Mindfulness DOI 10.1007/s12671-016-0591-z

ORIGINAL PAPER

Eight-Week Mindfulness Training Enhances Left Frontal EEG Asymmetry During Emotional Challenge: a Randomized Controlled Trial Renlai Zhou 1 & Lei Liu 2

# Springer Science+Business Media New York 2016

Abstract Mindfulness is defined as nonjudgmental attention to experiences in the present moment. Frontal EEG asymmetry can be considered as a biological indicator of affective style. Up to date, most studies examined how mindfulness training influenced frontal EEG asymmetry at resting baseline; however, few studies investigated how mindfulness training influenced frontal EEG asymmetry during emotional challenge. The present study examined simultaneously how mindfulness training influenced frontal EEG asymmetries at resting baseline and during emotional challenge in nonclinical population. In order to test the effect of mindfulness training, the present study tested the mindfulness-based cognitive therapy (MBCT) group (n = 16) and the waitlist control (WC) group (n = 20) at baseline, 4 weeks, and 8 weeks. A testing phase (baseline, 4 weeks, and 8 weeks) × time point (the first sad film phase, relaxation phase, the second sad film phase, and the meditation intervention phase) × group (MBCT and WC), ANOVA was conducted for the tasking EEG data. The results showed that there was significant testing phase × group interactive effect for frontal EEG asymmetry during emotional challenge. Further analysis showed that frontal EEG asymmetry scores at 8 weeks were lower than at baseline and 4 weeks for the MBCT group but not significant differences for the WC group. However, mindfulness training did not influence frontal EEG asymmetry at resting

* Renlai Zhou [email protected]

1

Department of Psychology, School of Social and Behavioral Sciences, Nanjing University, Nanjing 210023, China

2

Department of Psychology, College of Teacher Education, Ningbo University, Ningbo, China

baseline. Therefore, it is concluded that 8 weeks of mindfulness training can enhance left frontal EEG asymmetry during emotional challenge. These results indicated that frontal EEG asymmetry during emotional challenge may be an index of affective style. Keywords Mindfulness training . Frontal EEG asymmetry . Emotional challenge . Resting baseline . EEG

Introduction Mindfulness is typically defined as nonjudgmental attention to experiences in the present moment (Bishop et al. 2004; KabatZinn 1994). A large body of research has documented the efficacy of mindfulness training in the treatment of clinical disorders, including anxiety, depression, substance abuse, eating disorders, and chronic pain (Bowen et al. 2006; Garland et al. 2010; Kabat-Zinn 1982; Kabat-Zinn, Lipworth, and Burney 1985; Kabat-Zinn et al. 1992; Kristeller, Baer, and Quillian-Wolever 2006; Shapiro et al. 2003; Teasdale et al. 2000). Until recently, not only has mindfulness training successfully been used in the treatment of disorders but it has also been shown to produce positive effects on psychological wellbeing and to strength the individuals’ capacity for emotional regulation (Brown, Ryan, and Creswell 2007; Shapiro et al. 2006). Increasing evidence suggests that mindfulness and its psychological aspects are related to emotional regulation processes. Ortner, Kilner, and Zelazo (2007) showed that individuals with 7 weeks of mindfulness training showed a reduction in emotional interference from unpleasant images compared with relaxation meditation training or no intervention, and both mindfulness training and relaxation meditation training resulted in lower skin conductance responses to

Mindfulness

unpleasant images and increased well-being. According to self-report and questionnaire data, mindfulness training reduced self-reported negative affect, increased ratings of positive emotion, and reduced ruminative thoughts and reactivity to repetitive thoughts (Feldman, Greeson, and Senville 2010; Fredrickson et al. 2008; Hutcherson, Seppala, and Gross 2008; Jain et al. 2007; Jha et al. 2010). Similarly, mindfulness training improved performance on emotion recognition tasks, increased self-reported compassion for self and others, and led to improvements in self-reported emotional regulation (Kemeny et al. 2012; Mascaro et al. 2013; Jazaieri et al. 2014). Mindfulness training can also influence frontal EEG asymmetry. Frontal EEG asymmetry examines the difference of alpha band (8 ∼ 13 Hz) of the left and right frontal cortex. Previous studies have shown that enhanced left frontal EEG activity correlates to positive emotion or approach motivation, whereas enhanced right frontal EEG activity has been linked to negative emotion or withdrawal motivation (Coan and Allen 2003; Hagemann et al. 1998; Tomarken, Davidson, and Henriques 1990; Wheeler, Davidson, and Tomarken 1993). Frontal EEG asymmetry is also recognized as a biological indicator of affective style (Davidson 1995). Davidson et al. (2003) measured brain electrical activity after an 8-week training program in mindfulness meditation. The results showed a significant increase in left-sided anterior activation at the central sites, a pattern that was previously associated with a positive effect in meditators compared with non-meditators. Moyer et al. (2011) recently observed that a 5-week mindfulness training shifted frontal EEG asymmetry toward a pattern that is associated with positive, approach-oriented emotions. In contrast, Barnhofer et al. (2007) investigated the effects of meditation-based treatment on left frontal EEG pattern and preventing depression relapses. They found that the treatment-as-usual group showed a significant deterioration toward relatively decreased left frontal activation, whereas the mindfulness training group did not show such a change. These findings suggested that mindfulness training could help individuals with depression at high risk for suicide to retain a balanced pattern of brain activation. Moynihan et al. (2013) also found that the mindfulness-based stress reduction group demonstrated a reduction of right frontal alpha activation after intervention compared with the waiting list control group. These results showed that mindfulness-based stress reduction sustained left frontal alpha asymmetry. However, Keune et al. (2011) recently reported that the mean values of alpha asymmetry remained unaffected by mindfulness-based cognitive therapy training. These inconsistent findings indicate a need to further investigate the effect of mindfulness training on frontal EEG asymmetry. It is known that frontal EEG asymmetry includes two types of asymmetry: frontal EEG asymmetry at resting baseline and frontal EEG asymmetry during emotional challenge. The former indicates a tendency to predominantly respond with either

approach (indexed by relatively greater left frontal activity) or withdrawal (indexed by relatively greater right frontal activity)-related affect across all or most situations (Davidson 1998); the latter is a state-dependent concomitant of emotional response indicating that frontal brain activity during an emotional challenge is a more powerful detector of motivational differences than at rest because it may reflect the individuals’ capacity for emotional regulation in situations that demand it (Coan and Allen 2004; Coan, Allen, and McKnight 2006). This capability model has been supported by recent studies (Coan et al. 2006; Keune et al. 2011; Stewart et al. 2011). However, most studies have predominantly examined how mindfulness training affects frontal EEG asymmetry at resting baseline. Very few studies have been conducted to examine the effects of mindfulness training on frontal EEG asymmetry during emotional challenge (Keune et al. 2013). To our knowledge, no studies have examined the effects of mindfulness training on frontal EEG asymmetry in both resting and emotional challenge situations. The mindfulness training program adopted the mindfulness training curricula of Williams and Penman (2011). The program closely adhered to the core elements of mindfulnessbased cognitive therapy (MBCT) and was explicitly designed for people to cope with stress, elevate mood, and reduce anxiety (Williams and Penman 2011).Taylor et al. (2014) was the first groups of researchers who examined the effectiveness of MBCT on the reduction of emotion-related disorders among students. They found a significant reduction in the severity of depressive, anxiety, and stress symptoms, and significant improvements in life satisfaction, mindfulness, and selfcompassion for intervention participants in comparison to a waitlist control group. This study indicates that MBCT has the potential to be a low-cost, readily available, and highly acceptable intervention. In the present study, MBCT group and waitlist control (WC) group were tested at baseline, 4 weeks, and 8 weeks. The aim of the present study was to determine the effects of mindfulness training on frontal EEG asymmetry at resting baseline and during emotional challenge. In the study, participants first completed a 2-min open/close eye task, which was used to study frontal EEG asymmetry at resting baseline. After then, the participants completed a set of emotional film tasks, which were used to examine frontal EEG asymmetry during emotional challenge in the sad emotional state.

Method Participants Participants were recruited through online advertisements and word of mouth. Participants completed a preliminary telephone screening to determine eligibility. Participants were excluded if

Mindfulness

they self-reported the following: 1. has practice mediation before; 2. has experienced of head trauma, psychological, or neurological disorders; and 3. has any cardiac problems or were prescribed psychotropic medications. Forty college or graduate students were recruited to undergo the present study, and a total of 12 males and 28 females participated in the study. The average age of the participants was 22.5 years (SD = 3.16 years; range 19– 32 years). Participants were randomly assigned to the MBCT group or WC group. During the 8-week training program, four participants did not complete the mindfulness training (see Fig. 1). Thus, a total of 16 (M = 24.62 years, SD = 3.24 years) and 20 (M = 21.40 years, SD = 2.23 years) participants were examined in the MBCT group and WC group, respectively. Informed consent was obtained from all eligible students who volunteered for the study. The study was approved by the local ethics committee of Beijing Normal University.

Procedure To determine the effect of mindfulness training, the present study adopted group training and then tested the MBCT group and WC group at baseline, 4 weeks, and 8 weeks. The participants of the MBCT group and WC group completed the same testing tasks at baseline, 4 weeks, and 8 weeks. The baseline test was completed within 1 week before mindfulness training. Each of them came to the lab, signed the informed consent form, and completed the emotional questionnaires. The participants were then connected to electrode caps in a laboratory with electromagnetic shielding. (1) They first completed a 2-min open/close eyes task (recording EEG). (2) They completed the emotional films task (recording EEG). In the emotional films task, the participants were asked to watch two sad films and then follow the instructions for relaxation or meditation intervention. The procedure of emotional film task was the following: watching a sad film (3 min) → relaxation (3 min) → watching another sad film (3 min) → meditation intervention (3 min). For the relaxation phase, participants were asked to Bsitting quietly with your eyes closed, relaxing your muscles and calming your mind.^ The guided portion reiterated points from the introduction and instructed participants to Brelax your body, releasing tension from your muscles,^ to Blet your worries and concerns fall away, and let your mind grow calm.^ For the meditation intervention phase, participants were guided with a 3-min audio instruction for BBreathing Space meditation^ (Williams and Penman 2011). The tasks of 4 and 8 weeks were the same as the task of the baseline, and the order of the six films was balanced among the baseline, 4 weeks, and 8 weeks.

Measures Emotional Film Materials From the emotional movie library of our laboratory, six sad films were selected, and the length of each film was approximately 3 min. Film 1 was clip from Chang Jiang Qi Hao (during which the boy wept bitterly when his father died); film 2 was clip from The Message (during which Zhiguo Wu and Ningyu Li recalled sadly the death scene of Xiaomeng Gu); film 3 was clip from My Beloved (during which the boy broke away from his mother); film 4 was clip from 10 Promises to My Dog (during which the girl left her beloved dog); film 5 was clip from Gone Is the One Who Held Me Dearest in the World (during which the female recalled sadly her dead mother); and film 6 was clip from My Sisters and Brothers (during which the sister was sent to a couple by her brother). Twenty-six participants evaluated their subjective feelings to the six sad films on 20 dimensions on a nine-point scale. One-way ANOVA was performed for each film, and these results revealed that all of the main effects were significant: first film, F(20, 500) = 26.434, p < 0.001, η2 = 0.514; second film, F(20, 500) = 21.262, p < 0.001, η2 = 0.460; third film, F(20, 500) = 11.424, p < 0.001, η2 = 0.314; forth film, F(20, 500) = 23.534, p < 0.001, η2 = 0.485; fifth film, F(20, 500) = 17.118, p < 0.001, η2 = 0.406; and sixth film, F(20, 500) = 30.213, p < 0.001, η2 = 0.547. The post hoc tests showed that for each film, the sad emotion was higher than other emotions (ps < 0.05), which suggested that all the six films induced the sad emotion. Furthermore, one-way ANOVA was performed for the sad intensity of the six films, and the result showed that the main effect of sad intensity was not significant, F(5, 125) = 1.688, p = 0.157, η2 = 0.063, indicating that the six films could induce the same sad emotional intensity. Three-Minute Breathing Space Meditation Breathing space meditation emphasizes the core elements of the mindfulness program into three steps of roughly 1 min each. Step 1: becoming aware. If possible, close your eyes. Next, bring your awareness to your inner experience and acknowledge it. Step 2: gathering and focusing attention. Redirecting the attention to a narrow Bspotlight^ on the physical sensations of the breath. Follow the breath all of the way in and all of the way out. Use each breath as an opportunity to anchor yourself into the present. If the mind wanders, gently escort the attention back to the breath. Step 3: expanding attention. Expand the field of awareness around the breathing such that it includes a sense of the body as a whole, including your posture and facial expression. Questionnaire Materials The Beck Depression Inventory (BDI) is a self-report measure that is designed to assess depression (Beck et al. 1979). The Chinese BDI scale has Cronbach’s alpha coefficient of 0.89 and the split-half

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Fig. 1 Flow diagram

reliability coefficient of 0.88 (Zhang, Wang, and Qian 1990). The Beck anxiety inventory (BAI) is a self-report measure designed to assess anxiety (Beck et al. 1988). The Chinese BAI scale has very reliability coefficients, where Cronbach’s alpha coefficient is 0.95 (Zheng et al. 2002). The PANAS measures emotional states. Ten of the items measure PA; the other ten measure NA (Watson, Clark, and Tellegen 1988). For the Chinese PANAS scale, Cronbach’s alpha coefficient is 0.87 (Zhang, Diao, and Schick 2004). The emotional flexibility questionnaire measures individual emotional flexibility, which consists of ten items. The internal consistency reliability is 0.826, and the test-retest reliability is 0.622 (Yi 2010). The emotion regulation questionnaire (ERQ; Gross and John 2003) assesses the typical use of emotion suppression versus reappraisal. The scale has been shown to exhibit good psychometric properties (Gross and John 2003). Mindfulness Training Program The present mindfulness training program was based on the mindfulness training curricula of Williams and Penman (2011). Williams and

Penman’s architecture is evidence-based, and is fashioned into a coherent and compelling program for people who care about his or her own health and sanity. It consisted of 8-week programs. Week one: Mindfulness of the body and breath meditation; Week two: Body scan practice; Week three: Mindful movement meditation; Week four: Sounds and thoughts meditation; Week five: Exploring difficult meditation; Week six: Befriending meditation; Week seven: Individual tailoring of their formal meditation practice; Week eight: Start the day with mindfulness; and Maintain your mindfulness practice. Each session was 2 h long per week, and the participants had to practice the task for 20–30 min per day. Data Analyses Raw unfiltered EEG was recorded using the NeuroScan recording system and a 40-electrode head cap. EEG signals were acquired by a DC model with a sampling rate of 1000 Hz and a bandwidth of 100 Hz. Vertical and horizontal electro-oculograms (EOGs) were also recorded by electrodes

Mindfulness Table 1 Group demographics information and the means of emotional questionnaires (BAI, BDI, and PANAS) for the MCBT and WC groups MBCT (n = 16)

WC (n = 20)

M (SD)

M (SD)

Table 2 The descriptive results of emotional questionnaires (flexibility, suppression, and reappraisal) for the MBCT and WC groups Flexibility M (SD)

Suppression M (SD)

Reappraisal M (SD)

Demographics t

Age

24.62 (3.24) % (n)

21.40 (2.23) % (n)

3.528 χ2

Female

68.75 (11)

70 (14)

0.007

Education Bachelor’s degree

93.75 (15)

90 (18)

6.25 (1)

10 (2)

100 (16)

100 (20)

0

BAI BDI

M (SD) 29.13 (9.37) 9.88 (8.09)

M (SD) 29.00 (5.89) 11.10 (6.60)

t 0.049 −0.501

PANAS(P)

24.50 (6.59)

25.95 (7.84)

−0.591

PANAS(N)

17.69 (6.99)

18.30 (6.01)

−0.283

Master’s degree Occupation Student

MBCT (n = 16) Baseline 50.31 (7.41) 4 weeks WC (n = 20)

Questionnaires

placed above and below the outer orbits of the left eye. The electrode on the left mastoid served as a reference during the EEG recording, and the electrodes on the right mastoid were treated similarly to the other electrodes. For all of the electrodes, the impedance was kept under 5 KΩ. Offline analysis of EEG signals was re-referenced to the Cz electrode and was filtered using a 30 Hz bandwidth (24 dB/ octave slope). All data were inspected visually to eliminate intervals in which the ocular or muscle artifacts occurred. Power spectra were derived using a fast Fourier transform with a Hamming window (epoch length 1 s, 50 % overlap) for time point (the first sad film phase, relaxation phase, second sad film phase, and meditation intervention) at baseline, 4 weeks, and 8 weeks for each participant. Importantly, for consistency with previous research, we focused on the alpha band (8–13 Hz) in the frontal electrodes, F3 and F4. A laterality coefficient indexing relative right- versus left-sided activation was used because in this general context, the relative difference between the hemispheres is more important than the absolute level of independent left or right hemisphere activity per se. The EEG laterality coefficients (LC) were computed as the following: LC = ((R − L) / (R + L)) × 100. Positive values indicated higher alpha activity in the right compared to the left hemisphere (i.e., relatively greater left hemisphere cortical activity). The calculation of LC has a long tradition in laterality research because it separates the variance in asymmetry from the variance in general magnitude. An independent sample t test was completed between the MBCT group and WC group for BAI, BDI, and PANAS at baseline. Then, a testing phase (baseline, 4 weeks, and 8 weeks) × group (MBCT and WC) ANOVA was conducted

13.38 (4.27) 27.13 (4.38) 13.31 (4.81) 27.56 (4.70)

8 weeks 55.13 (10.13) 15.75 (6.14) 30.06 (6.15) Baseline 52.45 (5.54) 15.82 (4.74) 28.65 (4.51) 4 weeks 8 weeks

0.164

51.56 (5.90)

50.45 (8.22) 51.55 (6.87)

15.95 (4.93) 28.75 (3.45) 15.70 (4.09) 29.75 (3.70)

for emotional flexibility scores, suppression scores, and reappraisal scores, respectively. Testing phase was the withinsubjects factor; group was the between-subjects factor. Similarly, a testing phase (baseline, 4 weeks, and 8 weeks) × group (MBCT and WC) ANOVA was conducted for the resting EEG data. Testing phase was the within-subjects factor; group was the between-subjects factor. Furthermore, a testing phase (baseline, 4 weeks, and 8 weeks) × time point (the first sad film phase, relaxation phase, the second sad film phase, and the meditation intervention phase) × group (MBCT, WC) ANOVA was conducted for the tasking EEG data. Testing phase and time point were the within-subjects factors; group was the between-subjects factor. In addition, intraclass correlations (ICCs) were calculated among testing phase at baseline, 4 weeks, and 8 weeks for the emotional flexibility questionnaire, the emotion reappraisal questionnaire, and emotion suppression questionnaire, respectively.

Results The descriptive scores of emotional questionnaires that were collected are shown in Tables 1 and 2. Student’s t tests were conducted to assess group differences on the scores of emotional questionnaires (BAI, BDI, and PANAS). The statistical Table 3 The statistical results of emotional questionnaires (flexibility, suppression, and reappraisal) for the MBCT and WC groups

Flexibility

Suppression

Reappraisal

F

p

ηP2

Group

0.182

0.672

0.005

Testing phase Group × testing phase Group Testing phase Group × testing phase Group Testing phase Group × testing phase

1.844 2.416 1.310 2.327 3.256 0.393 4.979 0.991

0.166 0.098 0.260 0.105 0.047 0.535 0.015 0.363

0.051 0.086 0.037 0.064 0.087 0.011 0.128 0.028

Mindfulness Table 4

The descriptive results of frontal EEG asymmetry data at resting baseline and during emotional challenge for the MBCT and WC groups Resting M (SD)

MBCT (n = 16)

WC (n = 20)

Sad film 1 M (SD)

Relaxation M (SD)

Sad film 2 M (SD)

Meditation M (SD)

Baseline

1.23 (10.68)

2.31 (8.97)

0.49 (5.78)

2.72 (8.67)

1.06 (8.11)

4 weeks

3.72 (6.37)

4.76 (10.23)

2.30 (5.67)

3.07 (8.62)

2.06 (8.29)

8 weeks Baseline

−4.71 (6.78) −2.04 (18.13)

−4.80 (13.41) −1.58 (12.65)

−5.66 (6.05) −0.52 (9.17)

−2.69 (11.67) −2.08 (11.29)

−5.44 (7.22) −2.09 (11.35)

4 weeks

4.26 (13.29)

0.98 (13.30)

1.22 (9.88)

0.09 (10.83)

−1.17 (10.65)

8 weeks

1.03 (7.80)

−0.51 (7.59)

1.11 (8.45)

1.29 (8.35)

−0.33 (7.43)

results showed that there were no significantly differences on scores of BAI, BDI, and PANAS between the MBCT group and WC group (see Table 1). After then, a univariate analysis of variance (ANOVA) was conducted on scores of expression inhibition questionnaire. The results indicated a significant time point × group interaction effect, F(2, 68) = 3.256, p = 0.047, η2 = 0.087; further simple effect analysis showed that suppression scores at 8 weeks were higher than at baseline and 4 weeks (baseline vs. 8 weeks: p = 0.008; 4 vs. 8 weeks: p = 0.016) for the MBCT group, whereas there were not significant differences for the WC group (baseline vs. 8 weeks: p = 0.843; 4 vs. 8 weeks: p = 0.773). Similarly, for emotional flexibility scores, there was a marginally significant time point × group interaction effect, F (2, 68) = 2.416, p = 0.098, η2 = 0.086; further simple effect analysis showed that emotional flexibility scores at 8 weeks were higher than at baseline and 4 weeks (baseline vs. 8 weeks: p = 0.016; 4 vs. 8 weeks: p = 0.068) for the MBCT group, whereas there were not significant differences for the WC group (baseline vs. 8 weeks: p = 0.599; 4 vs. 8 weeks: p = 0.519).However, for reappraisal scores, all effects were not significant (see Table 3). In addition, ICCs among testing phase at baseline, 4 weeks, and 8 weeks ranged from 0.695 for emotional flexibility questionnaire, 0.799 for emotional reappraisal questionnaire, to 0.887 for emotional suppression questionnaire. The descriptive results of EEG data are shown in Table 4. Table 5 The statistical results of frontal EEG asymmetry data at resting baseline and during emotional challenge for the MBCT and WC groups

Resting baseline

During emotional challenge

For resting EEG data, the ANOVA results showed that the main effect of the group was not significant, F(1, 34) = 0.143, p = 0.708, η2 = 0.004; the main effect of testing phase was marginally significant, F(2, 68) = 3.033, p = 0.058, η2 = 0.082; the interaction effect of testing phase × group was not significant, F(2, 68) = 1.676, p = 0.197, η2 = 0.047. These results suggested that mindfulness training did not influence frontal EEG asymmetry at resting baseline. For tasking EEG data, the ANOVA results showed that the main effects of the group were not significant, F(1, 34) = 0.024, p = 0.879, η2 = 0.001; the main effect of testing phase was marginally significant, F(2, 68) = 3.168, p = 0.051, η2 = 0.085; the main effects of time point was not significant F(3, 102) = 0.540, p = 0.589, η2 = 0.016; the interaction effect of time point × group was not significant, F(3, 102) = 0.590, p = 0.560, η2 = 0.017; the interaction effect of time point × testing phase was not significant, F(6, 204) = 0.731, p = 0.551, η2 = 0.021; the interaction effect of testing phase × group was significant, F(2, 68) = 4.709 p = 0.014, η2 = 0.122; the interaction effect of testing phase × time point × group was not significant, F(6, 204) = 0.283, p = 0.860, η2 = 0.008. The simple effect analysis of testing phase × group showed that for the MBCT group, frontal EEG asymmetry scores at 8 weeks were lower than at baseline and 4 weeks (baseline vs. 8 weeks: p = 0.004; 4 vs. 8 weeks: p = 0.004), whereas there were not significant differences for the WC group (baseline vs. 8 weeks: p = 0.286; 4 vs. 8 weeks: p = 0.962) (see Table 5).

Group Testing phase Group × testing phase Group Testing phase Time point Group × testing phase Group × time point Testing phase × time point Group × testing phase × time point

F

p

ηP2

0.143 3.033 1.676 0.024 3.168 0.540 4.709 0.590 0.731 0.283

0.708 0.058 0.197 0.879 0.051 0.589 0.014 0.560 0.551 0.860

0.004 0.082 0.047 0.001 0.085 0.016 0.122 0.017 0.021 0.008

Mindfulness

Discussion The present study examined how 8 weeks of mindfulness training influenced frontal EEG asymmetry. The EEG data results showed that 8 weeks of mindfulness training influenced frontal EEG asymmetry during emotional challenge. The questionnaire data results showed that 8 weeks of mindfulness training enhanced emotional flexibility scores and emotional suppression scores. Furthermore, the effect of mindfulness training occurred in the last 4 weeks. Our finding that the level of frontal EEG asymmetry at 8 weeks was significantly lower than at baseline and 4 weeks for the MBCT group during emotional challenge provides physiological evidence of the effectiveness of mindfulness training in emotion regulation. It is important to point out that researchers have reported inconsistent findings in terms of the effect of mindfulness training on frontal EEG asymmetry. The findings from Davidson et al. (2003) and Moyer et al. (2011) are consistent with our finding that mindfulness training shifts frontal EEG asymmetry toward a pattern that is associated with positive, approach-oriented emotions. The findings from Barnhofer et al. (2007) and Moynihan et al. (2013) also confirm that mindfulness training could reduce the rightward shift of frontal alpha activation after intervention compared with the waiting list control group. However, Keune et al. (2011) showed that frontal EEG asymmetry remained unaffected by mindfulness training. The reasons of the inconsistent findings among studies are unclear. One postulation could be that the inconsistent findings could be related to the different emotion provoking procedures that have been used in previous studies. Researchers (Coan et al. 2006) have also proposed that research procedures on frontal EEG asymmetry should measure frontal EEG asymmetry measurements at both resting baseline and during emotional challenge. The capability model (Coan et al. 2006) posits that meaningful individual differences in frontal EEG asymmetry exist, but that these individual differences are best thought of as interactions between the emotional demands of specific situations and the emotion-regulatory abilities individuals bring to those situations. In the present study, the effect of mindfulness training on frontal EEG asymmetry during emotional challenge is better than on frontal EEG asymmetry at resting baseline. Thus, the present finding suggested that it is more likely that frontal EEG asymmetry during emotional challenge is an index of affective style, which may account for the inconsistent results obtained in previous studies. The present study also measured the subjective emotional flexibility and emotion suppression using questionnaires. The results from the questionnaire data showed that the scores of emotional flexibility and emotion suppression at 8 weeks were

higher than at baseline and 4 weeks for the MBCT group. This was consistent with findings obtained from previous studies. These studies showed that mindfulness training decreased negative mood states, improved positive mood states, reduced distractive and ruminative thoughts, and reduced reactivity to repetitive thoughts (Feldman et al. 2010; Jain et al. 2007; Jha et al. 2010). In addition, neural studies showed that mindfulness training reduced the activation of the amygdala regarding emotion processes and increased the activation of the parietal cortex regarding attention processes (Goldin et al. 2013). This present finding suggested that mindfulness training enhanced the capacity for emotional flexibility and emotional regulation. Thus, the results of emotional questionnaires were consistent with the results of frontal EEG asymmetry. Furthermore, the present study revealed that that the effect of mindfulness training on frontal EEG asymmetry and emotional questionnaires occurred within the last 4 weeks. Based on the classification of mindfulness of Lutz et al. (2008), these first 4 weeks for mindfulness training mainly focused on attention meditation which entails voluntarily focusing attention on a chosen object in a sustained fashion, and the last 4 weeks were mainly open monitoring meditation which involves the non-reactive monitoring of the content of an experience from moment to moment, primarily as a means to recognize the nature of the emotional and cognitive patterns (Bishop et al. 2004). Furthermore, most researchers agree that mindfulness involves increased present-centered attention and awareness as well as a nonjudgmental and accepting approach to cognitions, emotions, sensations, and environmental stimuli (Kabat-Zinn 1990; Miller, Fletcher, and Kabat-Zinn 1995), and the simultaneous lack of both present-centered awareness and nonjudgment would likely produce dysregulated, impulsive behavior or dissociation (both forms of acting without awareness) specifically in the presence of judged experiences, particularly negative affect and difficult interpersonal interactions. Recently, Peters et al. (2013) showed that increased attention can be deleterious in the presence of judging/reactivity, whereas increased attention can be helpful in the presence of non-judging/non-reactivity. Thus, the effect of mindfulness training on frontal EEG asymmetry occurred within the last 4 weeks, because the latter stages of the MBCT in the present study may focus more heavily on non-reactivity and the other qualities of mindful attention (as opposed to focusing on increasing attention itself). However, for the 8-week training program, it is a continuous process over the first 4 weeks and the last 4 weeks, and the practice content of the last 4 weeks also includes some practice content from the first 4 weeks. Due to the confound effects of techniques and time, further studies are warranted to examine the different roles for focusing breath meditation and open monitoring meditation.

Mindfulness Compliance with Ethical Standards Funding This study was supported by the Tsinghua University research fund of positive psychology (No. 0020344-2015-01-008) and the Key Project of Philosophy and Social Science Research in Colleges and Universities in Jiangsu Province (2015JDXM001). The authors wish to thank Professor Tian Po S Oei and Senqi Hu for revising the manuscript. Ethical Approval All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards. Conflict of Interest The authors declare that they have no conflict of interest. Informed Consent Informed consent was obtained from all individual participants included in the study.

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