The authors have declared that no competing interests exist.
Conceived and designed the experiments: YL XH. Performed the experiments: BL JL. Analyzed the data: YL ZY DW. Wrote the paper: YL XH ZY DW.
Why are some people happier than others? This question has intrigued many researchers. However, limited work has addressed this question within a neuroscientific framework.
The present study investigated the neural correlates of trait happiness using the resting-state functional magnetic resonance imaging (rs-fMRI) approach. Specifically, regional homogeneity (ReHo) was examined on two groups of young adults: happy and unhappy individuals (N = 25 per group).
Decreased ReHo in unhappy relative to happy individuals was observed within prefrontal cortex, medial temporal lobe, superior temporal lobe, and retrosplenial cortex. In contrast, increased ReHo in unhappy relative to happy individuals was observed within the dorsolateral prefrontal cortex, middle cingulate gyrus, putamen, and thalamus. In addition, the ReHo within the left thalamus was negatively correlated with Chinese Happiness Inventory (CHI) score within the happy group.
As an exploratory study, we examined how general trait happiness is reflected in the regional homogeneity of intrinsic brain activity in a relatively small sample. Examining other types of happiness in a larger sample using a multitude of intrinsic brain activity indices are warranted for future work.
The local synchronization of BOLD signal is altered in unhappy individuals. The regions implicated in this alteration partly overlapped with previously identified default mode network, emotional circuitry, and rewarding system, suggesting that these systems may be involved in happiness.
Although happiness is almost everyone's pursuit, the capacity to be happy varies widely across people. Why are some people happier than others? A large amount of correlational research addressed this question by examining the associations between happiness and a diverse range of factors, such as wealth, marriage, and life events
To date, limited work has investigated happiness within a neuroscientific framework
Resting-state fMRI research has been blooming during the last couple of decades
In the current study, we focused on obtaining preliminary knowledge on the neural signature of happiness by examining whether happy and unhappy individuals differed in ReHo. Building on the evidence that happy and unhappy individuals differed in self-evaluation, self-reflection, self-regulation, and person perception
This study was approved by the Ethics Committee of the Southwest University. Written informed consent was obtained from all participants and they were informed that they can quit at any time during the experiment.
A total of 422 undergraduate students were sampled at the Southwest University in China and assessed with the Subjective Happiness Scale (SHS)
The four-item SHS has been frequently used to assess overall dispositional happiness and is appropriate for different ages, occupations and cultural groups
The experiment was performed on a 3 Tesla Siemens Tim Trio system (Siemens, Erlangen, Germany). Functional images were acquired using a single-shot, gradient-recalled echo planar imaging sequence (TR = 2000 ms, TE = 30 ms, flip angle = 90°, 32 axial slices, FOV = 192 mm×192 mm, acquisition matrix = 64×64, slice thickness = 3 mm, with 1 mm gap, voxel size = 3 mm×3 mm×4 mm). For each participant, a total of 8 minutes of resting data were acquired. Participants were instructed to simply rest with their eyes closed, not to think of anything in particular, and not to fall asleep. To minimize head motion, participants' heads were restricted with foam cushions. For spatial normalization and localization, high-resolution T1-weighted anatomical images were also acquired in sagittal orientation using a 3D magnetization prepared rapid gradient-echo (MPRAGE) sequence (176 slices, TR = 1900 ms, TE = 2.53 ms, flip angle = 9°, resolution = 256×256, and voxel size = 1 mm×1 mm×1 mm) on each participant.
Following the scan, participants' affective states were assessed with the Positive and Negative Affect Schedule (PANAS)
Data were preprocessed using DPARSF (Data Processing Assistant and Resting-State FMRI, version 2.2)
To further rule out the residual effect of motion on ReHo, volume-level mean framewise displacement (FD) was computed
For each individual, a ReHo map was generated using REST (Resting state fMRI data analysis toolkit, version 1.8)
One-sample t-tests were first performed within each group to detect where the standardized KCC values were larger than the global mean KCC. The results were false discovery rate (FDR) corrected at
To examine the group differences in ReHo, voxel-wised two-sample t-tests were performed on ReHo maps using the statistical program in the REST toolkit. Mean FD and gender were included as nuisance covariates to remove the residual effect of motion and gender effect at the group level. Following previous studies
To explore whether ReHo correlates with the level of happiness, Pearson's correlations between the mean ReHo extracted from regions showing significant group differences and the subjective happiness scores (SHS and CHI) were computed for happy and unhappy group, respectively.
The demographical and neuropsychological results were presented in
Happy group ( |
Unhappy group ( |
||
Age (mean, |
20.16(1.34) | 20.36(1.38) | |
Gender (male/female) | 6/19 | 8/17 | |
SHS (mean, |
6.51(0.29) | 3.96(0.45) | |
PA (mean, |
33.96(3.9) | 29.84(5.21) | |
NA (mean, |
18.00(3.74) | 20.80(5.21) | |
CHI (mean, |
2.621(0.33)( |
2.21(0.30)( |
The mean ReHo maps for the happy and unhappy group are shown in
The t statistical maps for the Happy (top panel) and the Unhappy (bottom panel) group are presented (one-sample t-test,
The two-sample t-tests results indicated that several regions were implicated in the significant group differences in ReHo (
Anatomical region | Side | BAs | MNI | Voxel Size (voxels) | Peak T-value | ||
x | y | z | |||||
Medial Prefrontal Cortex | B | 10 | −6 | 72 | 6 | 39 | 3.39 |
Ventrolateral Prefrontal Cortex | R | 45 | 48 | 42 | −3 | 20 | 4.33 |
Posterior Cingulate Cortex/Retrosplenial Cortex | L | 18 | −9 | −57 | 3 | 19 | 3.25 |
Superior Temporal Gyrus | R | 22 | 51 | −57 | 12 | 22 | 3.55 |
Parahippocampa Gyrus | R | 18 | −12 | −18 | 25 | 3.86 | |
Hippocampus | L | −15 | −9 | −12 | 18 | 3.52 | |
Superior Frontal Gyrus | R | 9 | 21 | 36 | 48 | 23 | −3.53 |
Dorsolateral Prefrontal Cortex | L | 46 | −30 | 51 | 24 | 45 | −4.47 |
Middle Cingulate Gyrus | R | 6 | 15 | 0 | 51 | 120 | −5.77 |
Putamen | R | 27 | 9 | −6 | 23 | −3.4 | |
Thalamus | L | −6 | −21 | 15 | 41 | −4.27 |
For the happy group, there was a significant negative correlation between the CHI and the mean ReHo within the left TH (
The present study used resting-state fMRI approach to examine whether local functional homogeneity was modulated by happiness. Significant group differences in ReHo were observed within distributed brain regions over prefrontal cortex, temporal lobe, limbic system and certain subcortical regions, suggesting multiple brain regions were involved in trait happiness. In addition, the ReHo within left thalamus was correlated with CHI in happy group, suggesting that ReHo could be useful for indexing the extent of happiness. Recently, extensive studies have reported altered ReHo in clinical populations with emotional disorders, such as depression
ReHo measures the local synchronization of a given voxel with its nearest neighbors based on the assumption that if a brain region is responsible for a specific function, the voxels within this region were more temporally homogeneous when involved in that function
Compared to happy individuals, unhappy individuals exhibited decreased ReHo within MPFC, VLPFC, medial temporal lobe (MTL), STG, and PCC/RSP cortex. These regions largely overlap the DMN, suggesting the involvement of DMN in happiness. This is supported by the reports that ReHo was altered within the DMN in depression
Happy and unhappy individuals have their own characteristic temperaments. Individuals with lower level of happiness spent more time ruminating on negative feelings, thoughts and shortcomings
Converging evidence from lesion, neuroimaging and electrophysiological data supports the view that the prefrontal cortex (PFC) is a key component of the circuitry that implements both positive and negative affect
In contrast, higher ReHo was found within left DLPFC for unhappy individuals. One methodological limitation of the ReHo approach is that its biological significance is still unclear, thus it is difficult to interpret the exact meaning of the opposite effects observed within MPFC and DLPFC
Besides the DLPFC, the right MCG, right putamen, and left thalamus also exhibited increased ReHo in unhappy individuals. Furthermore, the ReHo score within the left thalamus is negatively correlated with CHI scores in happy group. Previous personality neuroimaging studies demonstrated that the volume of MCG was correlated with neuroticism
Both the striatum
Although the current results provided novel information on our understanding of happiness, several limitations need to be considered. First of all, general trait happiness instead of a specific type of happiness was assessed in the current study. As happiness can be divided into hedonic (pleasure attainment and pain avoidance) and eudaimonic components (meaning and self-realization)
In summary, we used a ReHo approach to investigate the differences in intrinsic brain activities between happy and unhappy individuals. We found that the local synchronization of intrinsic brain activities was altered in unhappy individuals within prefrontal cortex, temporal lobe, limbic system, and subcortical regions. These regions overlapped with the previously identified DMN, which suggests that DMN plays an important role in subjective happiness and is in support of the construal theory. Our findings also provide further evidence to support that core components of the emotional and rewarding network are involved in happiness.