPs) in the frontal-temporal-parietal
recording sites (Zangl & Mills, 2007). The IDS-induced
enhancement in neural activity may work jointly with
arousal, attention and affect to strengthen auditory
memory for phonological, syntactic and semantic cat-
egories. However, none of the previous infant studies
controlled the acoustic parameters to determine the
linguistic effects of formant exaggeration specific to
IDS independent of the prosodic affective effects pri-
marily drawn from fundamental frequency (f
0
) modi-
fications that are also found in pet-directed speech
(Burnham, Kitamura & Vollmer-Conna, 2002).
The present study utilized synthesized stimuli and
high-density EEG to investigate neural coding of vowel
formant exaggeration in infants. EEG records electrical
potential signals from electrodes placed on the scalp.
ERPs, which are derived from averaging EEG epochs
time-locked to stimulus presentation, provide a direct
noninvasive measure of postsynaptic activities with
millisecond resolution, suitable for studying the online
cortical dynamics of acoustic and linguistic processing
(Dehaene-Lambertz & Gliga, 2004; Ntnen & Win-
kler, 1999). High-density EEG, which records data from
64 or more electrodes, additionally allows for reliable
source estimation of high-quality ERP data (Izard,
Dehaene-Lambertz & Dehaene, 2008; Johnson, de
Haan, Oliver, Smith, Hatzakis, Tucker & Csibra, 2001;
Reynolds & Richards, 2005). Furthermore, advance-
ment in EEG time-frequency analysis has opened a new
venue for studying the event-related oscillations (EROs)
in infants (Csibra, Davis, Spratling & Johnson, 2000;
Csibra & Johnson, 2007). EROs reflect time-varying
neuronal excitability and discharge synchronization at
different rhythms subserving communications between
neuronal populations for various attentional, memory
and integrative functions (Klimesch, Freunberger,
Sauseng & Gruber, 2008). Studies have shown that delta
(1–4 Hz) and theta (4–8 Hz) activities, among other
EROs, are closely associated with linguistic processing
(Radicevic, Vujovic, Jelicic & Sovilj, 2008; Scheeringa,
Petersson, Oostenveld, Norris, Hagoort & Bastiaansen,
2009). It remains to be tested how formant-exaggerated
speech affects neural activation and synchronization in
the infant brain.
The experimental design followed a basic assumption
in auditory neuroscience – the average response for
repeated presentations of the same stimulus (or mul-
tiple instances of the same stimulus category) is
equivalent to the neural representation of the stimulus
(or the category), which codes its acoustic perceptual
features. An alternating block design with an equal
stimulus ratio was adopted for this purpose.
1
The
design took into account developmental changes in
infant ERPs. A number of studies have shown that
speech perception and auditory ERPs change dramat-
ically in the first year of life, and adult-like language-
specific perception occurs by 6 months of age (e.g.
Cheour, Ceponiene, Lehtokoski, Luuk, Allik, Alho &
Ntnen, 1998; Kuhl, Williams, Lacerda, Stevens &
Lindblom, 1992; Polka & Werker, 1994). The devel-
opmental changes in the latency, amplitude, polarity
and scalp distribution of ERP responses have led to a
better understanding of brain mechanisms that support
phonetic processing and language learning. There are
two salient auditory ERP components at this age,
P150, a positive peak at approximately 150 ms, and
N250, a negative peak at approximately 250 ms
(Dehaene-Lambertz & Dehaene, 1994; Fellman,
Kushnerenko, Mikkola, Ceponiene, Leipl & Ntnen,
2004; Kushnerenko, Ceponiene, Balan, Fellman, Huo-
tilaine & Ntnen, 2002; Novak, Kurtzberg, Kreuzer
& Vaughan, 1989; Rivera-Gaxiola, Silva-Pereyra,
Klarman, Garcia-Sierra, Lara-Ayala, Cadena-Salazar
& Kuhl, 2007; Zangl & Mills, 2007).
Although an exact neurocognitive model is not avail-
able to test how exaggerated speech affects neural pro-
cessing at the segmental level in infants, developmental
studies have provided important details about the neural
basis of speech perception early in life (Dehaene-
Lambertz & Gliga, 2004; Kuhl et al., 2008). Magneto-
encephalography (MEG) data show that phonetic
discrimination in infants at 6–12 months of age activates
the inferior frontal and superior temporal regions in the
left brain (Imada, Zhang, Cheour, Taulu, Ahonen &
Kuhl, 2006). Functional magnetic resonance imaging
(fMRI) data further reveal that activation for speech
stimuli in the Brocas area can be found even in 3-month-
old infants (Dehaene-Lambertz, Hertz-Pannier, Dubois,
Mriaux, Roche, Sigman & Dehaene, 2006). The
co-activation in Brocas and Werneckes areas is thought
to indicate perceptual-motor binding to promote speech
learning. Consistent with imaging results, ERP studies
suggest that both left and right auditory regions are
1
ERP research in speech perception has primarily used the mismatch
negativity (MMN) paradigm. The generation of MMN involves
repeated presentation of the same stimulus (or stimulus category),
which is occasionally replaced by another stimulus (or stimulus cate-
gory) (Ntnen, Paavilainen, Rinne & Alho, 2007). The MMN is a
powerful measure that reflects experience-dependent neural sensitivity
in detecting a sound change. A different paradigm, which requires less
recording time, presents stimuli in blocks with an equal occurrence ratio
to study neural coding of speech stimuli independent of discriminatory
sensitivity (e.g. Molfese & Molfese, 1980; Sharma, Marsh & Dorman,
2000; Tremblay, Friesen, Martin & Wright, 2003; Zangl & Mills, 2007).
The alternating block design combined important features of both
paradigms while keeping the stimulus presentation time relatively short
for the infant participants (Supplemental Figure 1). Unlike the MMN
experiment, the alternating block design shifted the focus to neural
coding of the standard stimuli alone.
Neural coding of exaggerated speech 567
2010 Blackwell Publishing Ltd.
sensitive to coding acoustic phonetic features of speech
stimuli with striking similarities between infants and
adults (Dehaene-Lambertz & Gliga, 2004). There exists
limited evidence for left-hemisphere dominance for
speech in infants, which may be attributable to a func-
tional asymmetry of the auditory system in processing
rapid acoustic transitions versus slow spectral changes
(Poeppel, 2003; Zatorre & Belin, 2001). For instance,
EEG and near-infrared spectroscopy data from newborn
infants show bilateral activation for speech-like acoustic
modulations and right-hemisphere dominance for
acoustic modulations at a much slower rate (Telkemeyer,
Rossi, Koch, Nierhaus, Steinbrink, Poeppel, Obrig &
Wartenburger, 2009). The vowel stimuli in the present
study did not contain rapid acoustic transition and
thus provided an opportunity to test functional asym-
metry for spectral processing in infants at 6–12 months
of age.
The general hypothesis was that formant exaggeration
would induce enhanced neural responses for speech
processing in the infant brain. Specifically, the present
study examined the effects of formant exaggeration in
two ERP components (P150 and N250), two ERO bands
(delta and theta), and three broad regions of interest
(frontal, temporal central, parietal) in both hemispheres.
There were four closely related questions. First, at what
time points, or in what ERP components, did the effect
occur? Second, was the hypothesized effect mediated by
differences in neural synchronization? Third, what cor-
tical regions were affected? Fourth, did the data support
early functional asymmetry for spectral processing of
formant exaggeration? Answers to these questions would
provide an initial account of the neural mechanisms
responsible for the facilitative role of formant exaggera-
tion in speech learning and acquisition.
Methods
Participants
Eighteen normally developing infants were recruited via
advertisement. Informed parental consent was obtained
in accordance with the procedures approved by the
institutional Human Research Protection Program. The
parents were paid $20. The infants were full term with
normal pregnancies and deliveries and no known audi-
tory or visual problems. Both parents were native English
speakers, and no immediate family members had a his-
tory of speech, language, or hearing deficits. Two infants
did not complete the entire recording session. Four
others were excluded due to excessive noise in their
EEG data. The remaining 12 infants (seven girls,
mean age = 9.2 months, range = 6.5–11.4 months) were
included in this report. Infants at this age range were
considered appropriate because their auditory N250
responses were known to be robust and sensitive to
phonetic learning.
Stimuli
The vowels were created with the HLsyn program (Sen-
simetrics Corporation, USA) (Figure 1). HLsyn allowed
quality synthesis with direct control of formants and
quasi-articulatory parameters based on the Klatt for-
mant synthesizer (Hanson & Stevens, 2002). The sounds
were 200 ms in duration (Supplementary materials,
Sounds 1 and 2). The F1 and F2 parameters were chosen
to simulate vowel space expansion based on previous
studies (Hillenbrand, Getty, Clark & Wheeler, 1995;
Kuhl et al., 1997). Only the male isounds were used for
this study.
2
Specifically, the center frequencies of F1 and
F2 were 342 Hz and 2322 Hz for the non-exaggerated
i. The exaggerated ihad F1 at 310 Hz and F2 at
2480 Hz. The f
0
, F3 and F4 were held constant at 138,
3000, and 3657 Hz for both sounds. Both vowels
included a rise fall time of 10 ms. They were resampled
at 44.1 kHz and normalized with equal average RMS
(root mean square) intensity.
Stimulus presentation
Infants sat on their parents lap in an acoustically and
electrically treated chamber (ETS-Lindgren Acoustic
Systems). Stimulus presentation used the EEvoke soft-
ware (ANT Inc., The Netherlands). The sounds were
played via a pair of loudspeakers (M-audio BX8a) placed
at approximately 1.2 m in front and 40 degrees to the
sides of the infant. The sound level was calibrated to be
65 dB SPL at the approximate location corresponding to
the center position of the subjects head. Alternating
blocks were presented for a total of 360 trials in 18
blocks, with each block consisting of 20 identical sounds.
Block order was counterbalanced among the subjects.
(a) (b)
Figure 1 (a) Simulated vowel triangle expansion in the F1-F2
space for the three point vowels, i,a, and u. Only the i
sounds were used in this study. (b) Spectral overlay plot for the
synthesized exaggerated and non-exaggerated istimuli.
2
The male voice was chosen in consideration of naturalness of the
synthesized vowel stimuli as judged by five adult native speakers of
English. The HLsyn software program is based on the Klatt formant
synthesizer (Klatt & Klatt, 1990), and male-based speech synthesis has
been used in previous infant speech perception studies (Kuhl, Tsao &
Liu, 2003; Liu et al., 2003). As pointed out by an anonymous reviewer,
female speech that typically contains greater acoustic exaggeration
could potentially produce a larger effect in infants than is reported in
the present study.
568 Yang Zhang et al.
2010 Blackwell Publishing Ltd.
The offset-to-onset interstimulus intervals were ran-
domized between 1.1 and 1.2 s, and the inter-block
silence period was 5 s. The short blocks (each less than 30 s)
and the relatively long randomized ISIs and inter-block
silence served to reduce habituation of the important
ERP components (Dehaene-Lambertz & Dehaene, 1994;
Woods & Elmasian, 1986).
EEG recording
Continuous EEG was recorded (bandwidth = 0.016–
200 Hz; sampling rate = 512 Hz) using the ASA-Lab
system with REFA-72 amplifier (TMS International BV)
and WaveGuard cap (ANT Inc., The Netherlands). Head
circumference was measured for each infant to determine
head size. The EEG cap used shielded wires for 65
sintered Ag AgCl electrodes in the international 10–20
montage system and the intermediate locations
(Figures 2, 3a, and 3b). The ground was positioned at
AFz, and the default reference for the REFA-72 ampli-
fier was the common average of all connected unipolar
electrode inputs. In the EEG cap, each electrode was
surrounded by a silicone ring to hold the conductive gel,
which allowed the electrodes to be prefilled with gel and
facilitated a smoother cap-fitting procedure. Adjust-
ments on individual electrodes were made to keep
impedances at or below 5 kX.
During the experiment, one or two research assistants
sat in front of the infant, silently playing with toys to
keep the infants attention. With parental permission,
a muted cartoon movie was also played on a 20-inch
LCD TV at 2.5 m away. The researcher in the control
room communicated with the assistant and parent via
intercom to coordinate the recording and initiate the
session when the infant sat relatively still. A surveillance
camera (Canon VB-C50iR) monitored the infants
behavior. The ASA-Lab automatically saved the online
video in synchrony with EEG recording for later
assessment of data quality and the infants alertness le-
vel. When necessary, sound presentation and EEG
recording would be paused until the infant sat relatively
still again. The entire EEG session, including prepara-
tion, lasted approximately 40 minutes.
ERP waveform analysis
ERP averaging was performed offline in BESA (Version
5.2, MEGIS Software GmbH, Germany) following rec-
ommended guidelines (DeBoer, Scott & Nelson, 2007;
Picton, Bentin, Berg, Donchin, Hillyard, Johnson, Mill-
er, Ritter, Ruchkin, Rugg & Taylor, 2000). The EEG data
were bandpassed at 0.5–40 Hz. The ERP epoch length
was 700 ms, including a pre-stimulus baseline of 100 ms.
The automatic artifact scanning tool in BESA was
applied in two steps to detect bad electrodes and noisy
signals. First, adjustments of rejection threshold
parameters were made on every subject to inspect the
effects on the entire recording session and the number of
accepted trials. Second, the automatic rejection criteria
were determined for all subjects. Epochs with a signal
Figure 2 Grand average ERP plot (linked-mastoid reference) for the two speech stimuli in all 64 electrodes.
Neural coding of exaggerated speech 569
2010 Blackwell Publishing Ltd.
level exceeding 150 lV from the segment baseline or a
slew rate exceeding 75 lVms were rejected. A subject
was excluded if the number of accepted trials for each
stimulus condition did not meet the minimum of 40. As
the focus of the study was on neural coding of the
standard stimuli, the first stimulus in each block was
excluded to avoid possible MMN elicitation from the
alternating blocks. The average number of accepted
trials was 58 for the exaggerated iand 55 for the
non-exaggerated i. Weighted averaging was calculated
for the grand average to minimize influences of individ-
ual subjects with fewer trials. A caution against weighted
averaging is that it might result in an undesirable bias
towards drowsy infants, who could have a large number
of uninformative epochs due to the decrease of ERP
amplitude. For the present study, online observation and
video recordings indicated that there were no alertness
problems for the infants (12 out of 18) included in grand
averaging.
To keep consistency with the majority of previous
infant ERP studies using speech stimuli (e.g. Ceponiene,
Haapanen, Ranta, Ntnen & Hukki, 2008; Fellman
et al., 2004; Rivera-Gaxiola et al., 2007; Zangl & Mills,
2007), the ERP data were re-referenced to linked
mastoids. Further analyses were performed in Matlab
(Version 7.6) with the EEGLAB software (Version
7.2.11) (Delorme & Makeig, 2004). To improve signal to
noise ratio, nine electrode regions were defined for
grouped averaging for each subject (Figure 3b): left
frontal (LF, including F7, F5, F3, FT7, FC5, and FC3),
middle frontal (MF, including F1, Fz, F2, FC1, FCz,
and FC2), right frontal (RF, including F8, F6, F4, FT8,
(a)
(d)
(b) (c)
Figure 3 (a) Photo illustration of an 11-month-old infant wearing the 64-channel EEG cap during a break in the recording session.
(b) Realistic head model with 64 standard electrode positions on. Nine grouped electrode sites were marked, representing frontal (F),
central (C), and parietal (P) regions in the left (L), middle (M), and right (R) divisions. (c) Grand average global field power data for the
exaggerated and non-exaggerated vowel stimuli. Significance for time-point-by-time-point comparison was shown by the black and
white bars on the x-axis. The black bars on the x-axis showed time windows of significant enhancement for the exaggerated sound
relative to the non-exaggerated sound, and the white bars showed time windows of significant reduction [p< .01]. (d) Grand average
ERP data (linked-mastoid reference) at the nine regionally grouped electrode sites for point-by-point comparison between the two
stimuli. The black bars on the x-axis showed time windows of significant negativity enhancement for the formant-exaggerated
speech [p< .01]. A cautionary note is needed for proper interpretation as the electrode site effects are not equivalent to those of
cortical regions in the brain.
570 Yang Zhang et al.
2010 Blackwell Publishing Ltd.
FC6, and FC4), left central (LC, including T7, C5, C3,
TP7, CP5, and CP3), middle central (MC, including C1,
Cz, C2, CP1, CPz, and CP2), right central (RC,
including T8, C6, C4, TP8, CP6, and CP4), left pos-
terior (LP, including P7, P5, P3, PO7, PO5, and PO3),
middle posterior (MP, including P1, Pz, P2, and POz),
and right posterior (RP, including P8, P6, P4, PO8,
PO6, and PO4) (Schneider, Debener, Oostenveld &
Engel, 2008). To derive an unbiased estimate indepen-
dent of electrode selection, global field power (GFP)
was calculated for comparison by computing the
standard deviation of the amplitude data across the
64 electrodes at each sampling point (Hamburger &
van der Burgt, 1991; Lehmann & Skrandies, 1980).
Repeated-measures ANOVA tests were performed on the
mean amplitudes of a 20 ms interval around peaks of
interest. The peak search windows (60–200 ms for P150
and 200–500 ms for N250) were confirmed by inspection
of the grand mean ERP overlay plots and topography.
Peak-to-peak (P150-N250) values were also calculated
for statistical comparison. The within-subject factors
were stimulus type (exaggerated and non-exaggerated),
hemisphere (left and right) and electrode region (frontal,
central, and parietal). Where appropriate, either Bon-
ferroni or Greenhouse-Geisser correction was applied to
the reported p-values.
There have been controversies regarding the use of
linked mastoids in ERP research (Picton et al., 2000).
The topographical voltage maps of linked-mastoid ref-
erence may be incorrect because the mastoids are not
neutral electrodes for auditory stimuli (Dehaene-
Lambertz & Gliga, 2004; Michel, Murray, Lantz,
Gonzalez, Spinelli & Grave de Peralta, 2004). To
examine the effects of reference selection, ERP wave-
form analysis using common average reference was also
performed. The common average reference method
assumes that the average of all recording electrodes on
a volume conductor is approximately neutral. However,
this assumption is valid only with accurate spatial
sampling of the scalp field, which requires a sufficient
number of electrodes with full coverage of the head
surface. The average of 64 electrodes with the standard
10-10 montage might be insufficient to qualify as a
truly neutral reference (Dien, 1998; Yao, Wang, Oost-
enveld, Nielsen, Arendt-Nielsen & Chen, 2005). To help
evaluate the differences between the two reference
methods, the ERP data were further transformed into
reference-free current source density (CSD) estimates
(Kayser & Tenke, 2006; Perrin, Pernier, Bertrand &
Echallier, 1989). In this approach, spherical spline
surface Laplacian transform was applied to identify
locations and relative magnitudes of current sources
and sinks. The CSD waveforms were submitted to
unrestricted Varimax-rotated temporal principal com-
ponents analysis (PCA) based on estimating the source-
current covariance matrix from the measured-data
covariance matrix (Kayser & Tenke, 2006). The CSD
estimates served to sharpen ERP scalp topography by
eliminating the volume-conducted contributions and
the dependence on reference.
Time-frequency analysis
Time-frequency representations (TFRs) were derived for
evoked EROs in the frontal (MF), central (MC) and
parietal (MP) electrode regions using continuous wavelet
transform (CWT) in Matlab (Csibra & Johnson, 2007;
Samar, Bopardikar, Rao & Swartz, 1999). CWT was
applied to the averaged ERP signals (referenced to linked
mastoids) with complex Morlet wavelets (band-
width = 1 Hz, center frequency = 0.5 Hz) in Matlab.
Morlet scalograms were plotted using the absolute values
of the squared coefficient of CWT, and the frequency
values on the plots were converted from the normalized
scale vector. The power data for delta (1–4 Hz) and theta
(4–8 Hz) bands at the MF, MC, and MP electrode sites
were subject to further analysis to examine the temporal
evolution of formant exaggeration.
Temporal evolution analysis
Two-tailed time-point-to-time-point t-tests were per-
formed to obtain accurate information about the tem-
poral evolution of significant differences between the
exaggerated and non-exaggerated stimuli (Guthrie &
Buchwald, 1991). Guthrie and Buchwalds method
requires three pieces of information to assess ERPs
temporal evolution at a chosen significance level:
(1) sample size (N, representing the number of subjects),
(2) number of sampling time points (T), and (3) the
autocorrelation value (ø). This parametric test method
relies on the computation of minimum number of con-
secutive sample points that need to show significant
differences, which depends on the autocorrelation esti-
mates and the total number of sample points in the ERP
data. The analysis was applied to the GFPs, ERPs, CSDs
and TFRs. For the present study, the data for an entire
epoch were decimated and assessed at two segments
(0–200 ms covering P150 and earlier components, and
200–600 ms covering N250 and later components) in
order to be consistent with Guthrie and Buchwalds
specific recommendations for the three parameters. For
N= 12, the ø values obtained from all IDS-versus-ADS
comparisons in the study were in the range of 0.38 to
0.74, corresponding to at least 5–9 consecutive sample
points at the significance level of 0.01. For a conservative
estimate, an interval would not be considered to differ
significantly in this study unless at least nine consecutive
sample points (approximately 18 ms) showed significant
differences at the level of 0.01.
Source localization analysis
Minimum norm estimation (MNE L2-norm) was applied
to the averaged ERP data (Hmlinen & Ilmoniemi,
1994; Izard et al., 2008). The MNE analysis approximated
Neural coding of exaggerated speech 571
2010 Blackwell Publishing Ltd.
the current source space using hundreds of prefixed,
discreet and distributed dipoles directly within the cortex
and searched for the optimal estimate with the smallest
norm to explain the measured ERP signals. Unlike CSDs
that are second-order spatial derivatives representing the
current sources and sinks on the scalp, MNEs are
modeled as true reference-free estimates of cortical cur-
rent activities. The implementation for the present study
included the following steps:
1. The infant electrode montage was calculated by scal-
ing the standard positions for the WaveGuard EEG
cap to fit the average head circumference of the 12
infant subjects.
2. A three-shell layer model was used to approximate the
infant head (Reynolds & Richards, 2005). To improve
the focality and reliability of the source activities, the
MNE procedure included both depth weighting and
spatio-temporal weighting (Dale & Sereno, 1993; Lin,
Witzel, Ahlfors, Stufflebeam, Belliveau & Hmli-
nen, 2006) to avoid bias towards superficial sources.
Noise regularization used the lowest 15% values, and
baseline noises were weighted by the average over the
64 electrodes. The entries in the main diagonal of the
noise covariance matrix were equally proportional to
the average noise power over all channels.
3. The total activity at each source location was com-
puted as the root mean square of the source activities
of its three components. The total activity solutions
were projected to the standard realistic brain model in
BESA. The current source data for 750 prefixed
locations (x, y, z coordinates covering the entire brain
space) at all latencies (358 sample points for the
)100–600 ms epoch) were further analyzed for
temporal and spatial interpretations.
4. In the temporal analysis, the total MNE activities in
each hemisphere were summed at each time point for
each stimulus. The MNE differences between the two
stimuli at each sample point were then subjected to
two-tailed z-test relative to the baseline mean and
variance. To examine regional contributions to the
total MNE activities, standard anatomical boundaries
in the Talairach space were used to define the spatial
masks for each region of interest (ROI) in the brain
space (Lancaster, Woldorff, Parsons, Liotti, Freitas,
Rainey, Kochunov, Nickerson, Mikiten & Fox, 2000).
The anatomical ROIs allowed a crude estimation for
frontal, temporal, and parietal activities in the two
hemispheres separately (Zhang, Kuhl, Imada, Kotani
& Tohkura, 2005).
5. In the spatial analysis, temporal integration was per-
formed. The MNE differences between the two stimuli
at each location and each time point were converted
to z-scores relative to the distribution of baseline
activities. The average z-scores were then calculated
for the two selected time windows (0–200 ms and
200–600 ms) at each source location and plotted using
Matlab visualization functions.
Results
ERP waveform results for linked-mastoid reference
The ERP data for the vowel stimuli showed clear audi-
tory N50, P150 and N250 responses and traceable
deflection for N450 in the sustaining negativity in the
frontal-central sites (Figures 2, 3). The P150-N250
complex was identified in all 12 infant subjects. A clear
N50 response was observed in 10 out of the 12 subjects.
Compared with the non-exaggerated i, the exaggerated
ielicited more negative ERP responses in both the early
time window (prior to 200 ms) dominated by P150 and
the late window (subsequent to 200 ms) dominated by
N250. Point-to-point t-test on the global field power
(GFP) showed significantly larger ERPs for the exag-
gerated sound in three components, N50 (44–62 ms),
N250 (228–356 ms), and sustaining negative activities
(392–431 ms) [p< .01] (Figure 3c). Significantly smaller
GFP values were found at 78–97 ms and 136–166 ms for
the exaggerated sound, indicating relatively more nega-
tive responses in the positive window.
Repeated-measures ANOVA results were obtained
from P150, N250, and peak-to-peak (P150-N250)
amplitude data on the LF, LC, LP, RF, RC, and RF sites.
For P150, there were significant main effects of stimulus
type [F(1, 11) = 5.28, p< .05] and region [F(2,
22) = 11.30, p< .01]. For N250, significant effects were
observed for stimulus type [F(1, 11) = 20.45, p< .001],
hemisphere [F(1, 11) = 7.66, p< .01] and region [F(2,
22) = 5.75, p< .01], and hemisphere-by-region inter-
action [F(2, 22) = 4.99, p< .05]. The P150 and N250
responses got progressively closer to baseline in the
frontal-central-parietal direction, and the right hemi-
sphere showed dominance in N250. In the peak-to-peak
(P150-N250) analysis, significant effects were found for
stimulus type [F(1, 11) = 29.97, p< .001] and region
[F(2, 22) = 24.38, p< .001], suggesting that the effects in
P150 and N250 were not due to an overall shift of the
P150-N250 complex.
Regional breakdown of the point-to-point compari-
sons between exaggerated and non-exaggerated is
showed detailed contributions from the frontal, central
and parietal recording sites at different time intervals
(Figures 3d and 6a). Significant effects were observed for
N50 (at RP), P150 (at MC, MP, and RC), and N250 (at
LF, MF, MC, MP, RF, RC, and RP) as well as for N450
(at LF, LC, MF, MC, MP, RF, RC and RP).
ERP waveform results for common average reference
The ERP data with common average reference showed
clear auditory N50, P150, N250 and N450 responses in
the frontal electrode sites (LF, MF and RF) (Figures 4
and 6b). Unlike the ERPs with linked-mastoid reference,
the parietal sites (LP, MP, RP) uniformly showed
polarity reversal to the frontal sites and the vertex,
572 Yang Zhang et al.
2010 Blackwell Publishing Ltd.
suggesting the existence of dipole current sources in the
left and right temporal cortices. The scalp potential dis-
tribution pattern created problematic averaging for the
LC and RC sites because the co-occurring sources and
sinks at the same sampling time point would cancel each
other out in these two electrode groups.
Repeated-measures ANOVA showed similarities and
differences between the two reference methods. For P150
amplitudes, there were significant effects for stimulus
type [F(1, 11) = 9.86, p< .01] and region [F(2,
22) = 13.49, p< .001]. For N250 amplitudes, there were
main effects of region [F(1, 11) = 32.98, p< .0001],
hemisphere [F(1, 11) = 5.28, p< .05], and stimulus-
by-region interaction [F(1, 11) = 17.25, p< .001]. As in
the linked-mastoid reference analysis, the right hemi-
sphere was dominant for N250.
Point-to-point comparisons between exaggerated i
and non-exaggerated ishowed enhanced N250 in MF,
RF, and MC. However, there was no such N250 effect in
LF. Contrary to the linked-mastoid reference, the sig-
nificant differences were more extensive in the parie-
tal electrodes than the frontal electrodes. The N250
enhancement showed up as increased positivity in LP
and RP sites due to polarity reversal. Significantly more
negative responses prior to N250 were observed in the
middle line frontal and central sites (MF and MC), and
this effect was observed with reversed polarity in left
temporal-parietal sites (LC and LP). Significant
enhancement in sustaining negativity after N250 was
found in LF, MF, MC and RF with polarity reversal in
LP, MP and RP sites. No significant differences were
found in any electrode sites for the N50 response.
CSD waveform results
The reference-free CSD estimates showed polarity
reversal in the frontal-parietal sites, which was similar to
ERP data with common average reference. The existence
of dipole current distribution in the bilateral temporal
regions produced problematic averaging for LC and RC
electrodes (Figures 5 and 6c). Unlike the ERP data, the
CSD estimates closely approximated localization of
cortical activities, and did not show strong P150 and
N250 responses in the midline electrodes.
Repeated-measures ANOVA results showed consistent
significant effects in P150 for stimulus type [F(1, 11) =
11.27, p< .01] and region [F(2, 22) = 9.76, p< .01].
There were main effects in N250 for region [F(1, 11) =
23.37, p< .0001], hemisphere [F(1, 11) = 5.28, p< .05],
and stimulus-by-region interaction [F(1, 11) = 27.51,
p< .001]. Point-to-point comparisons showed significant
contributions to P150 differences in MC, LP, and MP
regions, N250 differences in LF, RF, MC, RC, LP, and
RP regions, and sustaining negativity differences in MF,
RF, MC, LP and RP regions.
Topography and TFR results
The grand average ERP and CSD overlay plots and
topographical maps for the two vowels illustrated the
existence of N50, P150, N250 and N450 components
(Figures 6a, 6b, 6c). Despite the differences in the sta-
tistical comparisons among linked-mastoid reference,
common average reference and the reference-free CSDs,
the P150 amplitudes were consistently maximal at
Figure 4 Grand average ERP data (common average reference) at the nine regionally grouped electrode sites for point-by-point
comparison between the two stimuli.
Neural coding of exaggerated speech 573
2010 Blackwell Publishing Ltd.
bilateral frontal electrode sites, and the N250 was domi-
nant in temporal-parietal sites with functional asymmetry
in favor of the right hemisphere. The N250 enhancement
effect was clearly visible at temporal-parietal sites for
exaggerated irelative to the non-exaggerated i.
The dominant evoked EROs (linked-mastoid refer-
ence) were found in the delta (1–4 Hz) and theta
(4–8 Hz) bands corresponding to the P150-N250 com-
plex in time with linked-mastoid reference (Figure 6d).
Differences between the two stimuli were observed with
stronger ERO power in favor of the exaggerated sound.
The overall ERO power got progressively weaker in the
frontal-central-parietal direction. Strong delta and theta
activities were present at the frontal and central electrode
sites, and the parietal site predominantly showed delta
activity. In point-to-point comparisons, significantly
enhanced delta activity was found for the exaggerated i
sound at all three sites (154–425 ms at MF, 152–388 ms
at MC, and 152–502 ms at MP) [p< .01]. Differences in
theta band power were significant only in the frontal and
central sites (162–281 ms at MF, and 201–283 ms at MC)
[p< .01].
MNE results
The MNE plots showed activation in frontal, temporal,
and parietal regions for the two vowel stimuli in both
hemispheres (Figures 7a and 7b). Point-to-point com-
parisons showed significantly enhanced total cortical
activity (N250 and sustaining activity following N250)
for the exaggerated sound in both left (231–440 ms,
537–571 ms) and right hemispheres (190–482 ms,
500–600 ms) [p< .01]. As expected, temporal regions
were the primary contributor to the enhancement in
N250 and sustaining activities. There were also contri-
butions from frontal and parietal regions in the two
hemispheres. Unlike the waveform data, the total MNE
amplitude data showed no significant difference in P150
for either hemisphere. The lack of significance in total
activity for P150 could arise from regularization of cor-
tical source diffusion and depth weighting in the MNE
calculation. Significant regional P150 differences were
found bilaterally in the frontal MNE activities [p< .01].
Although the peak amplitude for N250 appeared to show
right-hemisphere dominance, the mean MNE differences
(total MNE activity between the two stimuli in the
window of 270–450 ms) were slightly larger in the left
hemisphere than in the right.
The significant MNE differences for the early window
(0–200 ms) were localized primarily in the inferior fron-
tal area in both hemispheres. In the late time window
(200–600 ms), the significant differences were found in
the left inferior frontal cortex as well as right anterior
temporal cortex extending posteriorly to superior tem-
poral, inferior frontal and middle frontal regions
(Figure 7c).
Discussion
Speech scientists have long stressed the importance of
formant exaggeration in infant-directed speech for pho-
netic learning (Burnham et al., 2002; Kuhl et al., 1997).
The ERP waveforms (including CSD waveforms), TFRs,
Figure 5 Grand average CSDs (current source density) at the nine regionally grouped electrode sites for point-by-point comparison
between the two stimuli.
574 Yang Zhang et al.
2010 Blackwell Publishing Ltd.
and MNE data here provided three lines of evidence in
support of this view. Despite striking differences in ERP
waveforms due to reference choice, significant enhance-
ment in N250 and sustaining activity following N250 for
exaggerated speech was confirmed in all the analyses.
The reduced P150 effect, on the other hand, was not
consistently found. Unlike N250, the early P150 com-
ponent presumably reflected acoustic mapping of the
spectral differences between the stimuli (Rivera-Gaxiola
et al., 2007). This functional distinction was partly sup-
ported by the scalp distribution of the components in all
three topographical calculations using linked-mastoid
reference, average reference, and the reference-free CSD
approach. The P150 was dominant in the frontal sites,
and the N250 extended posteriorly from frontal to tem-
poral-parietal electrode sites. The timing and scalp dis-
tribution of the enhanced negativity in the 200–600 ms
window were consistent with the notion that the N250
and sustaining negative responses are linked with pho-
netic and lexical processing in infants at 6 months of age
or older (Mills, Prat, Zangl, Stager, Neville & Werker,
2004; Rivera-Gaxiola et al., 2007; Zangl & Mills, 2007).
An alternative interpretation is that the P150 and N250
responses do not necessarily serve the strict bifurcation
of auditory vs. linguistic processing. Rather, these two
components co-occur and behave similarly in many
experimental situations, and may thus reflect connected
processes. In line with both of these interpretations, a
missing or diminished N250 was found to be associated
with lower level of cognitive and linguistic development
and diverted central auditory processing (Ceponiene
et al., 2002; Fellman et al., 2004; Tonnquist-Uhlen,
1996).
Differential patterns of neural activity for IDS and
ADS have been reported in previous infant studies (Saito
et al., 2007; Santesso et al., 2007; Zangl & Mills, 2007).
Saito and colleagues employed near-infrared spectro-
scopy in examining neonatesresponses to naturally
spoken sentences in the two speech styles. They found
that IDS increased frontal activation in neonates, which
was mainly attributable to the prosodic exaggeration of
IDS and its socio-affective impact. Santesso et al.
showed that in 9-month-old infants, the overall frontal
activation in terms of EEG power was linearly related to
affective intensity of natural sentences spoken in IDS.
Zangl and Mills compared ERPs for words spoken in
IDS and ADS in 6- and 13-month-old infants and found
larger N600–800 responses to IDS than to ADS in both
(a) (b)
(c) (d)
Figure 6 (a) Grand average ERP overlay plots of 64 electrodes (linked-mastoid reference) and topography maps of N50, P150, N250
and N450 responses for the two vowel stimuli. (b) Grand average ERP overlay plots (common average reference) and topography maps
for the two stimuli. (c) Grand average CSD overlay plots and topography maps for the two stimuli. (d) Grand average Morlet scalograms
at the MF (frontal), MC (central), and MP (parietal) sites for the exaggerated and non-exaggerated isounds in the study.
Neural coding of exaggerated speech 575
2010 Blackwell Publishing Ltd.
age groups. In the older infants, familiar words addi-
tionally showed enlarged N200–400 response to IDS.
Given that the IDS stimuli in the previous study were
significantly longer in duration and higher in funda-
mental frequency, maximum pitch, and frequency range
than ADS stimuli (Zangl & Mills, 2007), the increased
brain activity for IDS would presumably reflect a com-
posite effect of both prosodic and linguistic exaggera-
tions. By controlling acoustic exaggeration other than
formants in IDS and ADS, the new ERP data here
demonstrated that formant exaggeration alone at the
segmental level could produce significant enhancement
in neural activation in 6–12-month-old infants, which
may serve to strengthen associations between phonetic
processing and word learning (Swingley, 2009).
The mechanism for the observed enhancement in
N250 and sustaining negativity appears to rely on neural
synchronization of evoked EROs time-locked and phase-
locked to stimulus presentation. In the literature, the
adult theta activity has been linked with arousal orient-
ing responses and working memory of verbal stimuli
(Basar, Basar-Eroglu, Karakas & Schrmann, 1999;
Hwang, Jacobs, Geller, Danker, Sekuler & Kahana,
2005; Klimesch, Hanslmayr, Sauseng, Gruber, Brozinsky,
Kroll, Yonelinas & Doppelmayr, 2006; Scheeringa et al.,
2009; Summerfield & Mangels, 2005). In infants, delta
(1–4 Hz) and theta (4–8 Hz) activities are both affected
by linguistic processing with increased theta power for
affective speech (Orekhova, Stroganova, Posikera &
Elam, 2006; Radicevic et al., 2008; Santesso et al., 2007).
As the pitch level was controlled in the present study, the
observed increases in delta activity at frontal-central-
parietal sites, as well as in theta activity at frontal-central
sites, could not be due to prosodic processing. Rather, it
could be a composite effect of attentional and phonetic
encoding processes in response to the acoustically more
salient and phonetically more distinct speech (Kuhl et al.,
1997). As attention was not controlled in the present
study, it remains to be tested whether formant exagger-
ation alone makes speech more attractive to infant lis-
teners.
The MNE differences between the stimuli revealed a
bilateral cortical neural network sensitive to formant
exaggeration, including the Brocas area in the left brain
and frontal-temporal-parietal regions in the right. Bro-
cas activation for speech processing has been reported in
imaging studies of infants at 3–12 months of age
(Dehaene-Lambertz et al., 2006; Imada et al., 2006),
suggesting the existence of early perceptual-motor
binding in support of language acquisition. The present
(a)
(b) (c)
Figure 7 (a) Temporal evolution of total MNE activity and regional (frontal, temporal and parietal) MNE activities for the two stimuli
in the two hemispheres. The black bars on the x-axis show time windows of significant MNE enhancement for the exaggerated
speech relative to baseline current activities [p< .01]. (b) Spatial localization in top, left and right views of the MNE estimates (in
nAm) for the two vowel stimuli. The MNE activities were integrated over 0–600 ms at each prefixed spatial location. (c) Spatial
localization in top, left and right views of the MNE differences between the two vowel stimuli. The MNE differences were converted
into z-scores relative to baseline and integrated over two time windows (0–200 ms and 200–600 ms).
576 Yang Zhang et al.
2010 Blackwell Publishing Ltd.
MNE data further indicate that formant-exaggerated
speech leads to enhanced Brocas activation, which may
drive speech learning via interactions with the percep-
tual-motor system involving temporal, frontal, and
parietal cortices in both hemispheres. It is interesting to
note that the infant MNE activation patterns for passive
listening to speech show striking resemblance to adult
fMRI data during passive listening to music (Lahav,
Saltzman & Schlaug, 2007). Adult imaging research has
also shown that auditory listening alone can recruit
production-related regions including Brocas area (Love,
Haist, Nicol & Swinney, 2006; Meyer, Steinhauer, Alter,
Friederici & von Cramon, 2004; Skipper, Nusbaum &
Small, 2005; Wilson, Saygin, Sereno & Iacoboni, 2004).
The adult data were thought to reflect more general
mnemonic and integrative functions for the Brocas area
in making associations between motor actions for sound
generation (not just speech) and the acoustic product.
However, passive listening to nonsense syllables does not
reliably elicit inferior frontal activation in adults (Zhang,
Kuhl, Imada, Iverson, Pruitt, Stevens, Kawakatsu,
Tohkura & Nemoto, 2009; Zhang et al., 2005). As no motor
component of speech is measured for comparison in the
present design, it remains purely speculative that passive
listening to speech might elicit motor activities in the
developing minds to mediate phonological acquisition.
The ERP, CSD and MNE data all indicated greater
involvement of the right hemisphere for the N250 effect
than the left. This result was consistent with a recent
study that showed early functional asymmetry of spectral
processing in newborns (Telkemeyer et al., 2009). There
is a growing literature relating the right hemisphere with
speech processing at the prelexical and paralinguistic
levels (e.g. Bristow, Dehaene-Lambertz, Mattout, Soares,
Gliga, Baillet & Mangin, 2009; Homae, Watanabe,
Nakano, Asakawa & Taga, 2006; Scott & Wise, 2004;
Simos, Molfese & Brenden, 1997). However, the lateral-
ity result directly contradicted previous findings about
left-hemisphere dominance in significantly enhanced
N200–400 and N600–800 responses for familiar words
spoken in IDS relative to ADS (Zangl & Mills, 2007).
The laterality inconsistency can be explained by the
functional asymmetry model – spoken words involve
fine-scale temporal processing of the rapid acoustic
transitions in the left brain, and processing steady spec-
tral cues in simple vowel stimuli primarily depends on the
right brain (Poeppel, 2003; Zatorre & Belin, 2001).
Nevertheless, this model did not specify the time course
of functional asymmetry or the time course of interac-
tions of cortical regions in auditory processing. A simple
extrapolation would predict the same pattern of func-
tional asymmetry regardless of the time course of brain
activities, which was not supported by the current results.
Further research is necessary to determine left right
functional asymmetries at different cortical regions and
in different time windows and how asymmetry in brain
activation varies as a function of stimulus properties,
task variables, and subject characteristics.
The reference-dependent and reference-free approaches
in the present study showed similarities as well as striking
differences. The ERP research field has yet to adopt one
standard solution regarding the choice of reference.
Caution must be used in interpreting ERP results with
different reference methods (Dien, 1998; Yao et al.,
2005). The topographical map for common average ref-
erence was similar to the CSD map in terms of the
polarity reversal pattern. As all electrical activity pro-
duces dipolar fields, measurements from the two sides of
the dipolar activity will always be negatively correlated.
It is noteworthy that polarity reversal in the temporal-
parietal electrodes relative to frontal electrodes could
potentially cause problems in channel grouping and
interpretation. Compared with common average refer-
ence, the linked-mastoid reference appears to produce
biophysically unrealistic unipolar voltage fields. Although
linked mastoid reference was quite popular in the past, it
is recommended that researchers should switch to more
progressive approaches by adopting the common average
reference in future studies. While the CSD and MNE
solutions have the advantages of being reference-free,
these methods are highly susceptible to noise influence
and thus technically challenging to implement when
analyzing individual subjectsdata, especially those of
infants where there tends to be more noise.
As children learn to speak only the language(s) that
they are exposed to, defining the role of input and the
neurobiological mechanisms enabling this feat is central
to our understanding of the perceptual and computa-
tional processes that adaptively shape both the develop-
ing brain and the language outcome. There is cumulative
evidence that the acoustic and linguistic modifications in
IDS have important functions in the acquisition of
phonology and grammar (Burnham et al., 2002; Liu
et al., 2003; Morgan & Demuth, 1996; Werker, Pons,
Dietrich, Kajikawa, Fais & Amano, 2007). The present
results add a neural-level account of how formant
exaggeration in speech alters infantsbrain activities for
phonetic processing. This account is not without its
limitations in explaining the role of formant exaggeration
in language acquisition. Research has shown that not all
aspects of acoustic exaggeration in IDS necessarily aid
speech discriminability or learning (Trainor & Desjar-
dins, 2002). As the distributional and statistical proper-
ties of language input are embedded within an interactive
social learning environment (Meltzoff, Kuhl, Movellan &
Sejnowski, 2009), it seems unlikely that any single
property of IDS is indispensable to normal language
development.
Of particular interest to theory and practice is that the
effects of enriched language exposure, including formant
exaggeration, are not limited to infancy. IDS-based input
manipulation is conceptualized to be an agent of neural
plasticity regardless of age or experience (Zhang et al.,
2009). The benefits of various input manipulations have
been demonstrated in infants, children, and adults with
or without learning disabilities (Bradlow, Kraus &
Neural coding of exaggerated speech 577
2010 Blackwell Publishing Ltd.
Hayes, 2003; Kuhl et al., 2003; Tallal, 2004; Zhang et al.,
2009). Given that early brain measures have predictive
power for later language skills (Kuhl et al., 2008; Molf-
ese, 1989), more developmental studies are needed to
delineate the role of language input in the social context
of language acquisition or effective intervention. In
particular, an experimental design focusing on the dif-
ferent spectral and temporal aspects of IDS is necessary
to build a better understanding of cortical speech pro-
cessing and functional asymmetry. Both speech stimuli
and nonspeech control can be applied to further inves-
tigate the effects of acoustic versus phonetic processing
in populations of specific ages and neurological condi-
tions (Dehaene-Lambertz & Gliga, 2004).
In summary, the present study examined the effects of
formant exaggeration on cortical speech processing in
infants at 6–12 months of age. Despite methodological
differences, there was significant enhancement in N250
with right-hemisphere dominance in all reference-
dependent and reference-free analysis approaches.
Time-frequency analysis indicated increased neural syn-
chronization for processing formant-exaggerated vowel
stimuli in the delta band at frontal-central-parietal elec-
trode sites as well as in the theta band at frontal-central
sites. Minimum norm estimates further revealed a bilat-
eral cortical neural network (frontal, temporal and
parietal regions) in the infant brain sensitive to formant
exaggeration, which may facilitate learning via cortical
interactions in the perceptual-motor systems. Although
there was limited support for the early functional
asymmetry for spectral processing of formant exaggera-
tion in the right hemisphere, hemispheric laterality may
vary depending on the time course of neural activation.
Acknowledgements
This study was supported by funding sources to YZ,
including two Brain Imaging Research Awards from the
College of Liberal Arts (CLA) and the Grant-in-aid
Program at the University of Minnesota. The first author
would like to thank three anonymous reviewers for
revision suggestions, Drs Matti Hmlainen, Iku Nem-
oto, and Masaki Kawakatsu for technical discussions,
and CLA Associate Deans, Jo-Ida C. Hansen and Jen-
nifer Windsor, for support.
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Received: 29 September 2009
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Supporting Information
Additional Supporting Information may be found in the online
version of this article:
Supplementary Figure 1 Schematic illustration of two stimulus
presentation paradigms. (a) In the alternating block design, each
block contains 20 stimuli of one category (A) followed by a block
that contains stimuli of a different category (B). The blocks are
sequentially alternated to collect sufficient trials for both stimuli.
(b) In the bidirectional MMN paradigm, two long blocks are
presented to represent change directions (i.e. from A to B or B to
A). The MMN response to stimulus change is derived by sub-
tracting the brain responses to the same stimuli to control for
inherent acoustic differences between A and B (i.e. deviant A
minus standard A and deviant B minus standard B). In theory, the
bidirectional MMN paradigm allows the study of neural repre-
sentations of the auditory stimuli as well as sensitivity of the
neural system to change detection (Zhang et al., 2005). However,
the time length that is required to collect sufficient trials in the
bidirectional MMN design would exceed the limit for most infant
participants at 6–12 months of age.
Supplementary Sounds 1 and 2 The sound1.wav file was the
non-exaggerated i. The sound2.wav file was the formant-
exaggerated i. A preliminary survey on 15 adult native English
speakers (all were nave listeners who had not been previously
tested on synthetic speech in laboratory settings) indicated that
both sounds were heard as is. Twelve listeners reported the i
with expanded formants to be clearer or perceptually more
exaggerated than the other isound. The other three did not give
unequivocal judgment about which sound was perceptually
clearer or more exaggerated, which was probably due to unfa-
miliarity with synthetic speech.
Supplementary Movies 1 and 2 The MNE (minimum norm
estimate) movie files were generated in BESA software and
converted to compressed format. The movie1.wmv file showed
left-hemisphere MNE activities for the exaggerated i, which
were scaled to the maximum MNE peak in percentages and color-
coded. The movie2.wmv file showed right-hemisphere MNE
activities for the exaggerated iusing the peak-scaled color
coding scheme. A cautionary note is necessary here. As the peak
MMN values for different stimuli were not the same, a direct
comparison of the color-coded brain activation patterns could
lead to misinterpretation.
Please note: Wiley-Blackwell are not responsible for the
content or functionality of any supporting materials supplied
by the authors. Any queries (other than missing material)
should be directed to the corresponding author for the
article.
Neural coding of exaggerated speech 581
2010 Blackwell Publishing Ltd.
... In parallel and quite possibly due to these functions, infants with NH prefer to listen to IDS over ADS in their native (Cooper & Aslin, 1990;Fernald, 1985) and even in a foreign language (Fernald & Morikawa, 1993;ManyBabies Consortium, 2020;Werker et al., 1994). This preference may well focus their attention (Cooper & Aslin, 1990;Fernald & Simon, 1984) and facilitate processing of early linguistic input (Kalashnikova, Peter, Di Liberto, Lalor, & Burnham, 2018a;Peter, Kalashnikova, Santos, & Burnham, 2016;Zangl & Mills, 2007;Zhang et al., 2011). It is of particular interest here to determine whether this is also the case in infants with HL. ...
... For example, IDS vowels presented to nine-month-old infants result in a mature mismatch negativity response (MMN), whereas ADS vowels do not (Peter et al., 2016). Additionally, in six-to 12-month-olds input of vowels that are hyperarticulated versus those that are not result in increased neural synchronisation, which is important for phonetic encoding (Zhang et al., 2011). Moreover, the presence versus absence of vowel hyperarticulation in speech facilitates word recognition in 19-month-old infants (Song, Demuth, & Morgan, 2010). ...
... Such studies with NH infants have become increasingly possible in recent years through the use of neurophysiological methods (Männel & Friederici, 2010, 2013Peter et al., 2016;Saito et al., 2007;Santesso, Schmidt, & Trainor, 2007;Zangl & Mills, 2007;Zhang et al., 2011); methods that are now possible with infants with HL due to the development of optical imaging technologies (e.g., Functional Near Infrared Spectroscopy, Bortfeld, 2019), that are safe for use with infants and are not subject to electrical or magnetic interference from hearing devices. Such future research is essential for the understanding of early language experiences of infants with HL and for the optimization of the hardware and software for successful early language development. ...
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The majority of infants with permanent congenital hearing loss fall significantly behind their normal hearing peers in the development of receptive and expressive oral communication skills. Independent of any prosthetic intervention (“hardware”) for infants with hearing loss, the social and linguistic environment (“software”) can still be optimal or sub-optimal and so can exert significant positive or negative effects on speech and language acquisition, with far-reaching beneficial or adverse effects, respectively. This review focusses on the nature of the social and linguistic environment of infants with hearing loss, in particular others’ speech to infants. The nature of this “infant-directed speech” and its effects on language development has been studied extensively in hearing infants but far less comprehensively in infants with hearing loss. Here, literature on the nature of infant-directed speech and its impact on the speech perception and language acquisition in infants with hearing loss is reviewed. The review brings together evidence on the little-studied effects of infant-directed speech on speech and language development in infants with hearing loss, and provides suggestions, over and above early screening and external treatment, for a natural intervention at the level of the carer-infant microcosm that may well optimize the early linguistic experiences and mitigate later adverse effects for infants born with hearing loss.
... Frontal theta band modulations are also associated with memory processes (Begus et al., 2015;Jensen & Tesche, 2002) and are thought to reflect infants' learning of new information (Begus & Bonawitz, 2020). For instance, findings on neural processing of infantdirected speech provide evidence for an increase in infants' frontal theta power when listening to infant-directed speech compared to control conditions (Orekhova et al., 2006;Zhang et al., 2011). Thus, frontal theta band power is a promising neural measure to assess infants attentional processing during IDAs. ...
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... Implicit is that the infant's seemingly 'automatic' neural response to motherese mediates this development and learning. Typically developing (TD) infants attend to and prefer motherese over other forms of adult speech 3,[7][8][9][10] , and a small number of behavioural and neuroimaging studies suggest that TD infants may process motherese differently from non-speech sounds [11][12][13][14][15] . However, if such attention enhances social and language learning, then it would be predicted that enhanced or reduced neural responsiveness to such affective speech might be associated with enhanced or reduced early-age social and language ability. ...
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Affective speech, including motherese, captures an infant’s attention and enhances social, language and emotional development. Decreased behavioural response to affective speech and reduced caregiver–child interactions are early signs of autism in infants. To understand this, we measured neural responses to mild affect speech, moderate affect speech and motherese using natural sleep functional magnetic resonance imaging and behavioural preference for motherese using eye tracking in typically developing toddlers and those with autism. By combining diverse neural–clinical data using similarity network fusion, we discovered four distinct clusters of toddlers. The autism cluster with the weakest superior temporal responses to affective speech and very poor social and language abilities had reduced behavioural preference for motherese, while the typically developing cluster with the strongest superior temporal response to affective speech showed the opposite effect. We conclude that significantly reduced behavioural preference for motherese in autism is related to impaired development of temporal cortical systems that normally respond to parental affective speech.
... For each subject, GFP values of each condition were translated into z-scores with the pre-stimulus 200 ms as baseline. Significant differences (p < 0.05) in z-scores between conditions that persisted for at least 20 ms in either group were highlighted (Rao et al., 2010;Zhang et al., 2011). Raw data and Material are available in OSF 1 . ...
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The aim of the present study was to investigate how Chinese-Malay bilingual speakers with Chinese as heritage language process semantic congruency effects in Chinese and how their brain activities compare to those of monolingual Chinese speakers using electroencephalography (EEG) recordings. To this end, semantic congruencies were manipulated in Chinese classifier-noun phrases, resulting in four conditions: (i) a strongly constraining/high-cloze, plausible (SP) condition, (ii) a weakly constraining/low-cloze, plausible (WP) condition, (iii) a strongly constraining/implausible (SI) condition, and (iv) a weakly constraining/implausible (WI) condition. The analysis of EEG data focused on two event-related potential components, i.e., the N400, which is known for its sensitivity to semantic fit of a target word to its context, and a post-N400 late positive complex (LPC), which is linked to semantic integration after prediction violations and retrospective, evaluative processes. We found similar N400/LPC effects in response to the manipulations of semantic congruency in the mono- and bilingual groups, with a gradient N400 pattern (WI/SI > WP > SP), a larger frontal LPC in response to WP compared to SP, SI, and WI, as well as larger centro-parietal LPCs in response to WP compared to SI and WI, and a larger centro-parietal LPC for SP compared to SI. These results suggest that, in terms of event-related potential (ERP) data, Chinese-Malay early bilingual speakers predict and integrate upcoming semantic information in Chinese classifier-noun phrase to the same extent as monolingual Chinese speakers. However, the global field power (GFP) data showed significant differences between SP and WP in the N400 and LPC time windows in bilinguals, whereas no such effects were observed in monolinguals. This finding was interpreted as showing that bilinguals differ from their monolingual peers in terms of global field power intensity of the brain by processing plausible classifier-noun pairs with different congruency effects.
... Green et al., 2010), which shortens their vocal tract length and shifts their formant frequencies upward and further apart. In addition, neurophysiological studies indicate that this formant exaggeration facilitates the processing of speech by infants (Zhang et al., 2010). Although some have speculated that these modifications in the execution of IDS may reflect, at least in part, sociophonetic convergence between caregivers and their babies (Kalashnikova et al., 2017), adult caregivers clearly cannot shorten their vocal tract enough to match the vocal tract length of their infant. ...
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... One of the characteristics of motherese is the use of short clauses separated by pauses, and it has consequently more pauses than the adult-directed speech; therefore, it can help segmentation. It has been verified that 7,5-month-olds have a facility for recognizing words from the segmentation of the utterance since these words are located at the beginning or at the end of the sentence next to its pause 28 . ...
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