Genetic and Environmental Etiology of Infant Hemodynamic Response to Speech Stimuli: A Near-Infrared Spectroscopy Study of Twins

doi:10.4236/psych.2013.46A2003

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Psychology

2013. Vol.4, No.6A2, 14-18

Published Online June 2013 in SciRes (http://www.scirp.org/journal/psych) http://dx.doi.org/10.4236/psych.2013.46A2003

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

Email: [email protected], [email protected]

Received April 14th, 2013; revised May 16th, 2013; accepted June 13th, 2013

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;

Twins

Introduction

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

speech.

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-

K. SUZUKI, J. ANDO

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.

Methods

Participants

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.

Procedures

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-

uli.

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

cm.

Data Analysis

The concentrations of oxy- and deoxy-Hb were smoothed

using a 5-s moving average. If the data for a block included

K. SUZUKI, J. ANDO

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-

pants.

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.

Results

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.

K. SUZUKI, J. ANDO

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

−1.18E−07 .0271† .0546*

Right oxy-Hb (.0768) (.0143) (.0079)

3.72E−12 .20 .80

−1.53E−07 −1.78E−08 .0494*

Right deoxy-Hb (.0221) (.0147) (.0050)

9.57E−12 1.30E−13 1.00

−5.27E−08 .0171 .0609*

Left oxy-Hb (.0452) (.0240) (.0088)

6.94E−13 .07 .93

.0261* 5.30E−10 .0221*

Left deoxy-Hb (.0069) (.0178) (.0057) .58 2.40E−16 .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.

Discussion

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-

K. SUZUKI, J. ANDO

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.

Acknowledgements

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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