In comparing the two models, Model 2, where curiosity serves as the mediator, emerges as the preferred model. Intriguingly, this model also reveals a significant mediating effect. Since Curiosity and Fatigue are negatively correlated, using Curiosity as the mediator between Fatigue and FA-BHQ enhances the negative impact of Fatigue on FA-BHQ. The findings of this study expand our understanding of the complex relationship between curiosity, fatigue, and brain health. While prior research has extensively focused on fatigue’s detrimental effects on brain health in the way of reducing FA5 and increased mean diffusivity in white matter tracts10. This study not only proved the negative impact of fatigue towards brain but also uniquely highlights the positive role of curiosity in preserving and enhancing FA-BHQ.
Reassessing the significance of curiosity
In the face of the challenges posed by an aging population, especially in developed societies, the increasing longevity of individuals has elevated the importance of maintaining brain health in later years. The quest for a healthier brain and, by extension, a healthier life has become a global imperative. Understanding the lifestyle factors that contribute to improved brain health is crucial in addressing the multifaceted aspects of aging and promoting overall well-being. While past literature has extensively explored the connection between fatigue and FA-BHQ, the relationship between curiosity and its potential impact on FA or other indicators of brain health has received comparatively less attention. This research adds to the growing body of evidence of how curiosity may influence the structure of the brain by starting up with correlational research.
This research has revealed compelling evidence pointing to a positive correlation between curiosity and brain structure, as measured by FA-BHQ. This finding suggests that individuals who exhibit higher levels of curiosity tend to have healthier brain structures. Conversely, a negative correlation was observed between FA-BHQ and fatigue, indicating that increased fatigue is associated with less optimal brain health. Notably, there is a greater number of brain regions that are partially positively correlated with Curiosity, while controlling for age, sex, and GM-BHQ. This suggests that, based on the MRI data, curiosity plays a significant role in positively influencing the health of more brain regions while controlling the effect of age, sex and GM-BHQ. The intricate interplay between curiosity, fatigue, and brain health highlights the complexity of the factors influencing cognitive well-being.
Based on the regression results presented in Table 7, a significant negative correlation between curiosity and fatigue is evident (r = -0.151, p < 0.01). This finding implies that as curiosity increases, the level of fatigue in individuals tends to decrease. Higher curiosity level of an individual appears to be associated with a more active and driven lifestyle, fostering a greater inclination to explore the surrounding world. This finding also aligns with our expectations and is consistent with previous literature. For instance, Shen et al.46 discovered that fatigue is likely to reduce the exploration behavior of the individual. Valji et al.’s11 study linked curiosity to neural mechanisms involving reward processing and motivational behavior. Moreover, the finding that curiosity mitigates fatigue’s impact on FA-BHQ supports Garrosa et al.’s 201712 research showing curiosity’s protective effect against emotional exhaustion and disengagement.
Traditionally, literature has predominantly emphasized the link between fatigue and brain health, relegating curiosity to a secondary role. However, the findings of this study challenge this prevailing notion. Contrary to expectations, curiosity emerges as a potentially more influential factor concerning FA. This novel implication prompts a paradigm shift in prioritizing the exploration of curiosity’s impact on brain health, offering a fresh perspective for future research endeavors in the domain of neuroscience and cognitive health.
Implication behind fatigue and brain health
While the negative relationship between fatigue and FA-BHQ is established, the strength of this correlation intensifies when considering fatigue as a mediator in the model. As illustrated in Table 7, the direct effect between curiosity and FA-BHQ is 0.31 with p value < 0.05, but it increases to 0.36 when fatigue is included into the model. This result suggests that even when fatigue is considered, the influence of curiosity on maintaining a higher level of FA-BHQ is more pronounced, emphasizing the significant impact of curiosity on brain health.
Components of FA-BHQ
This study also showed that the FA-BHQ values of the corpus callosum, internal capsule, fornix, posterior thalamic radiation are significantly correlated with fatigue while corpus callosum, fornix, internal capsule, corona radiata, external capsule, cingulum and posterior thalamic radiation are significantly correlated with curiosity given control for sex, age and GM-BHQ.
The potential mechanism happening in the brain function could be considered as follows: Firstly, the corpus callosum serves as the bridge connecting the left and right hemispheres of the brain, facilitating the exchange of information between the two halves. Recent studies suggest that mental fatigue is becoming more commonly associated with the deviated reorganization of functional connectivity among different brain regions47; Secondly, the internal capsule (IC) which has a subcortical white matter structure with a location in the inferomedial portion of each cerebral hemisphere48. IC is part of limbic-thalamo-cortical circuitry which have been linked to impaired top-down emotion regulation systems in PTSD49,50. Additionally, the IC serves as a pathway to the thalamus, which, in turn, has white matter pathways connecting with the amygdala51,52. The research conducted by Grillon, et al., in 201553 suggests that engaging in cognitive tasks and experiencing subsequent mental fatigue can compromise the ability to regulate emotions. This result implies that mental fatigue is essential in emotion regulation. Next, posterior thalamic radiations, which is part of Thalamocortical radiations, connect the thalamus to the cerebral cortex. The thalamus serves as the central relay center of the brain, forwarding all sensory information, except for olfaction, to the cerebral cortex for additional processing54. Individuals with multiple sclerosis who exhibit high scores in sensory sensitivity also tend to experience greater levels of cognitive fatigue and a lower quality of life55. Lastly, fornix, which is the principal axonal tract of the hippocampus, has a connection between the hippocampus and various modulatory subcortical structures. Research indicates that the fornix plays a crucial role in memory formation by serving as a pathway for theta rhythms and acetylcholine56. In the meanwhile, research has shown that physical fatigue from a running exercise has negative effect on short-term memory57. Building on the aforementioned information, the relationship between FA-BHQ and fatigue becomes evident. When individuals experience physical or mental exhaustion, the brain’s ability to process information, especially in relation to complex tasks and the control of facial muscles, becomes challenging. This finding aligns with well-documented observations in the literature, emphasizing how fatigue adversely affects cognitive processes such as attention, memory, and decision-making.
Moving on to the curiosity side, we observed that there are seven brain parts that are positively correlated with curiosity, namely, corpus callosum, fornix, internal capsule, corona radiata, external capsule, cingulum and posterior thalamic radiation. Firstly, fornix, which contributes mnemonic representations to deep brain structures, is also influencing motivated behaviors56. Research has shown that curiosity naturally emerges in tandem with cognition and motivation and serves as a motivator for cognitive processes58. Secondly, corona radiata is a part of the limbic thalamo cortical circuits, encompassing thalamic projections from the internal capsule. The pathway connects the thalamus and the amygdala to the cortex51,52, specifically involving the prefrontal cortex gray matter areas associated with top-down emotion regulation systems59.In this way, the corona radiata has a function in emotional regulation processes60. In the research conducted by Leonard, N. H., & Harvey, M61. in 2007 has shown a close relationship between absorption curiosity and attention to emotions, clarity of emotions, and repair of emotions which are known as parts of emotional regulation process. Thirdly, the external capsule, situated between the putamen and claustrum, constitutes a gathering of white matter fibers. However, its structural organization and precise functional role are not fully understood at present62. Within the external capsule are striatal fibers, facilitating connections such as those between the primary sensorimotor cortex and the putamen, as well as between the supplementary motor area and the caudate nucleus63. As a result, it is believed that the external capsule plays a crucial role as a connection between cortical motor regions and the basal ganglia, contributing to the engagement of the basal ganglia in motor control. The basal ganglia participate in various parallel and functionally segregated cortical-subcortical circuits, supporting diverse sensorimotor, cognitive, and emotional-motivational brain functions. Litman15, has proposed an integrative I/D/wanting-liking model to prove that the nature of curiosity as an emotional-motivational state. Next, the cingulum bundle is a pathway through which the cingulate cortex sends projections to both the amygdala and prefrontal cortex. This neural pathway extends from the prefrontal cortex to the entire medial collateral limbic complex, including the parahippocampal region64. Recent research found that curiosity is triggered by substantial prediction errors that are assessed or evaluated. This cycle allows memory encoding by heightening attention, exploration, and the pursuit of information. Moreover, it strengthens the consolidation of information obtained during a state of curiosity through dopaminergic neuromodulation of the hippocampus which comprises parahippocampal region65. Moving to internal capsule, as abovementioned, it is part of limbic-thalamo-cortical circuitry that is associated with impaired top-down emotion regulation systems in PTSD. In the past literature, it has been proven that curiosity has a close relationship with emotional regulation. As during the search process, the individual is likely to experience both pleasure and pain and it is important for the individual to manage those feeling to navigate a pathway through the process66. Lastly, the relationship between the corpus callosum and curiosity lacks direct support due to the limited research on the brain mechanisms associated with curiosity. However, previous studies have established a positive correlation between openness and white matter integrity in the splenium of the corpus callosum67. Additionally, research by Tan et al.68 demonstrated a significant positive correlation between openness and curiosity. Consequently, we can conclude that, under the fMRI level investigation, there is a positive correlation between white matter integrity in the splenium and curiosity, mediated by the trait of openness. This conclusion is consistent with our findings.
Limitation
The limitations of this research contain four aspects. Firstly, the sample size and participant characteristics, as well as the analytical methods used, pose potential constraints. While our study included data from 578 healthy Japanese participants with an age range of 20 to 66, which is relatively large compared to some studies in this field, but the sample size still limits the generalizability of our findings. To establish the universality of our results, it would be better to include a more diverse participant pool with varying cultural backgrounds. Additionally, the use of cross-sectional data in this research prevents us from establishing causal relationships and understanding how the relationships between variables may change over time. Future research should aim to resolve these limitations by expanding the participant pool and employing diverse analytical approaches to further explore and confirm the identified relationships.
Secondly, the limitation of selection of the control variables used in the research might reduce the reproducibility of the results of this study. While we incorporated sex, age, GM-BHQ, and experiment venues as control variables, the exclusion of factors like income level, marital status, and knowledge level could potentially affect the robustness and social implications of our findings. Future research should consider expanding the set of control variables to enhance the comprehensiveness and applicability of the study.
Thirdly, the insufficient evidence in past literature hinders the validation of our findings. In our research we found corpus callosum is positively related with curiosity with statistical significance. However, as mentioned above, direct support for this finding in existing literature is lacking. Although we derive a conclusion through mediating factor of openness, the relationship between curiosity and corpus callosum as well as the brain mechanism on curiosity still worth exploration in future studies. This exploration is essential to substantiate and enhance the understanding of our observed results.
Next, the absence of D-type curiosity in our research framework limits the scope of our investigation into the multifaceted nature of curiosity. D-type curiosity, as described by Litman15,16, represents a drive to resolve specific informational gaps, often accompanied by a sense of deprivation or urgency. While our findings offer valuable insights into the relationship between curiosity and brain health, they are confined to the enjoyment-driven, exploratory aspects of curiosity measured by the CEI-II. This limitation restricts our ability to examine the broader dimensions of curiosity, particularly the unique ways D-type curiosity might impact brain function, cognitive persistence, or the resolution of uncertainty. Future research should incorporate measures specifically designed to assess D-type curiosity, such as the Curiosity as a Feeling-of-Deprivation (CFD) scales, to provide a more comprehensive understanding of how both dimensions of curiosity influence brain health and cognitive processes.
Lastly, it worths to highlight that the outcome of this study was based on a classical approach for mediation analysis. There are several limitations on the classic approach such as ignoring the confounding factor between mediator and outcome variables. Therefore, in future work, the researchers should adopt Causal Mediation Analysis or Counterfactual-based Mediation Analysis to further validate and refine these findings.
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