1. Introduction
To relieve negative emotions and stress in our daily lives, we employ various strategies. For example, when feeling depressed after making a mistake at work, we might try to reset by thinking that we have learned something from this failure. This thought style is an emotion regulation (ER) strategy called “positive reappraisal”. ER includes all the processes whereby individuals influence their emotions, as well as when and how they experience and express these emotions [
1]. ER consists of extrinsic and intrinsic processes responsible for monitoring, evaluating, and modifying emotional reactions, especially their intensive and temporal features, to accomplish goals [
2]. It involves changes in the latency, rise time, magnitude, duration, and offset of responses in the behavioral, experiential, and physiological domains [
1]. Negative emotions are related to stress [
3], so this study focused on stress reduction in ER.
ER can be strongly influenced by motivational factors, as well as other cognitive domains [
4]. For example, exercises that participants choose to perform [
5], memories that participants choose to memorize [
6], and stopwatches that participants choose to use for time measuring [
7] produce better results than same strategies when participants are forced to perform, memorize, or use them. This effect of self-choice can be explained in terms of self-determination theory [
8], which argues that the need for autonomy is essential for facilitating growth, which is the basis for self-motivation [
9]. Therefore, self-choice enhances intrinsic motivation and cognitive performance [
8]. One of the factors affecting ER choice, which involves whether ER should be performed and the strategy to adopt, is motivation. A systematic review indicated that the goal of ER (i.e., whether to decrease, increase, or avoid the emotion) motivates ER choice [
10].
Recent studies have revealed a positive effect of self-choice on ER. For instance, choosing to inhibit or feel negative emotions can reduce negative emotions better than being forced to inhibit or feel negative emotions after seeing certain images [
11]. The authors interpreted this result to mean that the act of voluntarily choosing whether to feel passively or to inhibit facilitated the effect of experienced emotions. Under daily life stress, the effects of the promotion of self-choice ER can be observed. However, previous studies have focused only on the choice of ER performance.
The framework of studies on ER choice involves choosing which strategy to use, namely reappraisal or distraction. Five families of ER strategies were outlined in [
12], including cognitive change and attentional deployment. Attentional deployment comprises distraction and rumination, and cognitive change comprises cognitive reappraisal and acceptance [
12]. In [
13], when the ER goal was “to feel better”, distraction was chosen over reappraisal, unlike when the ER goal was “to perform better” during negative feedback with respect to work situations. Some studies focusing on the intensity of stimuli have shown that people tend to choose reappraisal rather than distraction when seeing low-intensity pictures compared to high-intensity pictures [
14,
15]. Reappraisal affordance also predicts the choice preference for reappraisal over distraction [
16]. Moreover, people who tend to choose reappraisal over distraction exhibit high levels of resilience and well-being [
17]. However, if people are in conflict, reappraisal is often not chosen among the various available strategies [
18]. From these perspectives, two ER choices, namely reappraisal or distraction, are insufficient. Motivation is enhanced when there are three to five alternatives [
8]; therefore, the situation of choosing among five ER strategies depending on the place may maximize the ER effect. This is an important and innovative aspect of this study. We also focused on the cognitive aspect of ER because of its adaptiveness [
19].
Numerous neuroimaging studies have been conducted on ER. The inferior frontal gyrus (IFG) is involved in enhancing the effect of ER during ordinary emotion downregulation because its activity reflects the inhibition of motor responses associated with emotional reactivity [
20]. The authors of [
20] reported that increasing activity in the superior frontal gyrus indirectly leads to decreased activity in the amygdala. In addition, the supplementary motor area (SMA) is involved in the implementation of an internal model to compute the value of emotional regulatory actions and guide behavior [
21]. When negative emotions are downregulated by reappraisal, activation of the lateral temporal cortex and modulation of the bilateral amygdala occur [
22]. The insula is also involved in the ER network [
23].
Adding a self-choice perspective, that is, when participants chose to inhibit their emotions, caused stronger activation in the dorsomedial prefrontal cortex than when they were forced to do so [
11]. Refs [
24,
25] argued that self-choice becomes rewarding, per se, because choosing by oneself can induce one’s confidence. Neuroimaging studies such as that reported in [
26] have shown that the caudate, which is involved in reward processing, is related to self-choice. Furthermore, the ventromedial prefrontal cortex (vmPFC) plays an important role in promoting the cognitive performance of self-determined choices [
7]. However, no study has compared the brain activation associated with performing a chosen ER strategy among multiple alternatives to that associated with performing a forced strategy. However, since the inhibition of amygdala activation by the vmPFC leads to successful ER [
27], self-choice may encourage this effect. If self-choice ER leads to specific brain activation with better stress reduction than forced ER, we suggest that self-choice ER among multiple options is superior to forced ER for mental health. Therefore, we aimed to uncover the neural basis for self-choice ER among multiple ER strategy alternatives.
In this study, we focused on two issues uncovered in self-choice ER. First, we aimed to clarify whether the ER strategy, which was selected by the participants themselves from multiple options, reduced stress levels better than forced strategies. Second, we aimed to elucidate the neural basis of this effect. In this study, we employed cognitive ER strategies [
19] for self-choice and forced ER. We expected that a self-choice ER strategy would reduce stress, which is evoked by daily stressful situations, better than forced ER because of the self-choice effect [
8]. In terms of brain activation, we expected that the left caudate would be activated when people performed self-choice ER because self-choice contains inherent rewards [
28], as well as the activation of brain areas related to ER, including the IFG, vmPFC, and SMA.
4. Discussion
This study examined the effect of self-choice ER on stress and its underlying brain mechanisms. This study has four main findings. First, at the behavioral level, self-choice ER reduced stress better than forced ER, supporting our expectation. Secondly, we found shared brain activation between self-choice and forced ER, including in the left opercular part of the IFG. Thirdly, we found that weaker activation of the left opercular part of the IFG and SMA was associated with higher stress reduction only in self-choice ER. Fourthly, we observed significant caudate brain activation when choosing the ER strategy. The results of the brain imaging analysis were partly consistent with our expectations. The reason for this partial consistency might have occurred, to some degree, because of a carry-over of expectation.
Participants who chose ER strategies reduced stress better than those who could not choose strategies in our task. Our findings are consistent with those of previous studies, such as [
11]. According to [
11], if we choose to inhibit negative emotions and perform inhibition when we can choose either to perform inhibition or do nothing, better regulation occurs than under forced inhibition. Our results correspond to previous work because self-choice ER resulted in a better reduction than forced ER. This result might also support the effect of flexible ER, which is defined as the ability to effectively regulate emotions by applying different ER strategies (chosen from a broad repertoire) in different situations depending on their features, including context-related features—in other words, being adaptive to situation characteristics, as reflected in one’s strategy choice [
35]. Because motivation is considered a function of the expectation of success [
36], an expectation of the successful effect of ER strategies could reduce stress. Specifically, revealing that self-choice ER, chosen among multiple alternatives, has a larger effect than forced ER is a novel finding of this study. We used a single-item measure for stress in this study. The single-item measure is well-suited for fMRI tasks because fMRI studies need a regularity of time and a short duration for participants to answer questions in experiments [
37]. Therefore, several previous fMRI studies of self-choice ER have used a single-item stress measure. Nevertheless, the validity and reliability of this scale requires further consideration.
While performing self-choice ER, we observed significant activation of the SMA, IFG, MTG, and caudate. This result was consistent with that of a previous study (e.g., [
23]). During forced ER, we observed significant activation of the SMA, middle frontal gyrus, IFG, MTG, and fusiform gyrus.
Conjunction analysis showed that the left opercular part of the IFG was activated during both self-choice and forced ER, suggesting that the left IFG is important for ER, which is consistent with [
38], the authors of which considered that the association of the left IFG with the MTG was due to the habitual use of reappraisal, supporting the goal-driven control of subjective emotion. The authors also reported that the left IFG participates in emotional performance monitoring. During self-choice or forced ER strategies, participants actively regulated the induced negative emotions, including by monitoring their own emotions. Focusing on one’s own emotions might have evoked significant activity in the IFG in both the choice and forced conditions. We also found significant activation of the caudate in both conditions. Our findings are consistent with those of [
39], the authors of which reported that the caudate was involved in recovery from discomfort. Our task described daily stressful situations, and we asked participants to regulate the negative emotions evoked by the scenarios. Significant activation of the caudate in the choice and forced conditions may reflect the process of evoking and regulating negative feelings. In addition, we observed significant activation of the SMA. Our results support the findings of [
23], which showed SMA activation regardless of the type of regulation strategy used. These common areas of activation are located in the left hemisphere. Supporting the ER brain network in the left part plays the role of reappraising negative emotions and language processing [
40]; thus, these results suggest that performing cognitive ER involves significant thinking with language by repeating strategies and reappraising circumstances, which people face positively, regardless of strategy
Our correlation analyses revealed that when stress was substantially reduced, the IFG and SMA were less activated. This result may be explained by neural efficiency, whereby performance improvements are associated with decreased brain activation [
41]. The IFG plays an important role in performing ER [
38]. Given that the IFG and SMA are involved in ER, those who could self-choose ER and reduce their stress levels required less activation of the regions because of their sophistication in ER.
Nevertheless, our findings do not fully support our prediction. We hypothesized that some specific regions would be involved in self-choice ER. However, we did not find significant differences in brain activation between the choice and forced conditions. Although the results were uncorrected, we found that the supramarginal gyrus and superior parietal lobule showed greater activation in the choice condition than in the forced condition. The supramarginal gyrus and superior parietal lobule are associated with visual and spatial attention [
42,
43]; thus, this might indicate stronger attention to the task. Activation of the superior parietal lobule when choosing whether to perform ER has also been reported (e.g., [
33]). Accordingly, the specific activation of the supramarginal gyrus and inferior parietal lobule during the choice condition indicated that participants might have paid more attention to the stressful situation, driving them to perform self-choice ER strategies rather than forced ER strategies.
In contrast, the fusiform gyrus and calcarine cortex showed greater activation in the forced condition than in the choice condition. This might be due to differences in the strategies used. In the forced condition, participants were often compelled to use the “refocus on planning” strategy. On the other hand, in the choice condition, “refocus on planning” was chosen on 36% of occasions. Thus, the fusiform gyrus, which is related to visual processing of language [
44], and the calcarine cortex, which is related to visual images [
45], could be related to the strategy of refocus on planning.
In addition, we observed significant brain activation in the caudate, frontal pole, lingual gyrus, right superior frontal gyrus, left middle frontal gyrus, and cuneus during ER choice. The caudate codes for expected reward magnitude [
46]. In the context of our study, given the self-determination theory, self-choice ER, itself, seems to include a reward process. In the choice condition, participants could choose one strategy to regulate their negative emotions. The self-determination theory argues that deciding what to do based on one’s own thoughts is rewarding. The activation of the caudate during the choice condition might reflect the expectation of a reward, which includes personally choosing what to do and succeeding in stress reduction. Other activated regions were included in the neural network of cognitive ER [
47]. Accordingly, the activation of these regions reflected the consideration of strategies with the expectation of successful stress reduction. This result indicates that when people face situations of interpersonal stress in daily life, they carefully consider which strategy is appropriate, with the expectation of self-choice strategy success.
Our study has eight main limitations. First, we aimed to reveal the effect of self-choice ER on stress reduction in daily stressful situations; therefore, we employed previously developed scenarios [
29]. It is possible that we did not evoke stress in the participants’ own experiences. This is still insufficient because these stresses are not realistic enough, owing to the use of imagination in each stressful scenario. However, several reports have indicated that reappraisal after distress and negative emotions is negatively correlated with daily life [
48]. Future research should uncover whether the effect of self-choice on stress reduction is enhanced when participants perform self-choice ER strategies in their daily stressful situations. Secondly, we only considered the immediate effect of self-choice ER. However, the regulatory effect has been shown to last for ≥1 week in the ventromedial prefrontal cortex [
49]. Therefore, a longitudinal fMRI study is required to elucidate the long-term effects of self-choice ER. Thirdly, we examined the effect of self-choice ER on healthy university students. We excluded university students who had depressive symptoms, so we could only generalize our results to non-depressed undergraduates. Future studies should examine whether self-choice ER strategies reduce stress in both clinical and subclinical populations. For example, people with subclinical paranoia used the cognitive ER strategies of blaming others and catastrophizing habitually, and paranoia scores were predicted by self-blaming [
50]. Thus, the enhancing effect of self-choice ER may be different between subclinical and healthy people. Other symptoms such as anxiety are also related to ER according to previous research. For example, anxiety is predicted by less use of positive reappraisal [
51]. Therefore, individual differences in anxiety tendency could be considered in future studies. Fourthly, we could not identify the specific activation in the choice and forced conditions because we did not find a significant difference in brain activation between these conditions. Our behavioral results showed significant expectations of the effectiveness of stress reduction only in self-choice ER. Although we used a within-subject experimental design and counterbalanced it, this expectation might be a carryover from the choice to the forced condition. A between-subject design could help detect differences in brain activation between self-choice and forced strategies. Moreover, the proportions of “refocus on planning” strategies used were so discrepant that the difference between self-choice and forced ER might not be displayed in brain imaging results. Reappraisal and distraction are both cognitive ER; however, their neural bases differ. For example, distraction decreases amygdala activation more than reappraisal [
52], and the orbitofrontal cortex is especially activated during reappraisal [
53]. From these perspectives, in our study, the proportion of strategies used differed significantly between the choice and forced conditions such that the brain imaging contrast of each condition might have been reduced. We only asked about the participants’ subjective sense of accomplishment in performing ER; therefore, the difference in the ability to perform ER between participants might have also influenced the results. The proportion of ER strategies used should be equalized, and an objective ER ability should be obtained in future studies. Fifthly, in this study, we revealed the effect of self-choice ER but could not focus on the choice of the most appropriate strategy for each situation. Future studies are needed to examine differences in effects between good and not-good choices or which individual differences lead to the skills associated with choosing optimal ER strategies depending on circumstances. Sixthly, we did not ask participants in the control condition if they really did nothing. We also did not ask participants in the forced condition if they performed a forced emotion regulation strategy adequately. Such a manipulation check is needed in future studies. Seventhly, we could not consider the probability of consistency in choosing strategies. Some studies have pointed out that emotion regulation is affected by a belief in the malleability of emotions and that this tendency is observed in children (e.g., [
54]). Such participant beliefs might influence whether self-choice emotion regulation becomes flexible or rigid; therefore, this belief should be taken into account in future studies. Finally, we only tested stress. In daily life, we have to regulate various negative emotions, such as anger and sadness. Therefore, in future research, whether self-choice strategies are effective for such negative emotions should be examined.