Perceived happiness level influences evocation of positive emotions

doi:10.4236/ns.2011.38095

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Vol.3, No.8, 723-727

doi:10.4236/ns.2011.38095

(2011) Natural Science

Perceived happiness level influences evocation of

positive emotions

Masahiro Matsunaga1*, Hiroki Murakami2, Kaori Yamakawa3, Tokiko Isowa4,

Seisuke Fukuyama5, Jun Shinoda5, Jitsuhiro Yamada5, Hideki Ohira3

1Division of Cerebral Integration, Department of Cerebral Research, National Institute for Physiological Sciences, Aichi, Japan;

*Corresponding Author: [email protected]

2Department of Psychology, Stony Brook University, New York, USA;

3Department of Psychology, Graduate School of Environmental Studies, Nagoya University, Aichi, Japan;

4Faculty of Nursing, Graduate School of Medicine, Mie University, Mie, Japan;

5Chubu Medical Center for Prolonged Traumatic Brain Dysfunction, Kizawa Memorial Hospital, Gifu, Japan.

Received 12 April 2011; revised 30 April 2011; accepted 5 May 2011.

ABSTRACT

Happiness is a positive feeling characterized by

satisfaction, joy, pleasure, or love. Perceived

happiness level is likely associated with health

and well-being. However, the neurobiological

mechanisms underlying the positive effects of

happiness on psychological and physiological

wellness remain obscure. In this study, we

found that the medial prefrontal cortex (MPFC)

was strongly activated and positive emotions

were strongly evoked in individuals with high

perceived happiness levels compared to those

with low perceived happiness lev els when either

looked at a favorite person. Moreover, positive

mood changes were positively correlated with

MPFC activity. These results show that per-

ceived happiness level is associated with posi-

tive emotion evocation by means of the MPFC

function, which may be involved in psycho-

logical and physiological wellness. Therefore,

happiness may play an important role in psy-

chological and physiological wellness.

Keywords: Happiness; Medial Prefrontal Cortex;

Attraction; Positron Emission Tomogr aphy

1. INTRODUCTION

Stress and anxiety can lead to depression [1]. Psycho-

neuroimmunological studies have shown that psychoso-

cial stressors can activate the sympathetic nervous sys-

tem and that prolonged elevation of sympathetic nervous

activity weakens immune defense mechanisms [2]. There-

fore, stress and anxiety may cause or facilitate not only

depression but several other diseases as well, including

cancer [3].

The idea that perceived happiness level may be asso-

ciated with health and well-being has recently been ex-

plored. Individuals with a high level of perceived happi-

ness have reduced sympathetic activation after exposure

to psychological stressors compared with those having

low levels of perceived happiness [4]. A recent study has

also indicated that perceived happiness is negatively

correlated with depression and anxiety [5]. Therefore,

perceived happiness level is involved in both psycho-

logical and physiological well-being. The neurobiologi-

cal mechanism that explains these positive effects of

happiness on psychological and physiological well-being

remains obscure. The medial prefrontal cortex (MPFC)

is one of the brain regions that may be involved in psy-

chological and physiological wellness. It has been shown

that electrical stimulation of the MPFC has an antide-

pressant effect by increasing synaptic availability of se-

rotonin [6], suggesting that MPFC maintains positive

mood states. From this viewpoint, it is suggested that

individuals with high levels of perceived happiness have

stronger MPFC activity and stronger positive emotion

evocation than individuals with low levels of perceived

happiness upon exposure to positive emotional stimuli.

To test this hypothesis, participants in this study were

required to watch films featuring people that they con-

sidered attractive, and we simultaneously recorded their

brain activity using 15O-water positron emission tomo-

graphy (PET) during various mood states.

2. METHODS

2.1. Participants

We recruited 18 healthy right-handed male volunteers

(age range, 20 - 29 years). None of the participants had

M. Matsunaga et al. / Natural Science 3 (2011) 723-727

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chronic illness and none were taking medications known

to affect immunity. Only men were included to avoid

gender-related endocrine system and immune variations

caused by the female menstrual cycle. To screen indi-

viduals with higher or lower levels of perceived happi-

ness, we requested them to use the Japanese version of

the subjective happiness scale (JSHS) [5]. The JSHS is a

4-item scale that measures relatively stable perceived

happiness. Internal consistency, test-retest reliability,

convergent validity, and discriminant validity of the

JSHS have been confirmed [5]. Results of this screening

investigation indicate that the mean JSHS score for the

participants was 4.83 with a standard deviation (SD) of

0.9, similar to those reported earlier [5]. According to

the mean participant JSHS score, we designated partici-

pants with scores >5.0 as higher happiness respondents

(n = 9) and those <4.9 as lower happiness respondents (n

= 9). The participants were age-matched. All participants

signed informed consent forms prior to participating in

the study in accordance with university policy, and the

study was approved by the Kizawa Memorial Hospital

ethics committee.

2.2. Task Procedure

Each participant was instructed not to eat for 2 h be-

fore the scanning session but was allowed nonalcoholic

and caffeine-free fluids. Either an emotionally neutral

film (control film) or a film featuring people that the

participant considered attractive (positive film) was

viewed for 4 min on a 15-inch display positioned ap-

proximately 60 cm from the participants. PET scanning

(duration, 60 s) was performed during the first 2 - 3 min

of the screening. To obtain reliable PET data, each par-

ticipant was assigned 2 positive and 2 control films and

4 PET scans were obtained per participant. The control

film was a TV news program that featured a newscaster

that the participants did not consider particularly attrac-

tive. The participants watched the edited control and

positive films for the first time at the time of the experi-

ment. The emotional valence of the films had been pre-

viously evaluated [7]. The order of the 2 film types was

randomized across participants. The detailed PET pro-

cedure was described previously [7].

2.3. Image Acquisition by PET, Image

Processing, and Analysis

During each film-watching period, the distribution of

regional cerebral blood flow (rCBF) was measured using

an Advance NXi PET scanner (GE Healthcare Life Sci-

ences, Little Chalfont, England) operating in high-sensi-

tivity 3-dimensional mode [7]. SPM99 software [8] im-

plemented in Matlab (version 6.1; Mathworks, Sherborn,

MA) was used for spatial preprocessing and statistical

analyses as described previously [7]. To clarify signifi-

cant regional changes within groups (high vs. low) dur-

ing the positive condition, differences between the 2

groups were analyzed by subtracting the images of the

low happiness group from the images of the high happi-

ness group (each obtained by subtracting the control

condition images from those obtained during the positive

condition). The effects at each voxel were estimated

using a general linear model. Voxel values for each con-

trast yielded a statistical parametric map of the t-statistic

(SPM t), which was subsequently transformed to a unit

normal distribution (SPM z). The peak voxel value sig-

nificance thresholds were set at p < 0.001 (uncorrected)

and the cluster significance thresholds were set at 15

voxels.

Further, to examine the association between brain ac-

tivity reflected by the rCBF and positive mood, we cre-

ated statistical parametric maps to identify the brain re-

gions that were activated in synchrony with the positive

mood changes using the high happiness group data. For

these maps, the covariates option was selected, generat-

ing regression analyses that tested the linear relationship

between rCBF and positive mood, yielding a z score at

each voxel. We entered the value changes obtained in

the positive mood obtained by subtracting the value be-

fore watching a film from the value after watching a film

as covariates. For covariate analysis, the statistical

threshold was set at p < 0.0001 (uncorrected) for height

and clusters including >20 contiguous voxels were re-

ported. For ease of discussion, we refer to our findings in

terms of significant “correlations,” although the analysis

formally involved linear regression rather than correla-

tion assessment. After voxel-wise regression analysis,

the MPFC value was extracted using SPM99, and the

Pearson correlation coefficient was computed between

the MPFC response and the positive mood change. Both

control and positive conditions (2 × 9 data points) were

used in both the voxel-wise regression analysis and the

Pearson correlation coefficient computation.

2.4. Statistical Analyses

The positive mood before and after film watching was

compared using 3-way analysis of variance (ANOVA)

(group [high vs. low] × condition [control vs. positive] ×

period [before vs. after]), followed by paired t-tests. This

index was measured twice for each condition; thus, sta-

tistical analyses were conducted among participants after

each participant’s average was calculated.

3. RESULTS

Subtraction of the low happiness from the high hap-

M. Matsunaga et al. / Natural Science 3 (2011) 723-727

725725

piness group data revealed significant increases in the

rCBF of the MPFC (Brodmann area 9; x, y, z = –2, 58,

30), dorsal striatum (x, y, z, = –16, 10, –4), insula (x, y,

z = –36, 10, 0), and supplementary motor area (SMA; x,

y, z = –14, 8, 64) (Figure 1). With regard to the positive

mood, the ANOVA identified significant interactions of

group (high vs. low) × condition (control vs. positive) ×

period (before vs. after) (F [1,32] = 5.75, p < 0.05; Fig-

ure 2). Further statistical analyses indicated that watch-

ing the positive film significantly elevated the positive

mood in both groups: high: t(8) = 5.53, p < 0.01; low: t(8)

= –3.41, p < 0.01 (Figure 2). However, the positive

mood score after watching the positive film was higher

in the high happiness group than in the low happiness

group: t (8) = 3.46, p < 0.01 (Figure 2). We then ana-

lyzed the correlation between change in positive mood

and rCBF: positive correlations were identified in sev-

eral brain regions, including the MPFC (x, y, z = –2, 56,

38). The MPFC value, which was activated greatly in the

high happiness group, was then extracted and the Pear-

son correlation coefficient between the MPFC response

and the positive mood change was computed: r (18) =

0.51, p < 0.05 (Figure 3).

4. DISCUSSION

We hypothesized that the MPFC may be strongly ac-

tivated in individuals with high levels of perceived hap-

piness and that strong positive emotions may be evoked

upon sight of a favorite person compared to individuals

with low levels of perceived happiness. As expected, this

Figure 1. Brain regions activated by the

positive films in the high happiness group

as identified by PET and by subtracting

the low happiness group values from the

high happiness group values (p < 0.001,

uncorrected). PET: positron emission to-

mography; MPFC: medial prefrontal cor-

tex; SMA: supplementary motor area.

Figure 2. Change in positive mood state

after watching the films. p < 0.01 vs.

before watching by paired t-test. ††p <

0.01 vs. after watching the positive film in

the low happiness group by paired t-test.

Figure 3. SPM99 covariate analysis results (p < 0.0001, un-

corrected) showing a positive correlation between change in

positive mood and rCBF in the MPFC; scatterplot showing

MPFC activity and positive mood change from baseline to

after film screening (r(18) = 0.51, p < 0.05). rCBF: regional

cerebral blood flow; MPFC: medial prefrontal cortex.

study showed that the high happiness group had a

greater degree of MPFC activation. Moreover, strong

positive emotions were evoked in participants in this

group compared to participants in the low happiness

group when the participants looked at a favorite person.

Covariate analysis indicated that MPFC activation was

positively correlated with positive mood increases.

These results show that perceived happiness level is as-

sociated with positive emotion evocation via MPFC

function. Thus, individuals with high levels of perceived

happiness may maintain psychological well-being via

MPFC function. In addition, MPFC can control auto-

M. Matsunaga et al. / Natural Science 3 (2011) 723-727

726

nomic nervous system activity via brain autonomic cen-

ters such as the hypothalamus and midbrain through

direct neural projections [9]. Thus, it is possible that

reduced sympathetic activation in individuals with high

levels of perceived happiness after exposure to psycho-

social stressors [4] will also be attributable to the MPFC

function. A well-functioning MPFC may mediate the

association between perceived happiness and health and

well-being.

Happiness is a positive feeling characterized by con-

tentment, satisfaction, joy, pleasure, or love. Dopamine

plays an important role in the expression of positive

emotions [10-12]. The dopaminergic network within the

brain, which projects to the prefrontal cortex and stria-

tum from the midbrain region, is known as the “brain

reward system” [10,11], and our results indicated that

the reward-related brain regions including the dorsal

striatum were strongly activated in individuals with high

levels of perceived happiness. Findings have suggested

that dopaminergic function deficiency may be a possible

explanation for depression [13] and that antidepressant

treatment facilitated dopamine release in the nucleus

accumbens of a genetic animal model of depression [14].

Furthermore, activation of reward-related brain regions

can improve physiological functions including immune

functions [7]. Thus, brain dopaminergic function may be

a key factor in the positive effects of perceived happi-

ness levels.

The present study also indicated that activation of the

insula and SMA was higher in the high happiness group

than in the low happiness group when participants

looked at a favorite person. The insular cortex is in-

volved in the perception of signals arising from the body

and plays an important role in emotional experience

[15-17]. We have recently shown that parasympathetic

activity increases after an individual experiences positive

emotions and that activity of the insular cortex is associ-

ated with changes in peripheral autonomic activity [18].

Although the present study could not measure the pe-

ripheral autonomic activity during the task because of

technical limitations, it is possible that parasympathetic

activity was greater in the high happiness group than in

the low happiness group when positive emotions were

elicited, and the greater insula activity in the high hap-

piness group reflected this. Moreover, one study indi-

cated a positive correlation between SMA activity and

laughter/smiling; therefore, SMA activity reflects motor

activity associated with these actions [19]. The individu-

als with high levels of happiness probably smiled often

while watching the positive film compared with those

having low levels of happiness. Therefore, positive emo-

tion-related brain regions may have been highly stimu-

lated upon sight of a favorite person by participants of

the high happiness group compared with those of the

low happiness group.

Certain limitations of this study must be recognized.

Some reports have noted gender-related differences with

regard to perceived happiness level and physiological

reactivity [5,20], and we did not investigate the effect of

gender. Thus, the generalizability of the present findings

must be further tested using a larger sample size that

includes both male and female participants.

5. CONCLUSIONS

The findings of this study indicate that perceived hap-

piness level is associated with evocation of positive

emotions via MPFC function. High MPFC function in

individuals with high perceived happiness levels may

enhance psychological and physiological wellness.

6. ACKNOWLEDGEMENTS

This work was supported by Grant-in-Aid for Young Scientists (B)

from the Japan Society for the Promotion of Science (22700683 to

MM).

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