Advertisement

A Randomized Controlled Trial Examining the Effect of Mindfulness Meditation on Working Memory Capacity in Adolescents

      Abstract

      Purpose

      To investigate the effectiveness of a mindfulness meditation intervention on working memory capacity (WMC) in adolescents via a randomized controlled trial comparing mindfulness meditation to hatha yoga and a waitlist control group.

      Methods

      Participants (N = 198 adolescents) were recruited from a large public middle school in southwest United States and randomly assigned to mindfulness meditation, hatha yoga, or a waitlist control condition. Participants completed a computerized measure of WMC (Automated Operational Span Task) and self-report measures of perceived stress (Perceived Stress Scale) and anxiety (Screen for Childhood Anxiety Related Emotional Disorders) at preintervention and postintervention/waitlist. A series of mixed-design analyses of variance were used to examine changes in WMC, stress, and anxiety at preintervention and postintervention.

      Results

      Participants in the mindfulness meditation condition showed significant improvements in WMC, whereas those in the hatha yoga and waitlist control groups did not. No statistically significant between-group differences were found for stress or anxiety.

      Conclusions

      This is the first study to provide support for the benefits of short-term mindfulness practice, specifically mindfulness meditation, in improving WMC in adolescents. Results highlight the importance of investigating the components of mindfulness-based interventions among adolescents given that such interventions may improve cognitive function. More broadly, mindfulness interventions may be delivered in an abridged format, thus increasing their potential for integration into school settings and into existing treatment protocols.

      Keywords

      Implications and Contribution
      This study demonstrates a significant effect of mindfulness meditation on working memory capacity in adolescents. Mindfulness interventions may be feasibly abridged, while still delivering important benefits.
      In the last decade, mindfulness research in youth has grown rapidly. Mindfulness involves directing attention to the present-moment experience in a nonjudgmental and accepting way [
      • Kabat-Zinn J.
      Full catastrophe living: Using the wisdom of your body and mind to face stress, pain, and illness.
      ]. Mindfulness is often recognized as a “practice” or “training,” requiring a constant shift from an “automatic pilot” mindset to one that comprises attention and awareness [
      • Baer R.A.
      Mindfulness training as a clinical intervention: A conceptual and empirical review.
      ].
      The term mindfulness has been applied to numerous interventions, including multicomponent interventions such as Mindfulness-Based Stress Reduction (MBSR) [
      • Kabat-Zinn J.
      Full catastrophe living: Using the wisdom of your body and mind to face stress, pain, and illness.
      ], and to particular forms of mindfulness practice (e.g., mindfulness meditation). In general, mindfulness-based interventions have been shown to be effective in producing beneficial effects on physical and psychological functioning, particularly in adults. Although the methodological rigor of many treatment studies with youth remains questionable [
      • Baer R.A.
      Mindfulness training as a clinical intervention: A conceptual and empirical review.
      ,
      • Zenner C.
      • Herrnleben-Kurz S.
      • Walach H.
      Mindfulness-based interventions in schools—a systematic review and meta-analysis.
      ,
      • Tan L.B.
      A critical review of adolescent mindfulness-based programmes.
      ], the existing body of research suggests that mindfulness training may be feasibly adapted for use with adolescents in medical and community-based samples. Furthermore, mindfulness interventions have demonstrated beneficial effects on general well-being and physical health among adolescents [
      • Tan L.B.
      A critical review of adolescent mindfulness-based programmes.
      ,
      • Biegel G.M.
      • Brown K.W.
      • Shapiro S.L.
      • Schubert C.M.
      Mindfulness-based stress reduction for the treatment of adolescent psychiatric outpatients: A randomized clinical trial.
      ,
      • Black D.S.
      • Fernando R.
      Mindfulness training and classroom behavior among lower-income and ethnic minority elementary school children.
      ,
      • Burke C.A.
      Mindfulness-based approaches with children and adolescents: A preliminary review of current research in an emergent field.
      ,
      • Tan L.B.
      • Martin G.
      Taming the adolescent mind: A randomised controlled trial examining clinical efficacy of an adolescent mindfulness-based group programme.
      ,
      • Semple R.J.
      • Lee J.
      • Rosa D.
      • Miller L.F.
      A randomized trial of mindfulness-based cognitive therapy for children: Promoting mindful attention to enhance social-emotional resiliency in children.
      ,
      • Zoogman S.
      • Goldberg S.B.
      • Hoyt W.T.
      • Miller L.
      Mindfulness interventions with youth: A meta-analysis.
      ].
      Although the global benefits of mindfulness on physical and psychological functioning are encouraging, as researchers emphasize the positive outcomes of mindfulness interventions, it is imperative to begin identifying whether specific benefits may be uniquely associated with particular forms of mindfulness practice. One specific benefit of mindfulness training that has been explored exclusively in adult samples is the effect of mindfulness meditation on working memory. A growing number of neurologic studies have documented that mindfulness practices have been associated with changes in neural pathways [
      • Davidson R.J.
      • Kabat-Zinn J.
      • Schumacher J.
      • et al.
      Alterations in brain and immune function, produced by mindfulness meditation.
      ,
      • Fletcher L.B.
      • Schoendorff B.
      • Hayes S.C.
      Searching for mindfulness in the brain: A process-oriented approach to examining the neural correlates of mindfulness.
      ,
      • Lazar S.W.
      • Kerr C.E.
      • Wasserman R.H.
      • et al.
      Meditation experience is associated with increased cortical thickness.
      ]. Neurologic changes in the structure of the brain may affect specific cognitive functions such as attention and memory. Research has shown that mindfulness exercises, specifically mindfulness meditation, have a direct effect on higher-order cognitive processes, such as executive functioning [
      • Lutz A.
      • Greischar L.L.
      • Rawlings N.B.
      • et al.
      Long-term meditators self- induce high amplitude gamma synchrony during mental practice.
      ,
      • Slagter H.A.
      • Lutz A.
      • Greischar L.L.
      • et al.
      Mental training affects distribution of limited brain resources.
      ]. There has been particular interest in the impact of mindfulness on working memory because both constructs require moment-to-moment awareness [
      • Dehn M.J.
      Working memory and academic learning: Assessment and intervention.
      ]. Working memory is a mental workbench that allows an individual to hold information in awareness long enough for reasoning and comprehension to occur [
      • Dehn M.J.
      Working memory and academic learning: Assessment and intervention.
      ]. This cognitive function helps the brain shift information from short-term memory to long-term memory. Working memory also impacts performance on other higher-level cognitive tasks such as reasoning, comprehension, and executive functioning [
      • Dehn M.J.
      Working memory and academic learning: Assessment and intervention.
      ].
      The adult literature suggests that mindfulness meditation may be used as a tool to enhance working memory capacity (WMC) [
      • Chambers R.
      • Lo B.C.
      • Allen N.B.
      The impact of intensive mindfulness training on attentional control, cognitive style, and affect.
      ,
      • Jha A.P.
      • Stanley E.A.
      • Kiyonaga A.
      • et al.
      Examining the protective effects of mindfulness training on working memory capacity and affective experience.
      ,
      • Mrazek M.D.
      • Franklin M.S.
      • Phillips D.T.
      • et al.
      Mindfulness training improves working memory capacity and GRE performances while reducing mind wandering.
      ,
      • Zeidan F.
      • Johnson S.K.
      • Diamond B.J.
      • et al.
      Mindfulness meditation improves cognition: Evidence of brief mental training.
      ]. WMC is implicated in processes such as reasoning ability, mathematical problem solving, language, and reading comprehension, and many other aspects of learning [
      • Kyllonen P.C.
      • Christal R.E.
      Reasoning ability is (little more than) working memory capacity?!.
      ,
      • Swanson H.L.
      • Beebe-Frankenberger M.
      Relationship between working memory and mathematical problem solving in children at risk and not at risk for serious math difficulties.
      ]. Recent studies suggest that even brief mindfulness meditation training may improve WMC, mind wandering, and standardized test scores in nonclinical and college student samples [
      • Mrazek M.D.
      • Franklin M.S.
      • Phillips D.T.
      • et al.
      Mindfulness training improves working memory capacity and GRE performances while reducing mind wandering.
      ,
      • Zeidan F.
      • Johnson S.K.
      • Diamond B.J.
      • et al.
      Mindfulness meditation improves cognition: Evidence of brief mental training.
      ]. To date, however, the effectiveness of mindfulness training in improving WMC among adolescents has not been examined, although it would follow that strengthening WMC would be especially valuable during the developmental stage of adolescence, where the majority of the day is spent in an educational setting. Although school-based mindfulness interventions have been conducted [
      • Britton W.B.
      • Lepp N.E.
      • Niles H.F.
      • et al.
      A randomized controlled pilot trial of classroom-based mindfulness meditation compared to an active control condition in sixth-grade children.
      ,
      • Sibinga E.M.S.
      • Perry-Parrish C.
      • Chung S.
      • et al.
      School-based mindfulness instruction for urban male youth: A small randomized controlled trial.
      ], the incorporation of mindfulness practice in schools is inextricably linked to the availability of adequate resources. Briefly, low-cost programs integrated into existing curricula are likely to be the most feasible, particularly in schools with limited resources. [
      • Britton W.B.
      • Lepp N.E.
      • Niles H.F.
      • et al.
      A randomized controlled pilot trial of classroom-based mindfulness meditation compared to an active control condition in sixth-grade children.
      ].

      The present study

      The primary objective of this randomized controlled trial was to examine the impact of mindfulness meditation on WMC in adolescents. Based on previous research, we hypothesized that mindfulness meditation would result in significantly greater improvement in WMC compared to a hatha yoga intervention or a waitlist control condition. We chose to compare mindfulness meditation to hatha yoga given that both are formal mindfulness practices used in mindfulness-based interventions (e.g., MBSR [
      • Kabat-Zinn J.
      Full catastrophe living: Using the wisdom of your body and mind to face stress, pain, and illness.
      ]). Although theoretically similar in terms of their ability to enhance awareness of the present-moment, theoretical and experimental research suggests that mindfulness meditation may be particularly effective in promoting executive functioning [
      • Chambers R.
      • Lo B.C.
      • Allen N.B.
      The impact of intensive mindfulness training on attentional control, cognitive style, and affect.
      ,
      • Zeidan F.
      • Johnson S.K.
      • Diamond B.J.
      • et al.
      Mindfulness meditation improves cognition: Evidence of brief mental training.
      ]. By comparing mindfulness meditation to hatha yoga, a potentially important unique benefit of mindfulness meditation may be revealed. Given the proposed broad psychological benefits of mindfulness practice, our secondary aim was to examine the effectiveness of mindfulness meditation and hatha yoga on perceived stress [
      • Weinstein N.
      • Brown K.W.
      • Ryan R.M.
      A multi-method examination of the effects of mindfulness on stress attribution, coping, and emotional well-being.
      ] and anxiety [
      • Orsillo S.M.
      • Roemer L.
      • Barlow D.H.
      Integrating acceptance and mindfulness into existing cognitive-behavior treatment for GAD: A case study.
      ]. We hypothesized that adolescents in both the mindfulness meditation and hatha yoga groups would report significantly lower levels of stress and anxiety and greater mindfulness at postintervention compared to the waitlist control group.

      Methods

      Participants

      The clinical trial was approved by the Alliant International University institutional review board and was conducted at a large public junior high school in southern California between March 2013 and July 2013. The school consisted of students who were predominately from low-income minority households. About 80 percent of students were eligible for a free or reduced lunch. All parents or legal guardians provided written informed consent and youth provided assent. Inclusion criteria were (1) current school enrollment, (2) current age between 12 and 17 years, (3) English speaking, and (4) ability to attend weekly study sessions. Exclusion criteria were (1) hearing impairments, (2) injuries, or (3) physical disabilities that would hinder full participation in either intervention.

      Study design and interventions

      Participants completed a baseline assessment that included all study measures (see Measures). Participants were then randomly assigned to mindfulness meditation, hatha yoga, or a waitlist control condition. Participants in both intervention groups (mindfulness meditation and hatha yoga) met a total of eight times: twice weekly for 4 weeks. Each intervention session lasted 45 minutes, resulting in a total of approximately 6 hours of intervention time. Daily home practice was monitored through a written home practice log that was collected once a week during the intervention. Each intervention group comprised 10–13 participants. Both intervention arms were delivered during participants' physical education (PE) class periods. The sitting meditation intervention was held in the school's library, whereas the hatha yoga intervention was held in the school's gym. The waitlist control group attended regular PE classes. After the 4-week intervention period, all participants completed postintervention/postwaitlist measures. Waitlist control participants were then randomly assigned to receive the mindfulness meditation or hatha yoga intervention.

      Mindfulness meditation intervention

      Two female instructors with extensive training in mindfulness meditation facilitated the mindfulness meditation groups. Both instructors completed mindfulness-based training programs (i.e., Mindfulness Based Cognitive Therapy, MBSR) at various workshops and retreats, while maintaining their own personal practice. The instructors each had between 5 and 10 years of meditation experience. The curriculum was based on the MBSR program developed by Jon Kabat-Zinn [
      • Kabat-Zinn J.
      Full catastrophe living: Using the wisdom of your body and mind to face stress, pain, and illness.
      ]. Developmentally appropriate modifications included minor wording changes, shortening the session timeframe to fit the academic school schedule, and incorporating incentives for participation. To ensure intervention fidelity, instructors followed a mindfulness meditation curriculum and schedule. A researcher periodically attended sessions to verify that the intervention was being implemented with minimal variability.
      Each mindfulness meditation session consisted of (1) breathing techniques, (2) formal meditation, and (3) discussion. Participants learned new types of mindfulness meditation each week (Week 1, Breathing; Week 2, Being in the Body and Feelings; Week 3, Awareness and Leaves on a Stream; Week 4, Silent and Loving Kindness). Instructors were provided written scripts to help guide each of the weekly mindfulness themes (e.g., Being in the Body Meditation). Instructions on sitting posture, breathing, and wandering thoughts were repeated during each session. Each intervention session concluded with an instructor-led group discussion. Participants received a CD of meditation audio recordings to enable home practice. Participants were encouraged to practice meditation daily for 15–30 minutes and to record details of their practice in home practice logs.

      Hatha yoga intervention

      Two female yoga instructors trained in teaching child and adolescent yoga facilitated the hatha yoga groups. Both instructors were 200-hour yoga teacher training certified. The instructors each had practiced yoga for 6–10 years and maintained their own yoga practice while also teaching in private studios in the community. The hatha yoga curriculum was used with permission from Shanti Generation Yoga (2009). Hatha yoga sessions consisted of (1) breathing techniques, (2) yoga poses, and (3) discussion. Participants learned a series of new yoga poses each week and reviewed old poses (Week 1, Creating Happiness: Forward bends for flexibility; Week 2, Energy Amplified: Balancing and core strengthening postures; Week 3, Choosing Peace: Standing poses, backbends, and forward bends; Week 4, Being Sound: Twist, bend, and relax; and Voice Choice Possibility: Breathe, visualize, and move). Yoga poses included a series of mindful movements that required bending, stretching, and holding to increase muscle tone and flexibility. Yoga poses were used as tools to elicit moment-to-moment awareness. With each pose, adolescents practiced noticing without judgment the different sensations that arose in their mind and body. They were instructed to observe their thoughts, feelings, and mood as they transitioned into different postures. Instructors also encouraged participants to pay attention to their body language, facial expressions, and attitude to help them tune-in to their daily reactions. Participants received a DVD containing yoga lessons corresponding to the poses taught in the intervention. Participants were encouraged to practice yoga daily for 15–30 minutes and to record their practice in home practice logs.

      Measures

      Demographic data

      Brief demographic questionnaires were completed by participants and their parents to obtain background information including age, gender, grade level, race/ethnicity, socioeconomic status, and extracurricular activities.

      Automated Operation Span Task

      The Automated Operation Span Task (AOSPAN) [
      • Unsworth N.
      • Heitz R.P.
      • Schrock J.C.
      • Engle R.W.
      An automated version of the operation span task.
      ] is a computerized task used to measure WMC that comprises 15 sets of trials, each containing two simultaneous tasks: performing a memory task while answering math equations. In the memory section, participants were presented a series of letters appearing one at a time for 800 ms. Participants were asked to recall letters in the same order they were presented by clicking the box next to the appropriate letters in correct order. In the math portion of the task, math equations were presented in sets of simple questions such as “(1 × 2) + 1 = ?).” After solving the problem, participants were then asked to click to the next screen where a potential answer, such as “3” was presented. Participants clicked on the “true” or “false” box. There were 75 letters to recall and 75 math equations to solve. Although five scores were calculated, the OSPAN score was used as it is the most commonly used index of WMC [
      • Conway A.R.
      • Kane M.J.
      • Bunting M.F.
      • et al.
      Working memory span tasks: A methodological review and user's guide.
      ]. It is the sum of all recalled letters from letter sets that were recalled completely in the correct order. OSPAN scores have shown acceptable internal consistency (α = .78) and good test–retest reliability in nonclinical samples. This task has been used extensively in working memory research, and it has been shown to be appropriate for use with children and adolescents. [
      • Kaufman S.B.
      • DeYoung C.G.
      • Gray J.R.
      • et al.
      Associative learning predicts intelligence above and beyond working memory and processing speed.
      ].

      Perceived Stress Scale 10

      The Perceived Stress Scale 10 (PSS-10) [
      • Cohen S.
      • Williamson G.
      Perceived stress in a probability sample of the United States.
      ] is a 10-item measure that examines perceived stressful situations that may occur in daily life. All questions are rated on a five-point Likert scale, ranging from 0 (never) to 4 (very often) to assess how respondents felt during the past month. Overall scores range from 0 to 40, with higher scores indicating greater perceived stress. The PSS-10 has been used with diverse adolescent samples to examine perceptions of stress and has exhibited good internal consistency [
      • Carlozzi B.L.
      • Winterowd C.
      • Harrist R.S.
      • et al.
      Spirituality, anger, and stress in early adolescents.
      ] (α = .82). In the present study, internal consistency (α = .63) was lower than what has been previously reported. After omitting scores for 23 participants who endorsed the same response option for all items
      The 23 participants demonstrating an invalid response pattern (i.e., the same response option was chosen for every item) on the PSS-10 were excluded from analyses involving the PSS-10. However, these participants were not excluded from analyses involving the AOSPAN as it was not subject to self-report response biases. An independent t test found no significant differences in OSPAN scores between those with an invalid PSS-10 and the remaining participants (p = .86), suggesting that those with invalid PSS-10 scores did not score differently than other participants on the AOSPAN task.
      , scores from the remaining participants (n = 149) showed acceptable internal consistency (α = .71).

      Screen for Child Anxiety and Related Emotional Disorders

      The Screen for Child Anxiety and Related Emotional Disorders (SCARED) [
      • Birmaher B.
      • Khetarpal S.
      • Brent D.
      • et al.
      The screen for child anxiety related emotional disorders (SCARED): Scale construction and psychometric characteristics.
      ] is a 41-item self-report anxiety measure designed for youth ages 8–18 years. The SCARED parallels the five types of anxiety disorders described in the DSM-IV: generalized anxiety, social phobia, separation anxiety, panic disorder, and school phobia. Higher total scores indicate higher levels of anxiety and worry. Respondents use a three-point Likert scale: 0 (not true), 1 (sometimes true), or 2 (often true) after reading statements such as “I worry about other people liking me.” The SCARED has shown good psychometric properties in clinical and nonclinical youth samples. [
      • Hale W.H.
      • Crocetti E.
      • Raaijmakers Q.A.
      • Meeus W.H.
      A meta-analysis of the cross-cultural psychometric properties of the Screen for Child Anxiety Related Emotional Disorders (SCARED).
      ] In the present study, the SCARED Cronbach α = .93.
      Less than 5% of cases were missing data on the SCARED. Missing data were examined using listwise deletion methods in SPSS to determine if data were missing at random (MAR) or not missing at random (NMAR). The data was MAR; therefore, mean imputation was used to replace single missing item values. Scale means were computed for each participant using the mean of all other available item scores for that participant.

      Child Acceptance and Mindfulness Measure

      The Child Acceptance and Mindfulness Measure (CAMM) [
      • Greco L.A.
      • Baer R.A.
      • Smith G.T.
      Assessing mindfulness in children and adolescents: Development and validation of the child and adolescent mindfulness measure (CAMM).
      ] is a 25-item self-report measure that assesses the degree to which respondents observe internal experiences, act with awareness, and accept internal experiences in a nonjudgmental manner. A five-point Likert scale ranging from 0 (never true) to 4 (always true) is used in response to items such as “I try only to think about things that makes me feel happy.” Total scores range from 0 to 100. Higher scores indicate greater levels of acceptance and mindfulness. The CAMM has demonstrated good internal consistency previously [
      • Greco L.A.
      • Baer R.A.
      • Smith G.T.
      Assessing mindfulness in children and adolescents: Development and validation of the child and adolescent mindfulness measure (CAMM).
      ]; however, in the present study, it was extremely low (α = .33). Nineteen participants endorsed the same response option throughout the entire measure. Despite excluding these scores, the scores from the remaining participants (n = 150) yielded unacceptably low internal consistency (α = .41).

      Data analysis

      Analyses were completed using SPSS Statistics for Windows Version 20.0 (IBM Corp., Armonk, NY). A mixed-design analysis of variance was used to examine changes in WMC, as measured by OSPAN scores, between three conditions (mindfulness meditation, hatha yoga, waitlist control) from preintervention to postintervention. Mixed-design analyses of variance were also used to compare PSS-10 and SCARED scores among the three groups from pre intervention to postintervention. The CAMM was eliminated from further statistical analyses because of poor reliability (α = .41). A Bonferroni correction was applied to reduce the chance of a type 1 error. The number of hypothesized comparisons was 3; thus, α was adjusted to .016 (α/n). A power analysis (using G*Power) indicated that an estimated total sample size of 120 participants was needed to achieve 80% power and a medium effect size of .05.

      Results

      Participant characteristics

      One hundred ninety-eight adolescents completed the study. Figure 1 illustrates the flow of participants through the trial. Four hundred and twenty students received information about the study. However, because of space and study personnel limitations, only the first 30 to 35 students in each of the PE class periods who turned in their assent/consent forms were able to participate. Twelve participants (4.7%) dropped out after completing baseline measures, resulting in 186 participants being randomly assigned to mindfulness meditation, hatha yoga, or waitlist control. Participants who (1) completed both baseline and postintervention measures and (2) attended six or more mindfulness sessions (75% of the intervention) were deemed study completers and included in statistical analyses (N = 172). At baseline, no significant differences were found between those who dropped out after completing baseline measures, those who attended fewer than six sessions, and those who completed six or more sessions (OSPAN, F(2,197) = 2.7, p = .07; PSS, F(2,192) = .4, p = .70; SCARED, F(2,185) = 2.39, p = .10).
      Demographic characteristics for the 198 participants appear in Table 1. Participants were primarily female (62%) ranging in age from 12 to 15 years (mean age = 13.18, SD = .72). Most participants were Hispanic and had little or no experience with any mindfulness practices.
      Table 1Demographic characteristics of the study sample
      CharacteristicMeditation group (n = 54)Hatha yoga group (n = 65)Waitlist group (n = 53)
      Gender
       Female83.3 (n =45)63.1 (n = 41)52.8 (n = 28)
       Male16.7 (n = 9)36.9 (n = 24)47.2 (n = 25)
      Race/ethnicity
       Spanish/Hispanic/Latino72.2 (n = 39)67.7 (n = 44)56.5 (n = 30)
       Asian or Pacific Islander14.8 (n = 8)21.5 (n = 14)24.5 (n = 13)
       Biracial3.7 (n = 2)7.7 (n = 5)11.3 (n = 6)
       Caucasian0 (n = 0)1.5 (n = 1)1.9 (n = 1)
       African-American1.9 (n = 1)1.5 (n = 1)0 (n = 0)
       Native American5.6 (n = 3)0 (n = 0)3.8 (n = 2)
       Other1.9 (n = 1)0 (n = 0)1.9 (n = 1)
      Grade
       Seventh grade38.9 (n = 21)44.6 (n = 29)49.1 (n = 26)
       Eighth grade61.1 (n = 33)53.8 (n = 35)50.9 (n = 27)
       Ninth grade0 (n = 0)1.5 (n = 1)0 (n = 0)
      Period
       Second period11.1 (n = 6)23.1 (n = 15)0 (n = 0)
       Third period16.7 (n = 9)12.3 (n = 8)28.3 (n = 15)
       Fourth period14.8 (n = 8)13.8 (n = 9)26.4 (n = 14)
       Fifth period13.0 (n = 7)20.0 (n = 13)20.8 (n = 11)
       Sixth period22.2 (n = 12)9.2 (n = 6)24.5 (n = 13)
       Seventh period22.2 (n = 12)21.5 (n = 14)0 (n = 0)
      Previous meditation experience
       Never tried44.4 (n = 24)49.2 (n = 32)34.0 (n = 18)
       Tried once or twice44.4 (n = 24)38.5 (n = 25)52.8 (n = 28)
       Practice a few times a year5.6 (n = 3)4.6 (n = 3)7.5 (n = 4)
       Practice a few times a month1.9 (n = 1)1.5 (n = 1)0 (n = 0)
       Practice once a week1.9 (n = 1)1.5 (n = 1)3.8 (n = 2)
       Practice few times a week1.9 (n = 1)3.1 (n = 2)1.9 (n = 1)
       Practice daily0 (n = 0)1.5 (n = 1)0 (n = 0)
      Previous yoga experience
       Never tried55.6 (n = 30)53.8 (n = 35)47.2 (n = 25)
       Tried once or twice40.7 (n = 22)36.9 (n = 24)45.3 (n = 24)
       Practice a few times a year1.9 (n = 1)4.6 (n = 3)3.8 (n = 2)
       Practice a few times a month1.9 (n = 1)3.1 (n = 2)1.9 (n = 1)
       Practice once a week0 (n = 0)0 (n = 0)1.9 (n = 1)
       Practice a few times a week0 (n = 0)1.5 (n = 1)0 (n = 0)
       Practice daily0 (n = 0)0 (n = 0)0 (n = 0)
      Extracurricular activities
       No activities57.4 (n = 31)33.8 (n =22)30.2 (n = 16)
       1–2 Activities40.8 (n = 22)53.9 (n = 35)56.6 (n = 30)
       3–4 Activities1.9 (n = 1)12.3 (n = 8)13.2 (n = 7)
      Data is reported as percentages. One-way analyses of variance (ANOVAs) and chi-square analyses were conducted to determine if randomization produced three equivalent groups with regard to demographic information. There were no significant between-group differences in age, F(2,171) = .81, p = .45; grade F(2,171) = .54, p = .59; class period, F(2,171) = .30, p = .29; or race, F(2,171) = 2.0, p = .15. Similarly, no significant between-group differences were found with regard to previous yoga experience (p = .54) or previous mindfulness meditation experience (p = .79). There was a significant between-group difference in the number of reported extracurricular activities, F(2,171) = 4.40, p = .02. The mindfulness meditation group reported less participation in extracurricular activities compared to the hatha yoga and waitlist control groups. No statistically significant differences were found on any outcome variables when using a series of ANOVAs, suggesting that involvement in extracurricular activities was unlikely to be a confound. There was also a significant between-group difference in gender (p = .003). There were significantly fewer male participants in the mindfulness meditation group (n = 9 of 54, 16.7%) compared to the hatha yoga group (n = 24 of 65, 36.9%). Independent samples t tests were conducted to see if there were statistically significant gender differences on the dependent variables. No statistically significant differences were found on the PSS-10 or SCARED, suggesting that gender was unlikely to be a confound.

      Outcomes

      The primary study hypothesis was that the mindfulness meditation group would show significantly greater improvement in WMC compared to the hatha yoga and waitlist control conditions at postintervention. There was a significant time by group interaction, F (2,160) = 4.77, p = .01, ηp2=.04. A simple effects procedure was then conducted to determine the specific interaction effect and test the separate group conditions for significance. Results indicated that the mindfulness meditation group reported significant pre–post improvements in WMC, F(1,50) = 15.71, p < .001, ηp2=.24, whereas participants in the hatha yoga and waitlist control groups did not [hatha yoga: F(1,59) = 3.85, p =.11, ηp2=.04, waitlist: F(1, 51) = .50, p = .46, ηp2=.01; Figure 2]. Hypothesis 1 was supported, suggesting that mindfulness meditation was more effective in improving WMC than hatha yoga.
      Figure thumbnail gr2
      Figure 2Changes in working memory capacity (Operational Span Task) by group (mindfulness meditation, hatha yoga, waitlist control) from baseline to postintervention. Error bars represent one standard error above and below the mean.
      The secondary hypothesis was that the mindfulness meditation and hatha yoga groups would report significantly lower levels of stress and anxiety at postintervention compared to the waitlist control group. Differences in PSS-10 scores among the three groups preintervention to postintervention did not show a significant condition by time interaction effect, F(2,143) = 1.53, p = .22, ηp2=.02. There was a significant main effect for time, F(1,143) = 7.10, p = .01, ηp2=.05, with all three groups showing a reduction in perceived stress from preintervention to postintervention. The main effect of group was not significant, F(2,143) = .01, p =.99, ηp2=.00.
      Similarly, differences in SCARED scores among the three groups from preintervention to postintervention did not show a significant condition by time interaction effect, F(2,159) = .22, p = .80, ηp2=.00. There was a significant main effect for time, F(1,159) = 15.73, p < .001, ηp2=.09, with all three groups showing a reduction in anxiety from preintervention to postintervention. The main effect of group was not significant, F(2,159) = .18, p = .84, ηp2=.00. Hypothesis 2 was not supported, suggesting that mindfulness meditation and hatha yoga were not significantly different in their ability to reduce stress or anxiety. Means, standard deviations, and within-group pre–post effect sizes are presented in Table 2.
      Table 2Descriptive statistics of dependent variables by group condition at baseline and postintervention
      GroupVariableBaselinePost-interventiond
      nMSDMinMaxnMSDMinMax
      MeditationAOSPAN5434.1518.74.0075.005144.4518.913.0072.00−.55
      PSS4820.135.309.0033.004617.415.055.0030.00.52
      SCARED5322.9614.112.0069.005120.1513.823.0059.00.20
      Hatha yogaAOSPAN6535.9219.62.0075.006039.9621.05.0075.00−.20
      PSS5819.225.3610.0031.005718.195.825.0033.00.18
      SCARED6223.8015.03.0053.006121.1013.14.0047.00.19
      Waitlist controlAOSPAN5339.6420.77.0075.005236.9620.60.0075.00.13
      PSS4318.886.357.0033.004318.476.152.0032.00.06
      SCARED5023.7014.56.0064.005020.6013.10.0047.00.22
      AOSPAN = Automated Operational Span Task; d = Cohen's d for within-group pre-post effect sizes; M = mean; Min = minimum score; Max = maximum score; n = number; PSS = Perceived Stress Scale; SCARED = Screen for Childhood Anxiety Related Emotional Disorders; SD = standard deviation.
      Some participants did not complete all study measures. Nine participants skipped the AOSPAN at postintervention. Three participants did not complete the SCARED at postintervention. In addressing measures that were not completed in their entirety (i.e., whole measure), sample size was sufficient to use case pairwise deletion. This option excluded cases only if they were missing the data required for the specific analysis. The cases were still included in the analysis for which they had the necessary information; thus, there are different sample sizes (n) among the various analyses.

      Discussion

      The primary purpose of this randomized controlled trial was to evaluate the effectiveness of mindfulness meditation on WMC in adolescents. The broader goal was to advance our understanding of the potential benefits of mindfulness practice among adolescents. We hypothesized that mindfulness meditation training would result in greater improvement in WMC compared to hatha yoga and a waitlist control condition. This hypothesis was supported. Results revealed a significant increase in WMC for participants in the meditation group, whereas those in the hatha yoga and waitlist control groups did not exhibit significant changes. These findings suggest that mindfulness meditation may be an effective approach for improving WMC in adolescents, which, to date, has only been investigated in adults [
      • Chambers R.
      • Lo B.C.
      • Allen N.B.
      The impact of intensive mindfulness training on attentional control, cognitive style, and affect.
      ,
      • Jha A.P.
      • Stanley E.A.
      • Kiyonaga A.
      • et al.
      Examining the protective effects of mindfulness training on working memory capacity and affective experience.
      ,
      • Mrazek M.D.
      • Franklin M.S.
      • Phillips D.T.
      • et al.
      Mindfulness training improves working memory capacity and GRE performances while reducing mind wandering.
      ,
      • Zeidan F.
      • Johnson S.K.
      • Diamond B.J.
      • et al.
      Mindfulness meditation improves cognition: Evidence of brief mental training.
      ]. These results are consistent with the notion that the practice of meditation—which requires sustained attention while simultaneously redirecting attention back to the current experience—is closely related to the function of working memory [
      • Jenson C.G.
      • Vangkilde S.
      • Frokjaer V.
      • Hasselbalch S.G.
      Mindfulness training affects attention: Or is it attentional effort?.
      ].
      Although mindfulness invariably involves directing attention to the present moment, mindfulness is complex and can be practiced in myriad ways. Such complexity allows for the possibility that certain mindfulness practices may be uniquely associated with aspects of cognitive functioning and physical health [
      • Kabat-Zinn J.
      Full catastrophe living: Using the wisdom of your body and mind to face stress, pain, and illness.
      ,
      • Sauer-Zavala S.E.
      • Walsh E.C.
      • Eisenlohr-Moul T.A.
      • Lykins E.L.
      Comparing mindfulness-based intervention strategies: Differential effects of sitting meditation, body scan, and mindful yoga.
      ]. Thus, although the present study's findings suggest that mindfulness meditation may be particularly effective in strengthening WMC, practices that cultivate mindfulness in other ways (e.g., through mindful movement) likely afford their own distinctive benefits. For example, hatha yoga requires different physical involvements than sitting meditation. Research suggests that regular hatha yoga practice is associated with improved musculoskeletal strength and flexibility [
      • Kabat-Zinn J.
      Full catastrophe living: Using the wisdom of your body and mind to face stress, pain, and illness.
      ,
      • Oken B.S.
      • Kishiyama S.
      • Zajdel D.
      • et al.
      Randomized controlled trial of yoga and exercise in multiple sclerosis.
      ]. However, to our knowledge, no studies have investigated the impact of hatha yoga on WMC specifically or cognitive functioning more broadly. Finally, although present findings suggest that mindfulness meditation more effectively promotes WMC, it is also conceivable that learning to practice yoga mindfully was challenging for adolescents, but if done so mindfully, yoga may confer similar benefits to WMC.
      The secondary hypothesis was that adolescents receiving mindfulness training (either meditation or hatha yoga) would report lower levels of stress and anxiety at postintervention compared to waitlist controls. This hypothesis was not supported. Rather all three groups showed significantly reduced stress and anxiety postintervention. This challenges conclusions drawn in several uncontrolled studies reporting the benefits of mindfulness techniques to decrease stress and anxiety [
      • Hofmann S.G.
      • Sawyer A.T.
      • Witt A.A.
      • Oh D.
      The effect of mindfulness- based therapy on anxiety and depression: A meta-analytic review.
      ]. Because the present study included a waitlist control group, it is clear that such decreases in stress and anxiety cannot be solely attributed to the “active ingredients” of mindfulness interventions.
      One potential explanation for the nonsignificant results pertaining to perceived stress may be the low internal consistency on the PSS-10. Although the PSS-10 has exhibited reliability in previous studies, the majority of research was conducted with adult samples. In the present study, participants were adolescents from primarily ethnic minority backgrounds. Patterns of inconsistent responding on the PSS-10 were mainly (74%) found among Hispanic participants. As approximately 25% of participants' parents preferred completing study forms in Spanish, language barriers may have influenced participants' completion of self-report measures. Although data from inconsistent responders were excluded from analyses, results from the PSS-10 should be interpreted with caution.
      Another possible reason for the lack of group differences on stress and anxiety measures may be attributable to using a community, rather than clinical sample. Past research has found significant improvements in levels of perceived stress and anxiety with clinical samples [
      • Tan L.B.
      A critical review of adolescent mindfulness-based programmes.
      ,
      • Biegel G.M.
      • Brown K.W.
      • Shapiro S.L.
      • Schubert C.M.
      Mindfulness-based stress reduction for the treatment of adolescent psychiatric outpatients: A randomized clinical trial.
      ]; these measures are likely not as sensitive in a community sample. The present study included adolescents who scored similarly on the PSS (M = 19.28) to other community samples [
      • Lemon J.C.
      • Watson J.C.
      Early identification of potential high school dropouts: An investigation of the relationship among at-risk status, wellness, perceived stress and mattering.
      ]. Likewise, approximately half (53%) of participants in this study scored within the normal range on the SCARED. Thus, although it seems promising that moderate or high levels of stress and anxiety may be reduced through mindfulness training [
      • Biegel G.M.
      • Brown K.W.
      • Shapiro S.L.
      • Schubert C.M.
      Mindfulness-based stress reduction for the treatment of adolescent psychiatric outpatients: A randomized clinical trial.
      ], individuals whose level of distress is within the normal range may show less improvement [
      • Zoogman S.
      • Goldberg S.B.
      • Hoyt W.T.
      • Miller L.
      Mindfulness interventions with youth: A meta-analysis.
      ], which is consistent with the small (hatha yoga) to medium (mindfulness meditation) pre–post effect sizes found in the present study [
      • Semple R.J.
      • Lee J.
      • Rosa D.
      • Miller L.F.
      A randomized trial of mindfulness-based cognitive therapy for children: Promoting mindful attention to enhance social-emotional resiliency in children.
      ]. It may also be that more intervention sessions were needed to show greater benefit.
      It was initially hypothesized that participants in the mindfulness meditation and hatha yoga groups would report greater levels of mindfulness postintervention compared to waitlist controls. This could not be tested due to the unacceptable internal consistency (α = .41) of the CAMM. Problems with reliability may be partially explained by the inconsistent response patterns (e.g., acquiescence bias) evidenced by a subset of participants. Additionally, mindfulness is a complex, abstract construct that may be challenging to capture in a self-report measure. Thus, adolescents may have had difficulty understanding and responding to CAMM items. Studies have generally found the CAMM to display adequate reliability, and it is considered a developmentally appropriate measure for adolescents [
      • Greco L.A.
      • Baer R.A.
      • Smith G.T.
      Assessing mindfulness in children and adolescents: Development and validation of the child and adolescent mindfulness measure (CAMM).
      ]. However, research has been conducted with predominately Caucasian youth [
      • Greco L.A.
      • Baer R.A.
      • Smith G.T.
      Assessing mindfulness in children and adolescents: Development and validation of the child and adolescent mindfulness measure (CAMM).
      ]. The present findings suggest that the crosscultural generalizability of the psychometric properties of the CAMM should be further investigated.

      Limitations and directions for future research

      The present study is the first to date to explore the effectiveness of mindfulness interventions on working memory in adolescents. As the present study was conducted with an adolescent sample attending a public school in a lower–socioeconomic status (SES) neighborhood, intervention effects may not generalize to samples in other settings. Research has shown that lower-SES students score significantly lower than their higher-SES counterparts on tests of language, working memory, and executive functioning [
      • Noble K.G.
      • Norman M.F.
      • Farah M.J.
      Neurocognitive correlates of socioeconomic status in kindergarten children.
      ]. Given the relationship between WMC and broader executive functioning, which is integral to learning, improvement in this area may benefit adolescents from disadvantaged backgrounds. In general, future work should examine mindfulness meditation with diverse populations in terms of ethnicity, age, SES, geographic location, and other demographic variables.
      The timing of the intervention may have introduced a source of error. This study took place near the end of the academic school year. Students may have experienced a general decrease in stress and anxiety as they approached summer break. This may have contributed to the general reduction in stress and anxiety seen across all participants. Future studies need to examine whether differences in intervention effectiveness are seen at different times of the academic year.
      It may be beneficial to further investigate whether enhancements in WMC are associated with improvements in overall academic performance to further evidence improvements in cognitive functioning. Investigations of a broader range of school-related outcomes would be helpful in understanding whether gains from sitting meditation translate into improvements in academic performance or school functioning more broadly. Similarly, future research may also benefit from administering additional performance-based tasks, such as the N-back or Stroop task, to more fully evaluate the impact of mindfulness interventions on WMC and other neuropsychological functions.
      Despite the growing literature on mindfulness in adolescents, this is the first study to provide support for the benefits of short-term mindfulness practice, specifically mindfulness meditation, in improving WMC in adolescents. It is essential to continue the improvement, implementation, and empirical examination of mindfulness practices to improve our understanding of their usage and clinical utility.

      Acknowledgments

      The authors would like to extend our deepest appreciation to the children, parents, and school staff who participated in this study. The authors thank Janina Scarlet for providing feedback throughout the development of the project, Kiere Eichelberger for assistance with data collection, and Bethany Dawson for help with data management and scoring. They also like to thank the yoga studios that donated yoga equipment or offered participant incentives. This research was based on the first author's doctoral dissertation. Portions of the results of this study were presented at the Society for Pediatric Psychology Annual Conference (April, 2014).

      References

        • Kabat-Zinn J.
        Full catastrophe living: Using the wisdom of your body and mind to face stress, pain, and illness.
        Delacorte Press, New York1990
        • Baer R.A.
        Mindfulness training as a clinical intervention: A conceptual and empirical review.
        Clin Psychol. 2003; 10: 125-143
        • Zenner C.
        • Herrnleben-Kurz S.
        • Walach H.
        Mindfulness-based interventions in schools—a systematic review and meta-analysis.
        Front Psychol. 2014; 5: 1-20
        • Tan L.B.
        A critical review of adolescent mindfulness-based programmes.
        Clin Child Psychol Psychiatry. 2015; ([e pub ahead of print]. DOI: 10.1177/1359104515577486)
        • Biegel G.M.
        • Brown K.W.
        • Shapiro S.L.
        • Schubert C.M.
        Mindfulness-based stress reduction for the treatment of adolescent psychiatric outpatients: A randomized clinical trial.
        J Consult Clin Psychol. 2009; 77: 855-866
        • Black D.S.
        • Fernando R.
        Mindfulness training and classroom behavior among lower-income and ethnic minority elementary school children.
        J Child Fam Stud. 2013; 23: 1242-1246
        • Burke C.A.
        Mindfulness-based approaches with children and adolescents: A preliminary review of current research in an emergent field.
        J Child Fam Stud. 2009; 19: 133-144
        • Tan L.B.
        • Martin G.
        Taming the adolescent mind: A randomised controlled trial examining clinical efficacy of an adolescent mindfulness-based group programme.
        Child Adolesc Ment Health. 2015; 20: 49-55
        • Semple R.J.
        • Lee J.
        • Rosa D.
        • Miller L.F.
        A randomized trial of mindfulness-based cognitive therapy for children: Promoting mindful attention to enhance social-emotional resiliency in children.
        J Child Fam Stud. 2009; 19: 218-229
        • Zoogman S.
        • Goldberg S.B.
        • Hoyt W.T.
        • Miller L.
        Mindfulness interventions with youth: A meta-analysis.
        Mindfulness. 2014; 6: 290-302
        • Davidson R.J.
        • Kabat-Zinn J.
        • Schumacher J.
        • et al.
        Alterations in brain and immune function, produced by mindfulness meditation.
        Psychosom Med. 2003; 65: 564-570
        • Fletcher L.B.
        • Schoendorff B.
        • Hayes S.C.
        Searching for mindfulness in the brain: A process-oriented approach to examining the neural correlates of mindfulness.
        Mindfulness. 2010; 1: 41-63
        • Lazar S.W.
        • Kerr C.E.
        • Wasserman R.H.
        • et al.
        Meditation experience is associated with increased cortical thickness.
        NeuroReport. 2005; 16: 1893-1897
        • Lutz A.
        • Greischar L.L.
        • Rawlings N.B.
        • et al.
        Long-term meditators self- induce high amplitude gamma synchrony during mental practice.
        Proc Natl Acad Sci USA. 2004; 101: 16369-16373
        • Slagter H.A.
        • Lutz A.
        • Greischar L.L.
        • et al.
        Mental training affects distribution of limited brain resources.
        PLos Biol. 2007; 5: 138
        • Dehn M.J.
        Working memory and academic learning: Assessment and intervention.
        Jon Wiley & Sons, Hoboken, NJ2008
        • Chambers R.
        • Lo B.C.
        • Allen N.B.
        The impact of intensive mindfulness training on attentional control, cognitive style, and affect.
        Cognitive Ther Res. 2008; 32: 303-322
        • Jha A.P.
        • Stanley E.A.
        • Kiyonaga A.
        • et al.
        Examining the protective effects of mindfulness training on working memory capacity and affective experience.
        Emotion. 2010; 10: 54-64
        • Mrazek M.D.
        • Franklin M.S.
        • Phillips D.T.
        • et al.
        Mindfulness training improves working memory capacity and GRE performances while reducing mind wandering.
        Psychol Sci. 2013; 24: 776-781
        • Zeidan F.
        • Johnson S.K.
        • Diamond B.J.
        • et al.
        Mindfulness meditation improves cognition: Evidence of brief mental training.
        Conscious Cogn. 2010; 19: 575-605
        • Kyllonen P.C.
        • Christal R.E.
        Reasoning ability is (little more than) working memory capacity?!.
        Intelligence. 1990; 14: 389-433
        • Swanson H.L.
        • Beebe-Frankenberger M.
        Relationship between working memory and mathematical problem solving in children at risk and not at risk for serious math difficulties.
        J Educ Psychol. 2004; 96: 471-491
        • Britton W.B.
        • Lepp N.E.
        • Niles H.F.
        • et al.
        A randomized controlled pilot trial of classroom-based mindfulness meditation compared to an active control condition in sixth-grade children.
        J Sch Psychol. 2014; 52: 263-278
        • Sibinga E.M.S.
        • Perry-Parrish C.
        • Chung S.
        • et al.
        School-based mindfulness instruction for urban male youth: A small randomized controlled trial.
        Prev Med. 2013; 57: 799-801
        • Weinstein N.
        • Brown K.W.
        • Ryan R.M.
        A multi-method examination of the effects of mindfulness on stress attribution, coping, and emotional well-being.
        J Res Pers. 2009; 43: 374-385
        • Orsillo S.M.
        • Roemer L.
        • Barlow D.H.
        Integrating acceptance and mindfulness into existing cognitive-behavior treatment for GAD: A case study.
        Cogn Behav Pract. 2003; 10: 222-230
        • Unsworth N.
        • Heitz R.P.
        • Schrock J.C.
        • Engle R.W.
        An automated version of the operation span task.
        Behav Res Methods. 2005; 37: 498-505
        • Conway A.R.
        • Kane M.J.
        • Bunting M.F.
        • et al.
        Working memory span tasks: A methodological review and user's guide.
        Psychon Bull Rev. 2005; 12: 769-786
        • Kaufman S.B.
        • DeYoung C.G.
        • Gray J.R.
        • et al.
        Associative learning predicts intelligence above and beyond working memory and processing speed.
        Intelligence. 2009; 37: 374-382
        • Cohen S.
        • Williamson G.
        Perceived stress in a probability sample of the United States.
        in: Spacapam S. Oskamp S. The Social Psychology of Health: Claremont Symposium on Applied Social Psychology. Sage, Newbury Park, CA1988
        • Carlozzi B.L.
        • Winterowd C.
        • Harrist R.S.
        • et al.
        Spirituality, anger, and stress in early adolescents.
        J Relig Health. 2010; 49: 445-459
        • Birmaher B.
        • Khetarpal S.
        • Brent D.
        • et al.
        The screen for child anxiety related emotional disorders (SCARED): Scale construction and psychometric characteristics.
        J Am Acad Child Adolesc Psychiatry. 1997; 36: 545-553
        • Hale W.H.
        • Crocetti E.
        • Raaijmakers Q.A.
        • Meeus W.H.
        A meta-analysis of the cross-cultural psychometric properties of the Screen for Child Anxiety Related Emotional Disorders (SCARED).
        J Child Psychol Psychiatry. 2011; 52: 80-90
        • Greco L.A.
        • Baer R.A.
        • Smith G.T.
        Assessing mindfulness in children and adolescents: Development and validation of the child and adolescent mindfulness measure (CAMM).
        Psychol Assess. 2011; 23: 606-614
        • Jenson C.G.
        • Vangkilde S.
        • Frokjaer V.
        • Hasselbalch S.G.
        Mindfulness training affects attention: Or is it attentional effort?.
        J Exp Psychol Gen. 2012; 141: 106
        • Sauer-Zavala S.E.
        • Walsh E.C.
        • Eisenlohr-Moul T.A.
        • Lykins E.L.
        Comparing mindfulness-based intervention strategies: Differential effects of sitting meditation, body scan, and mindful yoga.
        Mindfulness. 2012; 4: 383-388
        • Oken B.S.
        • Kishiyama S.
        • Zajdel D.
        • et al.
        Randomized controlled trial of yoga and exercise in multiple sclerosis.
        Neurology. 2004; 62: 2058-2064
        • Hofmann S.G.
        • Sawyer A.T.
        • Witt A.A.
        • Oh D.
        The effect of mindfulness- based therapy on anxiety and depression: A meta-analytic review.
        J Consult Clin Psychol. 2010; 78: 169-183
        • Lemon J.C.
        • Watson J.C.
        Early identification of potential high school dropouts: An investigation of the relationship among at-risk status, wellness, perceived stress and mattering.
        JARI. 2011; 16: 16-23
        • Noble K.G.
        • Norman M.F.
        • Farah M.J.
        Neurocognitive correlates of socioeconomic status in kindergarten children.
        Dev Sci. 2005; 8: 74-87