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Effects of Object Color Stimuli on Human Brain Activities in Perception and Attention Referred to EEG Alpha Band Response

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This study was designed to investigate the physiological effects of color in terms of blood pressure and the results of electroencephalogram (EEG) as subjects looked at the sheets of paper of various colors. A questionnaire was also used to assess psychological effects. Three colors (red, green, blue) were shown to each subject in randomized order. The various colors showed distinctly different effects on the mean power of the alpha band, theta band, and on the total power in the theta-beta EEG bandwidth and alpha attenuation coefficient (AAC). Scores of the subjective evaluations concerning heavy, excited, and warm feelings also indicated significant differences between red and blue conditions. Against to our prediction, blue elicited stronger arousal than did red as expressed by the results of AAC and the mean power of the alpha band, which conflicted with the results of the subjective evaluations scores. This phenomenon might be caused by bluish light's biological activating effect. The powers of the alpha band, and the theta band, and the total power of the theta-beta bandwidth as measured by EEG showed larger values while the subjects looked at red paper than while they looked at blue paper. This indicated that red possibly elicited an anxiety state and therefore caused a higher level of brain activity in the areas of perception and attention than did the color blue. Red paper's effect to activate the central cortical region with regard to perception and attention was considerably more distinguishable than was the biological activating effect of bluish light in our study.
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Abstract This study was designed to investigate the
physiological effects of color in terms of blood pressure and
the results of electroencephalogram (EEG) as subjects looked
at the sheets of paper of various colors. A questionnaire was
also used to assess psychological effects. Three colors (red,
green, blue) were shown to each subject in randomized order.
The various colors showed distinctly different effects on the
mean power of the alpha band, theta band, and on the total
power in the theta-beta EEG bandwidth and alpha attenuation
coefficient (AAC). Scores of the subjective evaluations
concerning heavy, excited, and warm feelings also indicated
significant differences between red and blue conditions.
Against to our prediction, blue elicited stronger arousal than
did red as expressed by the results of AAC and the mean power
of the alpha band, which conflicted with the results of the
subjective evaluations scores. This phenomenon might be
caused by bluish light’s biological activating effect. The powers
of the alpha band, and the theta band, and the total power of the
theta-beta bandwidth as measured by EEG showed larger
values while the subjects looked at red paper than while they
looked at blue paper. This indicated that red possibly elicited
an anxiety state and therefore caused a higher level of brain
activity in the areas of perception and attention than did the
color blue. Red paper’s effect to activate the central cortical
region with regard to perception and attention was
considerably more distinguishable than was the biological
activating effect of bluish light in our study. J Physiol
Anthropol 26(3): 373–379, 2007 http://www.jstage.jst.go.jp/
browse/jpa2
[DOI: 10.2114/jpa2.26.373]
Keywords: color, electroencephalogram, alpha band power,
alpha attenuation test
Introduction
As culture and technical civilization has developed, so has
the human living environment. More than ever before, color
has come to play an important role in communication.
Advances in studies regarding the influence of color stimuli on
human brain function are expected to provide better color
design for improving the usability and the quality of our living
environment.
From long ago, the various psychological influences of color
on human beings have been a focus of attention. Regarding the
emotional effect of the object color, it has been reported that
warm colors in the red color system arouse genial, positive,
active feelings, and neutral colors such as green promote
moderate, calm, ordinary feelings, while the cool colors in the
blue color system produce cold, passive, quiet feelings
(Matsuoka, 2000).
In research involving colored lights, electroencephalogram
(EEG) responses were monitored while a subject was shown
different colors on a personal computer screen. It was found
that in comparison with white and red, beta wave intensity in
the occipital areas was inhibited when the subject was shown
blue, suggesting that blue has a more relaxing effect (Ueda et
al., 2004). Under different colored illuminations, blood
pressure and alpha band power (EEG) showed the highest
values under the blue condition, while pulse rate and skin
temperature showed the lowest values under a blue light, with
the degree of arousal increasing in the order of bluegreen
red light (Shimagami and Hihara, 1991, 1992).
The physiological effects of object color have also been
recently reported. Mishima (1996) found different EEG
responses to variously colored pieces of cloth (W: 270 cm, H:
190 cm) between male and female. Shen et al. (1999) reported
significant skin temperature differences when subjects exposed
to red, blue, black and white paper (A2 sized). They observed
beta2 waves in the parietal region and alpha1 waves in the
occipital and parietal regions when subjects were shown blue
paper; while when the subjects viewed red paper, beta2 waves
were mainly observed in the frontal region. It is surmised this
can be attributed to the exciting or calming effect of the colors.
However, statistical reports regarding the influence of object
Effects of Object Color Stimuli on Human Brain Activities in Perception and
Attention Referred to EEG Alpha Band Response
Ai Yoto
1)
, Tetsuo Katsuura
2)
, Koichi Iwanaga
2)
and Yoshihiro Shimomura
2)
1) Graduate School of Science and Technology, Chiba University
2) Graduate School of Engineering, Chiba University
color on background EEG are still far from enough, and the
relation between affective elements and cortical activities
according to color have not been clarified. In light of this, more
studies are necessary to elucidate EEG response changes due
to object color stimulation.
In background EEG, the delta band (f4 Hz) and the theta
band (4 Hzf8 Hz) correspond to the decrement of arousal
level which associated with sleepiness and the decline of
consciousness level. The alpha band (8 Hzf13 Hz) is seen
in occipital areas predominantly at the time of awakening,
generally appearing mostly in a state of relaxation and when
light is excluded by closing one’s eyes. The beta band
(13f30 Hz) corresponds to a state of high consciousness
level or excitement.
The purpose of this study was to clarify the relationship
between the results of the physiological indexes and the
subjective effect of color. Three sheets of colored paper—red,
green, and blue—were used in this experiment to test the
arousal-calming effect of color, using each EEG band power
and the total band power (betaalphatheta), and the alpha
attenuation test (AAT) as standard indices of arousal.
Methods
Subjects
11 university students whose ages ranged from 20 to 31
years (22.43.2 yrs (meanSD), 6 males, 5 females)
participated in the experiment. They had normal color vision.
The experimental procedures were fully explained to them
before beginning the experiments. They were informed they
would be in no physical danger due to the experiment, and that
they were free to quit at any time during the experiment. They
gave their informed consent orally for their participation in the
experiments.
Experimental environment
A chair with a back and elbow rests and a desk were
positioned in a climate chamber (room temperature 24–25°C,
relative humidity 50%). Sheets of paper of various colors to be
used as stimuli were laid on the desk. Daylight fluorescent
lamps (FLR 40S-D/M Matsushita Electric Industrial Co., Ltd.)
on the ceiling were used to provide an illuminance of 500 lx on
the surface of the colored paper. Pieces of colored paper in A2
size, commonly available commercially, were used. They were
also selected to be as similar as possible in regard to
luminosity and chroma. The chromaticity coordinates
(x, y, u, v, Y) of the selected colored paper were measured using
a spectroradiometer (HSR-8100, OPT Research Co., Ltd.)
under the experimental environment. They were blue (0.3086,
0.3658, 0.1823, 0.3241, 21), green (0.3589, 0.4537, 0.1858,
0.3523, 19), and red (0.5290, 0.3614, 0.3370, 0.3454, 18). The
irradiances from 440 nm to 490 nm of blue, green, and red
condition were 1.42
m
W/cm
2
, 0.65
m
W/cm
2
, 0.38
m
W/cm
2
,
respectively, at a distance of 10 cm from the center of the paper
toward the eyes of the subjects. Another piece of gray colored
paper (chromaticity coordinates: 0.4009, 0.3995, 0.2293,
0.3428, 26) was shown in the intervals between each color
stimuli.
Procedure
Experiments were carried out during daytime between
10:00 am to 5:00 pm in July. When the subject was in the
chamber, seated and resting quietly, the task was explained,
and electrodes were attached. As shown in Fig. 1, EEG (4
minutes) and blood pressure were measured and subjective
evaluation questionnaires were filled out by the subject to
assess their psychological condition when looking at the sheets
of color paper. In the EEG measurement for AAC, 1 minute of
open-eye rest and 1 minute of closed-eye rest were repeated
twice instead of the typical three times. This was to avoid
excessive drowsiness caused by fatigue or the monotonousness
of the task rather than by the effects of the color stimuli. Three
sessions in total were carried out involving three colors in
random order for every subject.
Measurements
EEG: Ag/AgCl electrodes were attached at the Fp1, Fp2, F7,
F8, C3, C4, O1, O2, T5, T6, Fz, Cz, and Pz locations
according to the international 10/20 system for EEG recording.
EOG was recorded at the right eye supra- and infra-orbitally
for monitoring the subjects’ ocular movements. EEG and EOG
data were amplified by a bioelectric amplification unit (Bio-
Top 6R12-4, NEC Sanei). Filters were set for EEG at Low-Cut
of 0.016 Hz, and a Hi-Cut of 30 Hz, and for EOG at a Low-Cut
of 0.16 Hz, and a Hi-Cut of 15 Hz. With a personal computer
(ThinkPad A21e, IBM Japan Ltd), data was A/D converted
using AcqKnowledgeIII for the MP100WS (BIOPAC Systems,
Inc.). The sampling rate was 100Hz.
EEGs were Fast Fourier transformed for each 5.12 seconds
of data which do not include artifacts such as ocular
374 Object Color Stimuli on Human Brain Activities
Fig. 1 Experimental protocol.
movement. The power density of each delta (1–4 Hz), theta
(4–8 Hz), alpha (8–13 Hz), and beta (13–30 Hz) band, and the
power density in the theta-beta bandwidth (thetaalphabeta)
when subjects’ eyes were open, taken in each electrode
locations under each color condition were averaged and used
for analysis.
The alpha attenuation coefficient (AACalpha band power
when eyes close/alpha band power when eyes are open) of each
color condition as an index of arousal level was calculated and
analyzed.
Blood pressure: Using a Riva-Rocci type mercury
sphygmomanometer, systolic and diastolic blood pressure were
measured in each subject’s left arm.
Subjective evaluations: A bidimensional scaling form for
each stimulus was filled out, stating ten pairs of items such as
relaxed versus agitated, feeling of lightness versus feeling
of heaviness, calm versus excited, comfortable versus
uncomfortable, refreshed versus gloomy, tired versus not tired,
cold versus warm, bright versus dark, sleepy versus not sleepy,
sharpness of mind versus dullness of mind. Subjects were
asked to reply to a 5-point scale between the two extremes of
each dimension, and these affective ratings were analyzed.
Statistical analysis
The EEG, blood pressure data, and subjectivity evaluation
were analyzed by means of a one-way repeated measures
analysis of variance (ANOVA), using the statistical program
SPSS. When the ANOVA proved significant, Bonferroni’s
post-hoc test was further performed.
Results
EEG
The ANOVAs showed that the AACs at Fp1, F7, F8, Fz and
Cz were significantly affected by color (F(2, 20)6.524, 8.298,
3.654, 6.843, 3.671, p0.007, 0.002, 0.044, 0.005, 0.044).
Bonferroni’s multiple comparison indicated that the AAC
values when looking at red were lower at Fp1, F7, F8, and Fz
than those when looking at blue ( p0.005, 0.003, 0.044,
0.004), and lower than the case of green in F7 (Fig. 2,
p0.017).
The ANOVAs regarding the alpha band power showed the
main effects of colors at Fp1, F7, F8, C3, T5, T6, Fz
(F(2, 20)6.090, 4.303, 3.597, 3.608, 5.567, 3.541, 6.098,
p0.009, 0.028, 0.046, 0.046, 0.012, 0.048, 0.009). Post-hoc
comparison showed that the power densities of red and green
were greater than blue at Fp1 (p0.019, 0.022), and red was
greater than blue at Fp1, F7, T5, and Fz (Fig. 3, p0.027,
0.011, 0.007).
The power density of the theta band at T5 and Pz during
color presentation showed significant differences with respect
to color stimuli (F(2, 20)3.743, 4.544, p0.042, 0.024).
Post-hoc comparison showed that red was greater than blue at
Pz (Fig. 4, p0.029).
No significant difference due to colors appeared in the beta
or delta band power at any electrode locations.
The total band power density in the theta-beta bandwidth
showed the main effects of colors at F7, F8, T5, Fz, and Cz
(F(2, 20)4.227, 3.778, 6.522, 4.266, 3.628, p0.029, 0.041,
0.007, 0.029, and 0.045). As shown by the Bonferroni test, the
power densities when looking at red were significantly greater
than blue at Fp1, F7, F8, and Fz (Fig. 5, p0.046, 0.049,
0.007, 0.037, 0.043).
Blood pressure
No main effect of color on blood pressure was found in this
experiment.
Subjective evaluations
Differences were found in the following item pairs: feeling
of lightness versus feeling of heaviness, calm versus excited,
cold versus warm (F(2,20)5.304, 5.714, 7.368, p0.014,
0.011, 0.004). The Bonferroni test indicated that the feeling
of lightness was stronger when looking at blue than red
Yoto, A et al. J Physiol Anthropol, 26: 373–379, 2007 375
Fig. 2 AAC values at Fp1, F7, F8, T5, Fz, Cz, Pz and O1 during color presentations. The AAC values at Fp1, F7, F8 and Fz when looking at
red were lower at these 4 locations than when looking at blue, and lower at F7 when looking at green.
376 Object Color Stimuli on Human Brain Activities
Fig. 3 Power densities of the alpha band at Fp1, F7, F8, T5, Fz, Cz, Pz, and O1 during color presentations. Greater values when looking at red
than when looking at blue were shown at Fp1, F7, T5, and Fz.
Fig. 4 Power densities of the theta band at Fp1, F7, F8, T5, Fz, Cz, Pz, and O1 during color presentations. Looking at red showed a greater
density than looking at blue at Pz.
Fig. 5 Total band power densities in the theta-beta bandwidth at Fp1, F7, F8, T5, Fz, Cz, Pz, and O1 during color presentations. At Fp1, F7, F8,
and Fz, the power densities when looking at red were greater than when looking at blue.
(p0.012), and looking at red excited people more than did
blue or green (p0.005, 0.026), while blue elicited colder
feelings than did red (Fig. 6, p0.012).
Discussion
First of all, with regard to EEGs during the colored paper
presentation, the AAC values, which were used as an index of
arousal level, were significantly higher in the frontal and
parietal areas while looking at blue paper than at red. Noguchi
et al. (1999) reported that bluish light with a high color
temperature elicited higher AAC values than did reddish light
with a lower color temperature, suggesting the reddish light
might possibly have less effects of arousal than the bluish one.
Our results showed the same phenomena as did their
experiments. In other words, contrary to our prediction based
on reports in the psychological field, it followed that the
arousal level elicited by the blue presentation was higher than
that elicited by the red presentation. This might be caused by
the different amount of irradiance from 440 nm to 490 nm (the
blue region of the spectrum) received by the eyes. Due to the
discovery of additional nerve connections from recently
detected novel photoreceptor cells in the eye to the brain, light
also mediates and controls a large number of biochemical
processes in the human body. Bluish, cool light has
biologically a larger activating (alerting) effect than does
warmer colored reddish light (Bommel and Beld, 2004). In our
study, the irradiance of the blue region of the blue colored
paper was 3.7-times greater than that of the red paper. Thus, it
could be considered that the stronger bluish light reflected by
the blue paper might have elicited a larger activating effect
than that elicited by the red paper. This might be the
mechanism explaining the higher arousal level in the subject
when shown blue paper.
From result of each EEG band power, the alpha band at FP1,
F7, T5, and Fz and the theta band at Pz showed higher power
during the red presentation than blue. In other words, looking
at the red paper had a less arousing effect than looking at the
blue paper. This result is the opposite of those of several
colored light studies (Ueda et al., 2004, Shimagami and
Hihara, 1991, 1992), that found that red significantly excited
people more than did blue. In Ueda’s study, color stimuli were
presented by reflecting the colors via a mirror, including red,
white, and blue created on a personal computer monitor. Their
results showed that beta wave intensity in the subject’s
occipital lobe when looking at color stimuli was red
whiteblue. Shimagami and Hihara used color filtered-
illumination lamps as stimuli in their two studies involving two
illuminance levels of 150 lx and 40 lx. The results at both
levels showed a greater alpha wave appearance rate with eyes
open under blue light. We suspected that the intensities of the
colored lights that entered the eyes in the above experiments
were weaker than was the case in our study, thus the biological
activating effects of bluish light seemed not efficient enough to
overturn the opposite results. It is considered that the visual
effects of color stimuli might distinguish their influences on
the human brain more than the non-visual effects of the bluish
light when the light intensity is not relatively strong enough.
On the other hand, regarding the sleepy feeling, no color
effect was found in the subjective evaluations. Furthermore,
the results of subjective evaluations showed that red paper
elicited more excited feelings than did blue, which stands in
opposition to the above-mentioned physiological data. There
are two ways to explain this phenomenon.
First, the psychological effects in which red causes excited
feelings while blue color elicits calm feelings might not at the
same time cause the physical arousal activities in EEGs as seen
in the results of present experiment. That is to say, the
expressions of the exciting effect of red and the calming effect
of blue do not respectively correlate with the high and low
levels of arousal described in the psychophysiological field.
The second explanation is that the lower AAC value in the
frontal and temporal areas and the higher EEG power in low
frequency bands such as the alpha band in frontal area and the
theta band in the parietal area might not be indicative of lower
arousal levels in our study. As proof, neither of the above EEG
indexes showed any significant effects of colors in the occipital
area, which is regarded as the most accurate index of arousal
level. Rather, no difference was shown in arousal levels under
all of the color conditions indicated by the low AAC values in
the occipital area (cf. Fig. 2). This corresponded with the result
of no significant difference in sleepy feeling from the
subjective evaluations (cf. Fig. 6). Considering these results,
more than reflecting arousal levels, there are possibly other
explanations for the lower AAC value and higher alpha band
power in the frontal and parietal areas while looking at red
colored paper.
According to Basar’s theory (1999), alpha rhythm is related
to perception and attention, and the alpha band power increases
when the surge of interest to environment caused by anxiety
Yoto, A et al. J Physiol Anthropol, 26: 373–379, 2007 377
Fig. 6 Subjective evaluation scores during color presentations. A higher
score indicated a stronger feeling regarding each item, where “5”
stood for the strongest feeling regarding the item, and “1” stood for
the strongest feelings opposite to the item. Looking at red elicited a
stronger feeling of heaviness and also more excited and warm
feelings than did looking at blue.
states or so on becomes stronger. It has also been found by
Vo gt et al. (1997), Klimesch (1999), Knyazev and Slobodskaya
(2003) and Knyazev et al. (2004) that the theta and alpha band
power participate in affective and recognition process,
respectively. Knyazev et al. suggested that differences of alpha
power were considered as differences in alpha system
preparedness for information processing, with higher power
being an indicator of higher vigilance. Vogt et al. reported that
good memory performers showed more alpha power even
during resting sessions with eyes open and closed. In
Klimesch’s study, large alpha power during a resting state with
eyes open indicated good cognitive performance. As well,
alpha synchronization in tasks associated with the maintenance
of attention during the anticipation of visual events was
reported by Orekhova et al. (2001), suggesting that alpha
synchronization over the posterior parietal cortex reflects an
active inhibition of certain parietal networks involved in
maintaining attention to peripheral visual fields rather than
merely an ‘idle’ state of this cortical area. Finally, in a study by
Takahashi et al. (2005), increases were observed in fast theta
power and slow alpha power on EEG predominantly in the
frontal area during Zen meditation with eyes open, with this
regarded as a reflection of enhanced internalized attention and
mindfulness. Thus, the increase of alpha activities in the
frontal and parietal areas possibly accounts for the affective
states of anxiety and the rise of the activating level of
perception or attention. With this in mind, our results showing
larger theta and alpha power recorded during the presentation
of red than that of blue might be related to the different levels
of such cognitive activity. That is to say, the red paper may
have increased anxiety, and thus brain activity such as
perception or attention became more active than in the case of
blue paper.
In summary, considering that the light intensities from our
color paper stimuli were not strong enough to incite the
biological activating effects of blue region irradiance, and that
no arousal differences were expressed by the EEG responses in
the occipital area, Basar’s theory that alpha band power
increases by stronger anxiety states seems to be more suitable
for explaining the lower AAC value and higher alpha band
power in the frontal and parietal areas while looking at red
colored paper.
Concerning the results regarding the total power in the theta-
beta EEG bandwidth, the red presentation elicited significantly
higher power at F7, F8, T5, Fz, and Pz than did the blue
presentation. Knyazev et al. (2005) reported that the total
bandwidth power of EEGs recorded with the subjects’ eyes
open increased with associations of uncertain state and anxiety,
in a wide range from the frontal to the occipital areas.
According to this point of view, again it can be said that the
electric activity of the nervous system possibly became more
active under red stimuli than under blue stimuli, reflecting
excitement due to anxiety. This almost correlates with Kuller’s
study (1986) in which red light activated the central nervous
system more strongly than did blue or green light.
In conclusion, it was suggested that the subjects’ EEG
responses in a resting state differed when shown different
colored paper. Comparing the difference in the subjects’
responses to red and blue, the low frequency band of the EEGs
in the frontal and parietal areas became active when looking at
red paper, indicating that blue had a stronger arousing effect
than did red, which contradicted our expectation, and
conflicted with the results of the subjective evaluations scores.
As discussed above, we support the possibility that red elicits
an anxiety state and therefore causes a higher level of brain
activity regarding perception or attention than does blue.
Further study is needed regarding these effects, particularly
involving higher levels of attention, and regarding these
effects’ practical application.
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This article was presented at the 8th International Congress of
Physiological Anthropology, 2006 (ICPA 2006), in Kamakura,
Japan.
Received: September 27, 2006
Accepted: January 14, 2007
Correspondence to: Ai Yoto, Ergonomics Section, Graduate
School of Science and Technology, Chiba University, 1–33
Ya y oicho, Inage-ku, Chiba 263–8522, Japan
Phone: 043–251–1111 (ext. 4523)
e-mail: yotoai@graduate.chiba-u.jp
Yoto, A et al. J Physiol Anthropol, 26: 373–379, 2007 379
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... Since then, analyzing the changes in alpha waves under different experimental conditions has been the usual form of investigation (Elliot 2019). Though few of those studies supported the driving hypothesis of red color associating with arousal (Ali 1972;Shen et al. 1999), majority either disagreed or remained inconclusive (Erwin et al. 1961;Caldwell and Jones 1985;Mikellides 1990;Yoto et al. 2007). Apart from red, the color that has been experimented with the most is blue, because of its shorter wavelength. ...
... This result is remarkable in terms of offering support to previous ideas. Though studies have previously reported the arousal during Blue (Lockley et al. 2006;Vandewalle et al. 2007;Yoto et al. 2007), but any consensus is yet to be reported, much less the how's and why's of the perceptual detail. Our study, backed by robust non-linear tools, could embolden the validity of these claims. ...
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... A difficulty in synthesizing results from different studies is caused by the variety of color sources used. Studies conducted in relation to the impact of color on brain activity, as measured using electroencephalography (EEG) and/or physiological response in humans include those which use a light source: halogen, incandescent (Park et al., 2013), light-emitting diode (LED) (Chinazzo et al., 2020;Stamps, 2010); or inkbased source: ambient color (painted surfaces) (Ainsworth et al., 1993;von Castell, Stelzmann, et al., 2018), and stimulus color (dyed or printed material) (Gao & Xin, 2006;Yoto et al., 2007). Furthermore, studies have either explored the individual dimensions of color: Chroma (saturation) (Dresp-Langley & Reeves, 2014; Zieliński, 2016), hue (pigmentation) (Mehta & Zhu, 2009), and value (brightness or darkness adjustment) (Knez, 2001;von Castell, Hecht, & Oberfeld, 2018a), or used an approach combining and reporting variations in dimensions. ...
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The study examined the suggestion that infant ability to maintain attention in anticipatory task and to sustain interference is related to the active inhibitory processes in cortical neural networks. The extent of selective EEG synchronization in the alpha range has been taken as a measure of cortical inhibition. EEG was registered in 60 infants aged 8-11 months during: (1) attention to an object in the visual field (externally controlled attention); (2) anticipation of the person in the peek-a-boo game (internally controlled attention). The infants who demonstrated longer periods of anticipatory attention had higher absolute spectral amplitude in the broad frequency range under both experimental conditions. It was suggested that the effect of 'overall' EEG synchronization is related to some stable individual differences in psychophysiological traits. To control for the effect of overall EEG synchronization the relation between relative alpha amplitudes in 6.4-10 Hz range and the duration of internally controlled attention was analyzed. The infants with longer compared to shorter anticipatory attention spans had relatively higher 6.8 Hz alpha synchronization at posterior parietal sites under this experimental condition. It was suggested that alpha synchronization over posterior parietal cortex reflects an active inhibition of certain parietal networks involved in maintaining attention to peripheral visual field rather than merely an 'idle' state of this cortical area. Such an inhibition appears to allow infants to avoid interference of concurrent visual stimulation at the periphery of the visual field.
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