Basic Taste Stimuli Elicit Unique Responses in Facial Skin
Hideaki Kashima1, Naoyuki Hayashi1,2*
1Graduate School of Human-Environment Studies, Kyushu University, Fukuoka, Japan, 2Institute of Health Science, Kyushu University, Fukuoka, Japan
Facial expression changes characteristically with the emotions induced by basic tastes in humans. We tested the hypothesis
that the five basic tastes also elicit unique responses in facial skin blood flow. Facial skin blood flow was measured using
laser speckle flowgraphy in 16 healthy subjects before and during the application of basic taste stimuli in the oral cavity for
20 s. The skin blood flow in the eyelid increased in response to sweet and umami taste stimuli, while that in the nose
decreased in response to a bitter stimulus. There was a significant correlation between the subjective hedonic scores
accompanying these taste stimuli and the above changes in skin blood flow. These results demonstrate that sweet, umami,
and bitter tastes induce unique changes in facial skin blood flow that reflect subjective hedonic scores.
Citation: Kashima H, Hayashi N (2011) Basic Taste Stimuli Elicit Unique Responses in Facial Skin Blood Flow. PLoS ONE 6(12): e28236. doi:10.1371/
Editor: Attila Szolnoki, Hungarian Academy of Sciences, Hungary
Received July 13, 2011; Accepted November 4, 2011; Published December 1, 2011
Copyright: ? 2011 Kashima, Hayashi. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: This work was supported by a grant from Yazuya Co. Ltd. to N.H. The funder had no role in study design, data collection and analysis, decision to
publish, or preparation of the manuscript.
Competing Interests: The authors have read the journal’s policy and have the following conflicts. This work was supported by a grant from Yazuya Co. Ltd. to
N.H. This does not alter the authors’ adherence to all the PLoS ONE policies on sharing data and materials.
* E-mail: firstname.lastname@example.org
The gustatory characteristics of foods exert effects on eating
behaviors and judgments of their acceptability so as to avoid the
ingestion of toxic and unpleasant foods . One way of conferring
these characteristics to one’s peers is by facial expressions, since
those associated with the intake of foods with pleasant and
unpleasant tastes are consistent among different cultures, races,
and individuals . This has led to facial expression being used to
evaluate objective gustatory senses . However, facial expressions
are also generated by voluntary muscles via the motor nervous
system, and can easily be modulated or masked by one’s intention.
This has led to a lack of objectivity regarding the evaluation of
facial expression in response to taste.
Contrary to the responses induced by voluntary muscles, it is
difficult to intentionally modulate or mask a response that is
induced by the autonomic nervous system (ANS), since it is not
under voluntary control. ANS activity is affected by hedonic
valence and emotion, and so responses identified by the ANS are
frequently used as lie detectors. Some previous studies have
suggested that the basic tastes also induce specific heart rate and
finger skin resistance responses according to the associated hedonic
valence or emotion [4–6]. These studies implicated the existence
of unique ANS responses to hedonic valence or emotion. Our
previous study also suggested that gustatory information increases
the blood flow in the gastrointestinal tract . However, such
responses cannot be used as a tool by which to communicate with
peers since most of them occur inside the body (i.e., playing a
functional role in digestion). It is unknown whether ANS responses
such as changes in facial skin blood flow mirror gustatory
The facial skin blood flow responses are induced by the ANS
[8,9] and can be visible to others . The blood flow response is
involuntary and rapid, and so it is readily relayed to peers. We
therefore hypothesized that facial skin blood flow reflects our
emotion related to the tastes we are experiencing. To test this
hypothesis, we observed the facial skin blood flow responses to five
basic tastes applied in three different concentrations to the oral
cavity. We found that pleasant and unpleasant tastes elicit unique
changes in facial skin blood flow that reflect subjective hedonic
The taste stimuli successfully provoked various hedonic
valences, with the hedonic scores to basic tastes differing
significantly from that to the water control (Fig. 1). The pleasant
score was higher for medium and high concentrations of sucrose
(sweet) stimuli than for the water control [repeated-measures
analysis of variance (ANOVA): F1, 15=12.89, P,0.05]. The
unpleasant score was higher for a high concentration NaCl (salty)
stimulus and all concentrations of quinine (bitter) than for the
water (repeated-measures ANOVA: salty, F1, 15=13.41, P,0.05;
bitter, F1, 15=57.88, P,0.05). Citric acid (sour) and monosodium
glutamate (umami) stimuli were rated as neither pleasant nor
unpleasant compared to the water (repeated-measures ANOVA:
sour, F1, 15=3.05, P.0.05; umami, F1, 15=2.09, P.0.05).
Sweet and umami stimuli were reflected in the eyelid skin blood
flow, such that the blood flow increased with the hedonic score
(Fig. 2a, b). We computed the relative change in facial skin blood
flow induced by each taste stimulus as compared to the baseline
before the stimulus. Table 1 indicates the relative changes in skin
blood flow to each taste stimulus. The eyelid skin blood flow
significantly increased for all concentrations of sweet and umami
stimuli. The hedonic score was significantly and positively
correlated with changes in eyelid skin blood flow for sweet
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(Fig. 3a; r=0.60, F1, 40=22.62, P,0.05) and umami (Fig. 3b;
r=0.57, F1, 43=20.35, P,0.05) stimuli. The slopes and intercepts
were similar for both relationships. Medium and high concentra-
tions of bitter stimuli were also reflected in the nose skin blood
flow, which decreased in all subjects (Fig. 2c), and were
significantly correlated with the hedonic score (Fig. 3c; r=0.52,
F1, 37=13.90, P,0.05).
While responses in facial skin blood flow were observed in other
facial regions (Table 1), their magnitudes were not related to
hedonic valence. Forehead skin blood flow significantly decreased
in response to all concentrations of sweet and high concentration
salty stimuli. Eyelid skin blood flow significantly increased in
response to all concentrations of sour and medium concentrations
of salty stimuli. Cheek skin blood flow significantly increased in
response to medium and high concentrations of sour and high
concentration of salty stimuli. Upper lip blood flow significantly
increased in response to all stimuli, whereas lower lip blood flow
significantly increased only in response to taste stimuli at medium
and high concentrations. The changes in skin blood flow in each
region were not linearly related to the solution concentration.
The heart rate, mean arterial blood pressure (MAP), and
electromyographic activities of the zygomaticus major and
corrugator supercilii muscles did not significantly change from
baseline in response to any of the taste stimuli.
The present study demonstrated that basic tastes elicit unique
responses in facial skin blood flow, and that the magnitude of these
responses is related to hedonic valence but not simply to the
solution concentration. Facial skin blood flow is altered by the
activity of the ANS [8,9]. The findings of the present study support
the hypothesis that gustatory information is reflected in charac-
teristic facial skin blood flow responses induced by the ANS.
Putting any substance into the oral cavity can elicit a blood flow
response in the upper lip, since even pure water increased skin
blood flow in this area. However, the responses elicited in other
regions must have been due to the actual taste stimuli.
The unpleasant rating for bitter stimulus was associated with
changes in nose skin blood flow. Unpleasant stimuli, such as
bitterness, are important cues to remind us of the danger of toxic
foods, making us experience a related negative emotion . Many
species including monkeys, cats, and human infants exhibit distaste
in response to bitterness . Thus, a reduction in nose skin blood
flow in response to a bitter stimulus may help to express the hazard
related to certain foods in humans as well as other species.
The pleasant rating of sweet and umami stimuli was associated
with an increase in eyelid skin blood flow. This increase is an
important cue to remind us of the essential macronutrients that
provide the energy we need to survive. The increase in eyelid skin
blood flow in response to sweet and umami stimuli may help to
inform peers of the utility of this food. The eyelid or face could
reflect the hedonic valence better than other areas of the body. A
previous study found that sweet stimuli associated with different
hedonic scores and taste intensities were not related to changes in
vascular resistance in finger skin . The differences between the
results of the present and previous studies may be attributable to
the different target areas studied—it has been shown previously
that facial skin area reflects emotion more effectively than does
finger skin .
The umami stimulus did not significantly change the hedonic
score in the present study, which is consistent with previous reports
. The umami taste is not pleasant for all subjects. Nevertheless,
the hedonic rating in response to the umami stimulus was
significantly correlated with the change in eyelid skin blood flow,
since hedonic valence was associated with the change in blood flow
during both the umami and sweet stimuli. This shows that the
umami stimulus itself was not necessarily responsible for the
increase in blood flow in the eyelid.
Sweet and umami stimuli activate similar cortical regions in the
brain . The hedonic valence to umami stimuli varies widely
among individuals, and those who perceived the umami stimulus
Figure 1. Hedonic scores (mean and SEM values) related to each taste, from 2 25=(the most unpleasant) to + +5 (=the most pleasant).
Each taste was compared to water by multiple comparisons (Bonferroni’s post-hoc test). *P,0.05. W, water; S, sweet; C, citric acid; N, NaCl; M,
monosodium glutamate; Q, quinine; l, low concentration; m, medium concentration; h, high concentration.
Basic Tastes Elicit Unique Facial Skin Blood Flow
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as a pleasant taste exhibited an increase in blood flow in the eyelid.
Eyelid skin blood flow must be affected by the hedonic score in
response to sweet and umami stimuli.
The mechanism by which bitter stimuli decrease the skin blood
flow in the nose area may be explained by anatomical and
physiological characteristics. The nose area has many arteriove-
nous anastomoses , and vasoconstriction in this area is elicited
by sympathetic nerve activation. An unpleasant taste, such as a
bitter stimulus, can increase sympathetic nerve activity and
consequently induce vasoconstriction, resulting in a decrease in
nose skin blood flow. Conversely, there is no anatomical or
physiological evidence to explain the observed eyelid skin blood
Sour and salty stimuli themselves increased the skin blood flow
in the eyelid and cheek, irrespective of the hedonic rating. The
increase may be due to a reflex mechanism independent of
hedonic information, unlike the case for the other taste stimuli. A
sour stimulus induces a sweating response, known as gustatory
flushing, while sweet and bitter stimuli do not . This flushing is
induced by the parasympathetic vasodilator reflex [9,18] via the
C-polymodal nociceptors  or transient receptor potential
vanilloid-1 (TRPV1) receptors [20,21]. TRPV1 receptors are
activated by sour and salty tastes such as citric acid and NaCl,
respectively, but not by sweet and bitter tastes such as sucrose and
quinine, respectively . Consequently, sour and salty stimuli
could increase skin blood flow via the parasympathetic vasodilator
reflex, independently of the hedonic rating.
The unique responses reported here were induced by changes in
the peripheral circulation rather than in the central circulation,
since no significant changes in either heart rate or MAP were
detected. Some previous studies have suggested that basic taste
stimuli increase the heart rate [4–6]; however, the subjects in these
studies were not instructed to keep the face motionless. The
formation of facial expressions such as angriness was shown to
increase heart rate and finger skin temperature . Since we
confirmed the lack of electromyographic activity during all taste
applications, the recorded facial skin blood flow responses were
not related to movements of the facial muscles.
The functional importance of facial skin blood flow arises from
the face easily conveying social signals. Facial expression may be
associated with facial skin blood flow, since we often use phrases in
daily conversational statements such as ‘‘you have a healthy
complexion’’, ‘‘you look pale’’, and ‘‘you turned red with anger’’
(and their equivalents in other languages). In fact, fear and disgust
decrease the skin temperature in the nose area of rhesus monkeys
[13,24], implying a change in skin blood flow. In addition, pain
stimulation in a tooth decreases the skin blood flow and
temperature in the nose area of humans . Of the ANS-
induced responses, the change in facial skin blood flow could be a
useful key for communication. In this context, we should confirm
in future research that peers are readily able to assess the rating of
hedonic valence induced by tastes, using changes in skin blood
flow as a key clue.
Differences between the study individuals did not affect the
obtained results. The baselines appear to differ among Fig. 2a, b,
and c, since data from different subjects are shown. We therefore
calculated the relative value to estimate the response in skin blood
flow whilst allowing for different baselines. Moreover, since the
light output in laser speckle flowgraphy is automatically settled by
the flowmeter, the baseline is arbitrarily determined. It is therefore
meaningless to compare these baselines.
In summary, the findings of the present study demonstrate that
sweet, umami, and bitter taste stimuli elicit characteristic facial
skin blood flow responses, showing that facial skin blood flow
mirrors a variety of complexions that reflect the hedonic valence.
The responses observed in the eyelid and nose skin blood flows
appear to represent signals of gustatory information. These
findings imply that facial skin blood flow is a potentially objective
method for assessing gustatory appreciation.
Sixteen healthy adults (ten males and six females; age 2665
years, mean6SD) participated in this study. The subjects were
normotensive, not taking any medication, and did not report any
gustatory disorder. The Ethics Committee of the Institution of
Health Science, Kyushu University, Japan, approved the exper-
imental protocol, and each subject provided written informed
consent to participate prior to the commencement of the study.
Five taste solutions were used: sweet, sour, salty, umami, and
bitter. Each taste was applied at three different concentrations
(Table 2). Subjects were randomly given 1 ml of one of the five
taste solutions at 37uC. They rated the taste as either pleasant or
unpleasant on an 11-point hedonic scale (where 0=no taste,
+5=the most pleasant taste, and 25=the most unpleasant taste).
Subjects arrived at the laboratory after having abstained from
exercise and alcohol ingestion for at least 1 day, and from eating,
Figure 2. Changes in facial skin blood-flow distribution in a
subject before (Pre) and during (Stimulus) gustatory stimula-
tion. The colored bar on the right side indicates the blood flow
magnitude, with the red and blue colors indicating higher and lower
blood flows, respectively. The white ellipses in the six facial regions
represent the target areas where blood flow was measured. Sweet (a)
and umami (b) stimuli characteristically increased eyelid skin blood
flow, while bitter (c) stimuli decreased nose skin blood flow.
Basic Tastes Elicit Unique Facial Skin Blood Flow
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smoking, and consuming caffeine for at least 2 h, and they had not
experienced sleep loss during the previous day. They were seated
in a quiet room at an ambient temperature of 24uC, and
participated in all of the trials. During the experiment, the subject
rested his/her chin on a pedestal to stabilize the face, and was
instructed not to move their facial muscles and to keep their eyes
closed. After 20 min of resting, a ‘‘taste’’ solution was applied to
the subject’s oral cavity via a straw placed in the mouth. The order
of the applied tastes was randomized, but the bitter solution was
given last since the bitter taste persists for a long time. The
concentrations for all types of taste were randomized, and the
subject did not know the order in which the solutions were
provided. The solution was kept in the mouth for 20 s before being
spat out, and the mouth was rinsed with water at 37uC until the
taste could no longer be perceived. The subject then reported his/
her perceived hedonic valence.
Facial skin blood flow was measured for 4 s using laser speckle
flowgraphy (LSFG-ANW-LL), before and 5 s after the onset of the
taste stimulation. To assess skin blood flow in the forehead, eyelid,
nose, cheek, upper lip, and lower lip, blood-flow images were
constructed from the speckle images using commercially available
software (LSFG Analyzer Cutaneous, Softcare, Fukuoka, Japan).
Each pixel in the blood-flow image is regarded as a vector of an
object’s movement, and their sum within a given area thus
corresponds to the relative value of the sum of that object’s
movements (i.e., blood flow). The target area in each subject’s
facial blood flow was kept constant.
The heart rate and MAP were measured. The beat-by-beat
MAP was monitored with an automatic sphygmomanometer on
the left middle finger (Finometer, Finapres Medical Systems,
Amsterdam, The Netherlands). Furthermore, since the electro-
myographic activities of the zygomaticus major and corrugator
Table 1. Changes in mean facial skin blood flow in response to various taste stimuli.
Taste concentrationForehead EyelidNoseCheek Upper lipLower lip
Medium062* 1163*362*762* 5564*1867*
2262* 1463*163*862* 5569*1365*
Data are mean and SEM values. n=16.
*P,0.05 vs. baseline.
Figure 3. Sweet, umami, and bitter stimuli were significantly correlated with hedonic rating and blood flow in specific facial skin
regions. Sweet (a) and umami (b) stimuli were significantly correlated with hedonic rating and eyelid skin blood flow (sweet: r=0.60, n=41; umami:
r=0.57, n=44). Bitter (c) stimuli were significantly correlated with hedonic rating and nose skin blood flow (r=0.52, n=38).
Basic Tastes Elicit Unique Facial Skin Blood Flow
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supercilii muscles relate to movements in response to pleasant and Download full-text
unpleasant feelings , movements of these muscles on the right
side were recorded.
The t-test revealed no gender differences in any of the study
variables, and hence subsequent analyses were performed after
combining data from male and female subjects. The hedonic
scores related to water and each of the taste solutions were tested
by one-way repeated-measures ANOVA and Bonferroni’s post-
hoc test. The facial skin blood-flow response was compared to a
prestimulus baseline using a paired t-test. The relationship
between hedonic score and facial skin blood flow was evaluated
by Pearson’s correlation. Heart rate, MAP, and integrated
electromyography signals of the zygomaticus major and corruga-
tor supercilii muscles were compared to the prestimulus baseline
values using a paired t-test. The level of statistical significance was
set at P,0.05. All statistical analyses were performed with SPSS
(PASW statistics 18).
Conceived and designed the experiments: NH. Performed the experiments:
HK. Analyzed the data: HK NH. Contributed reagents/materials/analysis
tools: HK NH. Wrote the paper: HK NH.
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Table 2. The tested concentrations for each taste substance.
Sucrose50 mM 300 mM1000 mM
Citric acid0.93 mM56 mM 100 mM
NaCl 4.0 mM 160 mM560 mM
MSG 3.0 mM 10 mM 60 mM
Quinine 0.01 mM0.1 mM0.5 mM
MSG: monosodium glutamate.
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