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| Study authors and years | Objectives | Measures | Participants | Intervention | Control | Key findings |
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| Klimecki et al. [52] | To investigate if compassion training produced functional neural plasticity | FMRI/report | 58 female volunteers | One day training in LKM held by a teacher with 10 years’ experience | Memory | Compared to memory training, compassion training increased positive affect, as did plasticity in the neural networks associated with love, affiliation, and positive valuation |
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| Klimecki et al. [53] | To determine if empathy and compassion training have distinct effects on neural functions, and if compassion training could overcome distress | FMRI/self-report | 53 female volunteers | One day training course in empathy and compassion, both of which held by teacher’s with >10 years’ experience in their particular field | Memory | In response to witnessing other’s distress, compassion training reduced negative affect, whilst increasing positive affect. Also, compassion training augmented neural activity in areas related to reward, affiliation, and positive affect, whereas the empathic response and negative affect increased after empathy training, also corresponding activity was seen in neural networks related to negative affect and self-experienced pain |
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| Polizzi et al. [58] | (1) To evaluate whether a single session of mindfulness meditation would elicit gains in psychological functioning
(2) Whether 2 sessions of LKM would produce greater positive outcomes than sitting quietly | Questionnaires scales/index | 91 university undergraduate volunteers, 77% female | (1) 15-minute meditation training followed by two weeks at home practice.
(2) 2 sessions LKM program | Sitting quietly | Brief mindfulness meditation enhanced positive affect compared to sitting quietly. LKM increased compassion for other compared to sitting quietly. Both studies reflected improved psychological outcomes |
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| Fredrickson et al. [63] | To test whether LKM generated positive emotions and built consequential personal resources | Survey/self-report | 139 volunteer IT company employees, 66% female. Males lost to attrition and/or disqualification | 6x 60-minute LKM group sessions led by a stress management expert | Waitlist | LKM practice generated positive emotions whilst decreasing negative emotions. Also, the personal resources of self-acceptance, mindful attention, and positive reactions to others increased, as did physical health |
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| Leung et al. [64] | To examine the difference in grey matter between novices and LKM experts | MRI | 26 Chinese men, 10 LKM experts, 15 novices | Novices completed 7 hrs of LKM under the instruction of an expert meditator | NA | LKM experts had considerably more grey matter in the posterior parahippocampal gyrus and right angular gyrus (regions associated with empathic responding emotion regulation). Posited that LKM may affect neural regions associated with emotion regulation |
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| Le Nguyen et al. [61] | To investigate if training in mindfulness meditation and LKM impact on telomere length (cellular aging), compared to a control | Blood samples/self-report | 176 novice volunteers | 6x 1-hour meditation workshops, plus 5 × 20 min. At home recorded guided meditation. Developed by meditation experts | Mindfulness meditation | Telomeres significantly shortened in the mindfulness and waitlist control groups. On average the LKM group, had no significant shortening and LKM appeared to buffer against attrition |
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| Kok et al. [67] | To investigate if cardiac vagal tone improved through the self-generation of positive emotions | To assess vagal tone, analysis of spectral frequency heart rate (HR) data was carried out to find high-frequency (HR) variability. Self-analysis scale | 65 novice faculty university staff. 66% female, 34% male | 6x 1-hour LKM workshops with trained medication teacher and at home practice | Waitlist | LKM increased self-perceived social connections, positive emotions, and cardiac vagal tone, compared to the control |
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| Engen et al. [60] | To assess the effects of long-term training on structural brain networks | fMRI/MRI | 17 long-term Tibetan LKM & compassion meditators | Previous participation in a 3 month to 3-year meditation retreat | No meditation experience | Long-term meditators (LTM) had increased thickness ranging from the prefrontal regions to the anterior insula cortex relative to the control. Hypothesised that LKM may generate positive emotional states, foster resilience, and buffer against negative stressors |
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| Engen and Singer [59] | To investigate the effects of compassion-meditation techniques, compared to the emotion regulation strategy of reappraisal, on neural affective and experiential responses whilst observing the distress of others | fMRI/subjective rating | 18 long-term LKM Tibetan meditators | Previous participation of at least 3 years at a full-time meditation retreat | NA | Positive affect increased during compassion, whereas reappraisal decreased negative affect. Compared to reappraisal compassion increased activation in neural regions associated with positive affect, reward processing, and affiliation |
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| Leung et al. [71] | To examine the amygdala’s functional connectivity in meditation experts during affective processing | fMRI/self-report | 24 Chinese men 10 experts, 14 novices | Novices completed 7 hours of at-home meditation practice | No meditation experience | There was greater connectivity between the left amygdala and the emotion processing regulation system (dACC) in the experts compared to the novices. The experts showed lower levels of negative emotions than the novices whilst processing negative emotions |
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| Valk et al. [66] | To investigate whether targeted cognitive training modules induced changes in the brain morphology | MRI/video task | 332 novice volunteers | 3-day intensive retreat followed by 8x 2-hr weekly group sessions and at-home practice, in either presence, affect (LKM), or perspective training (resource project) | NA | Performance in the theory of mind task increased for those trained in the perspective module. LKM increased compassion ratings after watching neutral or distressing videos, and increased regions associated with empathy and emotion regulation. All training resulted in structural brain plasticity |
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| Weng et al. [69] | To investigate if visual attention to suffering would increase or decrease amygdala activation, after training in either compassion or reappraisal | fMRI/eye tracking | 56 novices | Compassion or reappraisal training via guided audio instructions, 30-min per day for 2 weeks | NA | Compassion training decreased activation in the amygdala and regions associated with negative processing and arousing stimuli (right AI/OFC), whilst visually engaging in the suffering of others. This finding was not seen after reappraisal training |
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| Hildebrandt et al. [72] | To understand which specific training modules facilitated emotional regulation | Self-report | 332 novice volunteers | 13-week presence, affect (LKM) or perspective training modules (resource project) | NA | No emotion regulation was self-reported after the presence training module. Perspective training increased cognitive reappraising and acceptance. LKM increased acceptance and positive reappraisal whilst decreasing avoidance |
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| Trautwein et al. [73] | To investigate whether different mental training modules have specific or the same effects on cognitive functioning | EmpaTOM video task | 332 novice volunteers | 13-week presence, affect (LKM), or perspective training modules (resource project) | NA | The presence module neither increased compassion, nor theory of mind. The affect module increased compassion fostered positive affect, and enhanced attention. The perspective module enhanced theory of mind performance |
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| Lutz et al. [62] | To examine the neural circuitry whilst novices and expert meditators engaged in LKM | fMRI | 16 long-term Buddhist meditation experts | Previously completed 10–50,000 hrs of meditative practice | No previous meditation training | The neural circuitry-related empathic processing was modulated via the generation of, and expertise in LKM |
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| Frederickson [70] | To uncover the emotional and dose-response profiles, between and within individuals after meditation training | Self-report | 339 novice volunteers | 6x, 1 hr LKM or MM group sessions and at-home practice | NA | Both LKM and MM increased participants’ day-today experience of positive emotions over time. Regardless of meditation type increased practice resulted in feeling more positive emotions |
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| Seppala et al. [68] | (1) To determine if 10 minutes of compassion training changed affective responding to others
(2) How does LKM compare to other affective interventions | Self-report | 134 paid undergraduate university students | LKM instructions over headphones | Positive affect induction and neutral visualisation | LKM improved well-being and feelings of connection over and above the controls, while decreasing participants self-focus in less than 10 minutes |
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