All methods were performed in accordance with the relevant guidelines and regulations. Detailed information about the experiment is described below.
Participants
57 undergraduates (46 male and 11 female) were collected from a university in Nanjing by posting advertisements on campus and distributing advertisements in class, the sample size of this experiment28,29,30,31. The average age of the participants was 20.23 ± 1.30 (M ± SD) years. All the participants were right-handed, had normal vision or corrected vision, had no color blindness or color weakness, had no brain trauma or mental illness and were native Chinese speakers. All the participants volunteered to take part in this experiment. They signed the informed consent before the experiment. During t collecting EEG, if any physical discomfort occurs to the participants, they can stop the experiment at any time, and they can get the 70 yuan after completing the experiment. This experiment was approved by the ethics committee of Soochow University and met the ethical requirements.
Materials
Emotional face images
A total of 35 emotional images from the NimStim Set of Facial Expressions under the pleasant dimension were selected32, of which 4 were used for the practice experiment, and 31 for the formal experiment. Langeslag et al.33 confirmed that when the images in the NimStim Set of Facial Expressions were selected for emotional priming, the images with "open mouth" attracted more attention from individuals than the images with "shut mouth" and had a better effect in arousing emotions. Therefore, the emotional images selected in this experiment are all "open mouth" images.
In order to ensure that the selected images can effectively stimulate the participants' pleasant experience, before the formal experiment, 31 participants were recruited from a university in Suzhou to rate all the happy and open mouth images in the NimStim Set of Facial Expressions on a 9-point scale in terms of arousal and valence. Arousal was rated on a scale of 1 to 9, from extremely calm (1) to extremely uncalm (9). The ratings were rated on a scale of 1 to 9, from extremely unpleasant (1) to extremely pleasant (9). Finally, 15 female images with the highest valence (model No.: 1, 2, 3, 5, 6, 7, 8, 9, 11, 13, 14, 16, 17, 18, 19) and 16 male images (model No.:22, 23, 24, 25, 27, 28, 30, 31, 33, 34, 35, 36, 39, 40, 41, 43) for formal experiments. 2 female images (model No: 12, 15) and 2 male images (model No: 32, 43) were used for the practice experiment. There was no significant difference in the average valence and arousal of the 31 images used in the formal experiment, 7.07 ± 0.29 (M ± SD) and 6.32 ± 0.51 (M ± SD).
Chinese compound remote associate problems (CCRA)
The CCRA used in this experiment was compiled by Shen et al.34. A total of 128 items were included, of which 4 were used for practice experiments, and 124 were used for formal experiments. Each item includes three Chinese characters (e.g., 命/男/学; life/boy/student in English), and participants are asked to think of another character (e.g., 生; student in English) that can be combined with the three characters in the question to form three words (e.g., 生命/男生/学生; life/boy/student in English). Each question has one and only one answer.
In this study, material types were divided according to the third-party evaluation method35, and indexes such as problem-solving time, insight experience report rate, deadlock experience evaluation, and average difficulty of each problem were strictly controlled and matched. In the end, the 124 formal experiments included 64 that had an objective insight and 60 that did not; 55 objective deadlock, 69 objective no deadlock; 62 hard, 62 easy. CCRA is often used to assess convergent creativity and is a good tool to measure convergent creativity in college students36.
Positive and negative affective schedule (PANAS)
In order to control the possible interference caused by the participants' emotions in their natural state21,37,38, to reduce the possible impact of emotions on convergent creativity mentioned in the literature review, the PANAS was used to evaluate the participants' emotions before the experiment39,40. The scale includes two subscales, PA and NA, in which PA measures the participants' positive emotions, such as interested, enthusiastic, etc., in a total of 10 items. NA measures participants' negative emotions, such as nervousness and panic, in a total of 10 items. Each item was scored on a Likert 5-point scale from 1 (not at all) to 5 (extremely)39,40. In this experiment, the internal consistency coefficients of the full scale, PA and NA were 0.86, 0.75, and 0.92, respectively.
Emotion regulation questionnaire (ERQ)
In order to control the influence of the emotion regulation strategies used by the participants, the ERQ compiled by Gross and John11 was used to measure the participants' tendency to use emotion regulation strategies in daily life. The scale was divided into two dimensions: cognitive reappraisal and expressive suppression. The cognitive reappraisal scale included 6 items, such as "When I want to feel more positive emotions, I will change my way of thinking about the situation." The expressive suppression subscale includes 4 items, such as "When I feel negative emotions, I make sure I do not show them." Likert 7-point was used for each item, from 1 (strongly disagree) to 7 (strongly agree). In this experiment, the internal consistency coefficients of the full scale, cognitive reappraisal scale, and expressive suppression scale were 0.65, 0.80, and 0.64, respectively.
Procedure
In this study, a single factor and three levels (emotion regulation strategies: cognitive reappraisal, expression suppression, and control group) were used. Referring to previous studies41,42, the main meanings of the three emotion regulation strategies are as follows. Cognitive reappraisal: Please look at the following images with your heart and keep an objective attitude, imagining that they are all did by Photoshop and not real photos, and do not try to feel any emotions. In other words, the participants were asked to objectively look at the facial expression images, remove themselves from the emotion expressed by the facial expression, and reinterpret it. Expressive suppression: Please pay attention to the images below, while feeling emotions, do not show your feelings, try to hide your emotions, do not let others see your emotional experience. In other words, the participants were required to suppress and hide their emotions, not show them on their faces, and try to keep their face expressionless, so that others could not judge their true emotional feelings through facial expressions. Control group (free viewing): Please pay attention to the following images, that is, ask the participants to pay attention and carefully watch the images of emotional faces to be presented. After entering the laboratory, the participants were randomly divided into cognitive reappraisal group, expressive suppression group, and control group by Using Excel method. Before the EEG experiment, participants completed the PANAS, ERQ, and creativity self-efficacy scale and then completed the EEG experiment. The experimental program was produced and presented by E-Prime 3.0 (Psychology Software Tools, Inc., Sharpsburg, PA, USA). The specific experimental process is shown in Figure 1.
The formal experiment consisted of 124 trials, with participants completing 124 questions. The whole experiment can be divided into three stages, the first stage is the emotion regulation. At the beginning of the experiment, a fixation point ("+") appears in the center of the screen (500 ms). Subsequently, according to the random grouping before the experiment, each group was presented with the corresponding emotion regulation instructions. Next, images of happy faces were presented (1000 ms). When the images were presented, the participants were asked to adjust the emotions aroused by the images according to different commands. Then the participants need to rate their emotional experience after the images on a 9-point scale, with "1" meaning "very unpleasant" and "9" meaning "very pleasant" (1-4 indicates negative emotions, and the lower the number, the higher the experience of negative emotions; 5 indicates neutral emotion; 6-9 indicates positive emotions, and the higher the number, the higher the experience of positive emotions). Then participants go into two problem-solving stages. In the first stage, the CCRA (e.g., 台/光/电; station/light/electricity in English) will be displayed in the center of the screen, and the participants need to think of the answer as soon as possible within 20000 ms. If the answer comes to mind, they press the space on the keyboard immediately to enter the blank screen. The blank screen display time varies according to the time spent by the participants to solve the problem for the first time (5000 ms - first problem-solving time). If the participant cannot figure out the answer within the time limit, the program will automatically move on to the next question. In the evaluation stage, the correct answer to the question will be displayed in the center of the screen, and the participants need to compare the answer they think of with the correct answer. If the answer they think of is consistent with the correct answer (right), press "1"; if not (wrong), press "2". Finally, an empty screen of 600 ms to 800 ms is randomly presented. The scoring was based on Yuan's study35, but to better exclude the potential situations in which participants declare right (1) but in reality the answer was wrong (2), in the practice, the experimenter emphasized to the participants that the accuracy of your judgment in this experiment is very important for our experiment. There was no need to choose right when doing wrong in order to get a high score. In addition, during and after the practice, the experimenter emphasized this point again to ensure the accuracy of the participants' judgment of right and wrong. Finally, at the end of the whole experiment, the experimenter asked the participants again if there was any misjudgment, and if there was, we would remove the participants’ results.
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After the experiment was completed, the effectiveness of the emotion regulation strategy operation was tested43. The cognitive reappraisal group and the expression suppression group were required to complete a 9-point test question: "To what extent did you successfully adjust the emotion priming by emotional face images through cognitive reappraisal/expression suppression?" (A larger number indicates a higher degree). The control group had to answer the question: "Did you use any emotion regulation strategies throughout the experiment when you saw the images?". It takes about an hour and a half for the participants to complete the whole experiment.
In the practice experiment (including four trials), the participants will see the instructions of the experiment on the screen. In combination with the instructions, the experimenter will explain the instructions of emotional regulation in detail. In addition, the participants should understand that the experiment is to judge whether the answer they think up is consistent with the standard answer in the "right or wrong" interface, so as to know whether the answer is correct. If the participants doubt the experiment process during the first practice experiment, the experimenter will explain it patiently and conduct the second practice. Only after the participants have clarified the whole experiment process can they enter the formal experiment. The instructions, questions, prompts and judgment interface of the experimental program are presented in the center of the screen with the style "Song Style, Number 28, black, no bold".
Data recording and analysis
This study used the Vision Recorder 2.0 (Brain Product, Munich, Germany) with 64 scalp sites (10-20 system) to record the brain electrical activity. The FCz was used as the reference electrode for online recording, and the line of bilateral mastoid process was used as the average reference for offline analysis. Horizontal electrooculographies (EOGs) were recorded from an electrode placed 1 cm outer canthi of both eyes, while vertical EOGs were recorded from an electrode placed 1.5 cm below the left eye and were grounded on the forehead. The electrodes impedance was kept below 5 kΩ. Electroencephalogram signals were collected within 0.01–100 Hz, sampled at a rate of 500 Hz. Averaging of ERPs was computed offline with Vision Analyzer 2.0 (Brain Product, Munich, Germany). Trials with EOG artifacts (mean EOG voltage exceeding ± 100 μV) were excluded from averaging.
Firstly, the averaged of ERP induced by emotional stimulus in different emotion regulation strategy groups was analyzed, and the averaged for ERP was 1200 ms, including a 200 ms baseline (200ms before emotional images were presented). According to previous studies8,28,44 and the EEG topographic map and ERP waveform of this study, the analysis time window of electrode points and ERP components was selected. Specifically, CP1, CP2, C3, C4, FC3, and FC4 were selected to analyze the mean amplitude of N1 components during 90-130 ms of emotional stimulus initiation. The mean amplitude of P2 was analyzed at FC3, FCz, FC4, C3, Cz, C4, CP3, CPz, and CP4 (140-200 ms). The mean amplitude of LPP components were C1, Cz, C2, CP5, CPz, CP6, P3, Pz, and P4 (300-600 ms and 600-1000 ms). Then, the averaged of ERP by problem stimulus was analyzed, and the averaged for ERP was 2200 ms (200ms before CCRA presentation), including a 200ms baseline. According to previous studies45,46,47 and the EEG topographic map and ERP waveform of this study, the analysis time window of electrode points and ERP components was selected. Specifically, FC3, FCz, FC4, C1, Cz, and C2 were selected to analyze the mean amplitude of N1 during 90-150 ms presented by the topic. The mean amplitude of P2 was analyzed at F3, Fz, F4, FC5, FCz, FC6, C3, Cz, and C2 (180-250 ms). The LPP were F1, F2, FC3, FC4, C3, C4, CP1, and CP2 (350-650 ms, 650-1050 ms, 1050-1500 ms, 1500-2000 ms). To distinguishing, if the mean amplitude induced by the emotional stimulus and the problem stimulus had the same name, "E" (emotional stimulus) and "Q" (problem stimulus) were added before the name to distinguish them. For example, E-N1 was the mean amplitude of N1 by the emotional stimulus.
In this study, E-Prime 3.0 was used to automatically record the corresponding behavioral data, and all of the sorted data were analyzed by SPSS 20.0 (SPSS Inc., Chicago, IL, USA). Behavioral data were analyzed by one-way ANOVA, mainly to analyze the differences in CCRA score and response time when different emotion regulation strategies were adopted. The ERP data were analyzed by 3 (emotion regulation strategies: cognitive reappraisal, expression suppression, and control group) ×3 (front and back dimensions: front, middle and back) ×2 (left and right dimensions: left and right) repeated measure analysis of variances (ANOVAs). For the degrees of freedom that did not satisfy the hypothesis of the sphericity test, the Greenhouse-Geisser correction was used to the p values, correct, and the Bonferroni’s correction was used to correct all post-hoc tests.