2013. Vol.4, No.6A2, 14-18
Published Online June 2013 in SciRes (
Copyright © 2013 SciRes.
Genetic and Environmental Etiology of Infant Hemodynamic
Response to Speech Stimuli:
A Near-Infrared Spectroscopy Study of Twins
Kunitake Suzuki1, Juko Ando2
1Faculty of Human Sciences, Osaka University of Human Sciences, Osaka, Japan
2Faculty of Letters, Keio University, Tokyo, Japan
Received April 14th, 2013; revised May 16th, 2013; accepted June 13th, 2013
Copyright © 2013 Kunitake Suzuki, Juko Ando. This is an open access article distributed under the Creative
Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium,
provided the original work is properly cited.
For adults and children, genetic and environmental factors are known to affect brain structure and neural
activity necessary for conducting various cognitive tasks. However, little is known regarding genetic and
environmental contributions to individual differences in neural activity during the first two years of life.
Concentrations of oxygenated and deoxygenated hemoglobin were measured bilaterally over temporal
areas of 7 monozygotic and 17 dizygotic twin pairs using near-infrared spectroscopy. Results showed that
environmental influences on the concentration of hemoglobin were larger than for genetic influences.
Significant genetic and environmental influences were detected in different temporal areas. We discuss
the genetic and environmental influences on the hemodynamic response to speech stimuli during the first
two years of life.
Keywords: Environmental Factors; Genetic Factors; Infants; Near-Infrared Spectroscopy; Speech Stimuli;
Cerebral dominance for speech has been a popular research
topic, with left cerebral dominance observed within the major-
ity of speech-processing studies (Jansen et al., 2007; Knecht et
al., 2000). However, various types of cerebral dominance for
speech among infants have been observed. Left (e.g., Bortfeld,
Fava, & Boss, 2009; Kotilahti et al., 2010), right (e.g., Dehaene-
Lambertz et al., 2006; Homae, Watanabe, Nakano, & Taga,
2007), and no cerebral dominance (Novak, Kurtzberg, Kreuzer,
& Vaughan Jr., 1989) have all been reported. Although these
inconsistencies may result from the varied characteristics of
speech stimuli (Minagawa-Kawai, Cristià, & Dupoux, 2011),
discrepant cerebral dominance reported in previous studies sug-
gest larger individual differences among infant neural responses
to speech as compared to adults. For example, individual dif-
ferences during the first two years of life are related to the
speed of bilateral maturation within temporal brain regions (Chi-
ron et al., 1997), adaptation of neural systems to environmental
speech sounds (Cheour et al., 1998), and second language ex-
periences (Conboy & Kuhl, 2011). Specific factors associated
with individual differences in neural responses to speech may
have contributed to the varying results observed when testing
for cerebral dominance among infants. By accounting for these
differences, variance in neural responses to speech within the
two hemispheres can be validly determined. In the current study,
we decomposed individual differences into genetic and envi-
ronmental factors as a way to provide new evidence regarding
bilateral temporal responses to speech among infants.
Results related to the genetic and environmental influences
on neural responses to speech have not been assessed among
infants. However, these influences can be indirectly inferred
from known genetic and environmental influences on brain
structure and language ability. While brain structure is affected
by genetic factors more so than environmental factors in both
children and adults (e.g., Gilmore et al., 2010; Lenroot et al.,
2009), children’s language ability is thought to be largely asso-
ciated with environmental rather than genetic variations (e.g.,
Alarcón, Plomin, Fulker, Corley, & DeFries, 1998; Van Hulle,
Goldsmith, & Lemery, 2004). Nevertheless, these studies have
suggested that both genetic and environmental factors may
influence individual differences in infant neural responses to
We adopted a design using data from monozygotic (MZ) and
dizygotic (DZ) twins (twin design; Neale & Maes, 2004). Esti-
mating the degree to which individual differences are influ-
enced by additive genetic factors, shared environmental factors,
and unique environmental factors are possible with this type of
design. Additive genetic, shared, and unique environmental
factors independently contribute to twin phenotypes. The twin
design follows the basic logic that while MZ twins share all
their genes, and DZ twins, on average, only share half their
genes, the degree to which twins share environments (e.g. fam-
ily environment) is the same regardless of zygosity. If individ-
ual differences in the phenotype are based on additive genetic
factors, we would expect that the phenotype similarity between
MZ twin pairs to be twice as large as between DZ twins. Alter-
natively, if shared environmental factors are more critical, we
would expect MZ and DZ twin pairs to be equally similar. If
unique environmental factors are acting to make co-twins less
alike, we would expect phenotypes of MZ pairs to be dissimilar.
Measurement errors that randomly affect phenotypes of each
twin can cause false dissimilarity between twins, leading to
erroneous conclusions regarding unique environmental factors.
Here, we used near-infrared spectroscopy (NIRS; Maki et al.,
1995) to acquire hemodynamic responses (oxygenated hemo-
globin [oxy-Hb] and deoxygenated hemoglobin [deoxy-Hb]) to
speech stimuli within bilateral temporal brain regions because
this method is convenient and reliable. In the NIRS technique,
continuous near-infrared lasers are used, so that the technique is
non-invasive and requires relatively few physical constraints.
Moreover, the spatial resolution of the imaging data is rela-
tively high compared to that of an electroencephalogram. The
NIRS technique is particularly appropriate for infant research.
In this study, we estimated the degree that genetic and envi-
ronmental factors influence the concentration of not only oxy-
Hb but also deoxy-Hb within each bilateral temporal region.
Although the concentration of deoxy-Hb has not always been
analyzed in previous studies, changes in the concentration of
deoxy-Hb as opposed to oxy-Hb may indicate neural activation
required for conduction tasks (Kato, 2004). Furthermore, how
hemoglobin concentrations change among infants is still rela-
tively unknown. In general, decreases in deoxy-Hb concentra-
tion are often followed by increases in oxy-Hb. However, other
patterns of deoxy-/oxy-Hb concentrations have been reported
(Sato et al., 2007); thus, infant oxy- and deoxy-Hb concentra-
tions may contribute to different aspects of neural activation
required for a given task.
Subjects were twin infants recruited via the Tokyo Twin
Cohort Project (Ando et al., 2006). All twins were raised to-
gether by their biological parents, within their family home, in
the Tokyo area. Infants who demonstrated high levels of head
motion and crying during the experiment were removed from
the study (see data analysis section). To compare results ac-
cording to zygosity, we discarded any data that were not ob-
tained from both twin infants. With these parameters, data from
48 of 134 twins were available for this study. The zygosity of
the remaining 24 twin pairs was assessed via a standard ques-
tionnaire (Ooki & Asaka, 2004). This questionnaire revealed
that the sample included 7 MZ twin pairs (4 male-male and 3
female-female pairs) and 17 DZ twin pairs (5 male-male, 5
female-female, and 7 opposite sex pairs). The mean ages of the
MZ, same-sex DZ, and opposite-sex DZ twin infants were
12.16 months (SD = 5.06 months; range = 6.13 - 17.93 months),
11.43 months (SD = 4.29 months; range = 6.00 - 16.97 months),
and 11.9 months (SD = 4.89 months; range = 5.77 - 17.83
months), respectively. The mean gestational ages of the MZ,
same-sex DZ, and opposite-sex DZ twin infants were 35.71
weeks (SD = 1.32 weeks; range = 33 - 37 weeks), 36.20 weeks
(SD = .89 weeks; range = 34 - 37 weeks), and 34.57 weeks (SD
= 3.36 weeks; range = 29 - 37 weeks), respectively. The parents
of all infants voluntarily provided informed consent before the
infants participated in the study. The ethics committee at the
Faculty of Letters, Keio University, approved this study.
Apparatu s and Stimuli
The speech stimuli were recorded by a Japanese woman na-
tive to the Tokyo area in order to control for regional speech
intonation. The stimuli included news scripts describing the
Japanese public pension system and were considered too diffi-
cult for infants to understand. Thus, these stimuli could only be
processed by brain areas required for non-semantic phonologi-
cal characteristics of language. The speech stimuli were stored
in a digital portable recorder (PMD660, Marantz, Mahwah, N.J.,
USA) with a condenser microphone for input (KSM27, Shure,
Niles, IL, USA) and digitized at a sampling rate of 44.1 Hz
with a 16-bit resolution. The stimuli were edited into 20-s seg-
ments, and noise was reduced using Sound Forge 8 software
(Sony, Tokyo, Japan). These stimuli were presented through a
sound system that consisted of an amplifier (A100a, Yamaha,
Shizuoka, Japan) and a loudspeaker (Reveal, Tannoy, Scotland,
UK) at a mean intensity sound pressure level of 68 dB.
Each twin infant was supported by a parent sitting 170 cm in
front of a loudspeaker in a soundproof room. The infants were
awake or moderately sleepy without sedation. We presented
attractive materials, such as toy figures and cars, between the
infant and loudspeaker to maintain infant orientation toward the
speaker. The infant was then exposed to speech stimuli during
11 blocks, with each stimulus being presented for 20 s. Stimuli
were played backwards during the first block and alternated
between forward and backward playback thereafter, resulting in
6 backward (rest) and 5 forward (experimental) blocks. There
were no delays between blocks, and the total stimulus exposure
time was 220 s. If the infant was composed, this experimental
procedure was repeated a second time. If the infant tried to
move away from the parent’s lap during the speech stimulus
exposure, experimental staff attempted to calm the infant.
Stimulus presentation was terminated if the infant made large
motions or cried but was continued if the infant returned to a
calmer state. The co-twin rested outside the soundproof room
during the experiment and was unable to hear the speech stim-
Near-Infrared Spectroscopy Recordings
The concentration of oxy- and deoxy-Hb was measured
within both temporal brain areas at a 10 Hz sampling rate with
an optical topography system (ETG-7000, Hitachi Medical,
Tokyo, Japan). Four probes (two emitting probes and two de-
tecting probes) were attached within each bilateral temporal
area in a square alignment. The probe at the lowest and most
posterior position within each bilateral temporal area was at-
tached at each T7 and T8 position of the international 10-5
system (i.e., the extension of the 10-20 system; Oostenveld &
Praamstra, 2001). Another probe was attached on a line con-
necting points T7 and T8 via the Cz position. Each probe was
separated by a distance of 3 cm, so that penetration depth of
near-infrared light into the brain tissue was approximately 3
Data Analysis
The concentrations of oxy- and deoxy-Hb were smoothed
using a 5-s moving average. If the data for a block included
Copyright © 2013 SciRes. 15
artifacts, all data for the block were discarded by visual inspec-
tion of oxy- and deoxy-Hb concentration. If the concentration
of oxy- and deoxy-Hb at a channel included artifacts during
several blocks, then data at that channel were discarded. The
remaining concentrations of oxy- and deoxy-Hb were then av-
eraged more than twice, synchronously, to the forward-stimulus
blocks at each time point. Infants who had more than two con-
taminated channels within each bilateral area, and no clean
forward-stimulus data, were excluded from any further analyses.
According to these criteria, the mean number of forward-
stimulus blocks available for analysis was 4.42. The mean
number of channels was 3.90 and 3.75 from the left and right
temporal areas, respectively. Among eight cases, data were
obtained from two or three channels from each left and right
temporal area. All remaining infant pairs provided data from all
four channels within the left and right temporal areas.
Mean oxy- and deoxy-Hb waveforms were computed by av-
eraging concentrations of oxy- and deoxy-Hb at each time point
against the last 10 s of the backward-stimulus block, which
served as a baseline period at each channel for each infant. We
computed the mean values of oxy- and deoxy-Hb concentration
from 5 to 15 s after the onset of a forward-stimulus block at
each channel for each infant. This period was chosen because
we expected to acquire clean concentrations representing the
hemodynamic response. The oxy- and deoxy-Hb waveform
concentrations during the forward-stimulus period indicate the
hemodynamic responses that were relatively affected by neural
activation for speech stimuli. Next, the mean values acquired
from left-side channels were averaged to acquire representative
values of oxy- and deoxy-Hb for the left temporal areas, and the
same was repeated for the right-side channels. These represen-
tative Hb values resulted in a signal-to-noise ratio that was
greater than the mean oxy- and deoxy-Hb values from each
individual channel.
We used a one-sample t-test (two-tailed) to examine whether
the hemodynamic response from the temporal areas was sig-
nificantly different from baseline with a corrected significance
level (.05, Bonferroni correction). To test cerebral dominance
for speech stimuli, we used a paired t-test to compare the con-
centrations of hemoglobin between left and right temporal areas.
In addition, developmental changes in the concentration of oxy-
and deoxy-Hb for speech stimuli were tested by regression
analysis. Infant age was used as the independent variable, while
concentration of hemoglobin was the dependent variable. In the
paired t-test and regression analyses, a significance level of .05
was set. Twin and co-twin data were treated as single-partici-
Similarity in the hemoglobin concentration between twin and
twin siblings were estimated by calculating Pearson product-
moment correlation coefficients for each temporal area. The
correlation coefficients were calculated using a double entry
method in which each member of the twin pairs was used as
both the twin and twin sibling. The degree of genetic and envi-
ronmental influence on the hemodynamic response was esti-
mated through structural equation modeling (Neale & Maes,
2004) by OpenMx (version 1.1; Boker et al., 2011). The addi-
tive genetic, shared environmental, and unique environmental
factors were estimated for each oxy- and deoxy-Hb within each
temporal area. Path coefficients from the genetic and environ-
mental factors to each oxy- and deoxy-Hb within left and right
temporal areas were estimated. The proportions of genetic and
environmental variance to total variance for each hemoglobin
type were also estimated.
Grand average waveforms of the hemodynamic response to
speech stimuli across all left and right temporal positions are
presented in Figure 1(A). The mean concentrations of oxy- and
deoxy-Hb are presented in Figure 1(B). The mean representa-
tive oxy- and deoxy-Hb values at bilateral temporal areas did
not significantly increase or decrease from the baseline period
(left oxy-Hb, M = .008, SE = .009, t(47) = .94, p = .35; left de-
oxy-Hb, M = .0004, SE = .004, t(47) = .09, p = .92; right
oxy-Hb, M = .017, SE = .008, t(47) = 1.91, p = .06; right de-
oxy-Hb, M = .008, SE = .007, t(47) = 1.14, p = .26). Repre-
sentative oxy- and deoxy-Hb values across all infants ranged
from .11 to .21 mMmm and .07 to .09 mMmm, respectively,
in the left temporal area, and from .13 to .13 mMmm and .13
to .12 mMmm, respectively, in the right temporal area. Paired
t-tests showed no differences in the concentrations of hemoglo-
bin between the left and right temporal areas (oxy-Hb, t(47)
= .82, p = .42; deoxy-Hb, t(47) = .94, p = .35). No develop-
mental changes in oxy- and deoxy-Hb concentrations within
both temporal areas were observed by regression analysis (left
oxy-Hb, F(1, 46) = .49, p = .48; left deoxy-Hb, F(1, 46) = .02, p
= .87; right oxy-Hb, F(1, 46) = .46, p = .49; right deoxy-Hb,
F(1, 46) = .01, p = .90).
Pearson’s product-moment correlation coefficients for oxy-
Hb concentrations from the left and right temporal areas within
MZ pairs were .24 and .05, respectively; those for deoxy-Hb
concentrations were .69 and .24, respectively. Correlation coef-
ficients for oxy-Hb concentrations from the left and right tem-
poral areas for DZ twin pairs were similar, at .19 and .24, re-
spectively; those for deoxy-Hb were .10 and .29, respectively.
Table 1 shows that large path coefficients from the unique
Figure 1.
(A) Grand mean waveform of oxy- and deoxy-Hb from left and right
temporal areas; (B) Mean oxy- and deoxy-Hb concentrations. Note: (A)
Waveforms of oxy- and deoxy-Hb from left (A, left) and right (A, right)
temporal areas. The horizontal axis displays time in seconds, aligned to
the onset of the forward-playing speech stimulus; (B) Error bars indi-
cate standard errors of the mean for oxy- and deoxy-Hb.
Copyright © 2013 SciRes.
Copyright © 2013 SciRes. 17
Table 1.
Path coefficients and proportion of variance for each additive genetic, shared environmental, and unique environmental factors.
Path Coefficient Proportion of Variance
a c e a2 c
2 e
1.18E07 .0271 .0546*
Right oxy-Hb (.0768) (.0143) (.0079)
3.72E12 .20 .80
1.53E07 1.78E08 .0494*
Right deoxy-Hb (.0221) (.0147) (.0050)
9.57E12 1.30E13 1.00
5.27E08 .0171 .0609*
Left oxy-Hb (.0452) (.0240) (.0088)
6.94E13 .07 .93
.0261* 5.30E10 .0221*
Left deoxy-Hb (.0069) (.0178) (.0057) .58 2.40E16 .42
Note: *p < .05; p < .1, Numbers in parentheses indicate standard errors. a = Additive genetic factor; c = Shared environmental factor; e = Unique environmental factor; a2
= Additive genetic variance; c2 = Shared environmental variance; e2 = Unique environmental variance.
environmental factors to each hemoglobin concentration were
observed within both bilateral temporal areas.
Shared path coefficients to the oxy-Hb were moderate values
within each bilateral temporal area, but statistically significant
path coefficients were observed solely for the shared environ-
ment to right oxy-Hb. The proportion of shared environmental
variance to total variance in oxy-Hb concentration was .07
and .20 within the left and right temporal areas, respectively. In
contrast, a significant additive genetic path coefficient was
observed solely for left deoxy-Hb concentration. Additive ge-
netic variance largely contributed to the variance in concentra-
tion of deoxy-Hb within the left temporal area.
We demonstrated both genetic and environmental influences
on hemodynamic responses to speech stimuli. Both genetic and
environmental variance differentially appeared for each type of
hemoglobin within each bilateral temporal area. Environmental
variance in the hemodynamic response to speech was larger
than genetic variance, suggesting that environmental factors
may play an important role in infants’ neural processing of
speech stimuli. Moderate and low shared environmental influ-
ences on the concentration of oxy-Hb were observed within
right and left temporal areas, respectively. This suggests that
variations in infants’ hemodynamic responses to speech stimuli
may be partly dependent on characteristics that are present in
the twins and twin siblings environment. A shared environ-
mental influence on language ability has been reported in both
infants and children (e.g., Alarcón et al., 1998; Van Hulle et al.,
2004). The shared environmental influence on hemodynamic
responses to speech stimuli may be consistent with that of lan-
guage abilities in children.
There are some potential examples of shared environmental
factors affecting infants’ hemodynamic responses to speech
stimuli. One possibility is that the shared environmental factors
have been associated with an influence within families. Here,
twin infants are reared together in their own family home in
which intonation and stress of the Japanese language is homo-
geneous while matching local dialects. Twin and twin siblings’
neural processing of speech stimuli may be sensitive to the
same characteristics. Another possibility is that a family-level
environmental factor, such as socioeconomic status, which has
been previously reported to affect a child’s language ability and
cognitive function (e.g., Rowe & Goldn-Meadow, 2009), af-
fects hemodynamic responses to speech stimuli. These fam-
ily-level environmental factors may play important roles in an
infant’s neural processing of speech information.
Unique environmental factors had a much larger influence on
hemodynamic responses to speech than did shared environ-
mental factors, suggesting that twin and twin siblings differen-
tially, and individually, develop neural processing within the
same environment; here, they share the same family, socioeco-
nomic status, and exposure to acoustic characteristics. Within
the same environments, infants’ neural processing of speech
stimuli may differentially activate. Concrete factors associated
with unique environmental variance within infants’ neural proc-
essing for speech stimuli should be examined in future studies.
In contrast to environmental factors, no substantial additive
genetic variance in oxy-Hb within bilateral temporal areas was
detected, suggesting that genes may not contribute to infants’
acquisition of language ability to the same extent as environ-
mental factors. However, additive genetic factors, which ac-
counted for half of the total variance, were observed within the
left temporal area. According to Kato (2004), changes in the
concentration of deoxy-Hb may indicate the neural activation
required more deoxy-Hb than oxy-Hb. As such, genetic contri-
butions to infants’ neural activity in response to speech stimuli
may exist. Furthermore, additive genetic influences on left de-
oxy-Hb concentration also suggests infants’ left cerebral domi-
nance for processing speech despite no differences in the mean
hemodynamic response.
Findings of genetic and environmental influences provide
new implications for understanding infants’ hemodynamic re-
sponse to speech stimuli. In general, deoxy-Hb concentration is
nearly associated with oxy-Hb; thus, oxy-Hb concentration is
considered a useful index of neural activity as opposed to de-
oxy-Hb concentration. In this study, additive genetic and shared
environmental influences appeared within infant deoxy- and
oxy-Hb concentrations, respectively. The different variance
components among these hemoglobin types suggest that in-
fants’ oxy- and deoxy-Hb concentrations indicate different char-
acteristics of neural activity in response to speech stimuli.
Although the grand mean waveforms of hemodynamic re-
sponses to speech appeared to deflect within bilateral temporal
areas, significant changes in the variation of oxy- and deoxy-Hb
concentration were not observed. In addition, no cerebral domi-
nance or developmental changes in hemodynamic responses for
speech stimuli appeared. These results suggest that large indi-
vidual differences in hemodynamic responses to speech exist
during the first two years of life. However, large individual
differences are sometimes considered to be meaningless com-
ponents (e.g., measurement error). Thus, the lack of significant
differences in hemodynamic responses to speech appears to
provide no clear implications. However, in this study, the indi-
vidual differences in hemodynamic responses provide valid
variance as inferred from the degree of genetic and environ-
mental influences. As mentioned in the introduction section,
twin designs decompose variance into additive genetic, shared
environmental, and unique environmental variance. Measure-
ment error is only involved in unique environmental variance.
In this study, the individual differences in hemodynamic re-
sponses to speech have significant variance within the additive
genetic and shared environmental factors. Therefore, the large
individual differences in hemodynamic responses should be
meaningful components for right oxy-Hb and left deoxy-Hb.
This study highlights the genetic and environmental etiology
of hemodynamic responses to speech stimuli. The present find-
ings revealed that additive genetic factors, as well as shared and
unique environmental factors are significant contributors to
each type of hemoglobin within each bilateral temporal area.
These results provide new knowledge regarding infants’ hemo-
dynamic responses to speech stimuli. Further investigation into
the role of genetic and environmental variation within the infant
neural system, using a larger sample size, may provide more
important information for understanding the mechanisms un-
derlying infant neural maturation.
We are grateful to the families who participated in the study.
Data collection for this study was supported by a grant from the
Research Institute of Science and Technology for Society wi-
thin the Japan Science and Technology Agency. This manu-
script was supported by Grant-in-Aid for Young Scientists (B)
(#25730101) by the Japanese Ministry of Education, Culture,
Sports, Science and Technology.
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