The aim of the study was to examine whether and how specific facets of mindfulness play a protective role against depression vulnerability. We found that, among the five facets of the FFMQ, higher non-reactivity was inversely correlated with depression vulnerability, indicated by low rumination and less negative bias (i.e. better ability to inhibit a behavioral response to negative emotions). On the neural level, we did not find a significant correlation between non-reactivity and activation in the right IFC. Instead, non-reactivity was negatively correlated with activation in the left anterior insula during inhibiting and engaging in negative stimuli after the mindful breathing task, whereas rumination was positively correlated with activation in bilateral anterior insula activation after the stress task. These findings indicate that trait non-reactivity is a critical component of mindfulness that could protect against negative bias by reducing automatic emotional responding to negative stimuli reflected by reduced anterior insula activation under stress. Taken together, the data suggest plausible psychological and neural mechanisms that could explain how a specific facet of mindfulness—non-reactivity to negative stimuli—might buffer vulnerability to depression.
There are studies which have found greater cortical thickness in meditators compared with non-meditators in the right insula (Lazar et al., 2005
; Hölzel et al., 2008
) and other regions. Using different cognitive and affective paradigms, increased and decreased insular activation has also found to be associated with dispositional mindfulness or post-intervention mindfulness (Kumar et al., 2008
; Ives-Deliperi et al., 2010
; Slagter et al., 2011
; Zeidan et al., 2011
). To our knowledge, this study is the first to examine the neural mechanisms for subcomponents of mindfulness in protection against negative bias. The majority of neuroimaging studies on mindfulness in the literature used a unified score to measure mindfulness and have found increases in activation in attentional and executive function regions such as the superior/inferior parietal lobe (Brefczynski-Lewis et al., 2007
), dlPFC and dorsal anterior cingulate cortex (Farb et al., 2007
; Ives-Deliperi et al., 2010
; Manna et al., 2010
). However, in our examination of facets of mindfulness as measured by the FFMQ, we did not find a correlation between non-reactivity and activation in the executive control regions (dlPFC or IFC). Rather, non-reactivity was correlated with less activation to negative stimuli in the insula. Non-reactivity is the tendency to notice thoughts and emotions without getting engrossed in them and without reacting automatically (Baer et al., 2006
). We did not find support for a relationship between non-reactivity and effortful ‘top-down’ regulation of negative bias. Non-reactivity may reflect less automatic emotional response via less activation in the anterior insula.
There is ample evidence supporting the insula as the interoceptive cortex representing emotional arousal, feelings, empathy and internal body state and reflecting visceral states associated with emotional experiences (Damasio et al., 2000
; Craig, 2003
; Critchley et al., 2004
; Singer et al., 2009
). Low insula activation to negative stimuli in our study suggests that individuals with high non-reactivity scores may possibly use interoception to regulate automatic emotional responding. This result is consistent with recent experimental evidence linking trait mindfulness and decreased emotional reactivity (e.g. Brown et al., 2012
). The amygdala is often activated by emotionally salient stimuli and has been associated with emotional arousal. The fact that non-reactivity was associated with insula activation but not amygdala activation also supports our speculation that non-reactivity is effective through interoception to regulate automatic emotional responding.
Our overarching hypothesis is that different mindfulness skills are related to different cognitive processes as they relate to emotional responding (Slagter et al., 2011
). Each facet of mindfulness may have its own neural mechanism and confer different cognitive or emotional benefits. Our study does not imply that non-reactivity is superior to other facets of mindfulness. Rather, we recognize that non-reactivity was uniquely related to rumination and negative bias in this relatively small sample of healthy young males, which indicates its potential usefulness protecting against stress and depression vulnerability. Our findings warrant future studies in both males and females to confirm these results.
The major limitation of the study is that although we found a significant correlation between non-reactivity and insula activation to negative go and no-go stimuli, but not to neutral go or no-go stimuli, we did not find the correlation using the direct negative > neutral contrast in the whole-brain voxelwise analysis. Rather, the inverse relationship we found between nonreactivity and activation in the insula in the whole-brain voxelwise analysis was confirmed in a post hoc multiple regression analysis with the insula ROI (F1,15 = 6.19, P = 0.01; negative no-go, t = 3.27, P = 0.007; neutral, t = 1.35, P = 0.20). Because the post-hoc test on the insula activation can increase type I error, our finding that nonreactivity influences processing of negative but not neutral stimuli needs to replicated. Therefore, to further confirm whether non-reactivity was associated with negative bias on the behavioral level, future studies using larger sample size are necessary.
Another caveat of the study is that although we requested participants abstain from smoking (which might increase participants stress level for smokers), we did not include formal smoking measures. The study also lacked ratings of stress at baseline before the stress or mindful breathing tasks. This omission prohibited us from drawing conclusions about a specific stress-inducing effect of the stress task and/or stress-reducing effect of the mindful breathing task. However, our measures of positive and negative affect, state anxiety, heart rate and respiratory rate and cortisol were all comparable at baseline, which indicated that the pre-task stress levels were likely comparable between the two task sessions.
We did not formally collect information regarding prior mindfulness meditation experience, although the majority of study participants informally mentioned that they were meditation naive. Future studies should consider measuring the relationship between past mindfulness experience, trait mindfulness, and task-based measures of negative bias. In addition, future studies could compare training in mindfulness skills (e.g. non-reactivity) vs other emotion regulation skills, such as reappraisal, in novices to clarify the neural mechanisms associated with different pathways to reducing negative bias.
In summary, this study is unique in that it suggests the trait non-reactivity facet of mindfulness offers cognitive protection from rumination and negative bias on a task explicitly involving the interaction of emotion and cognition, and does so using a region of the brain traditionally involved with interoceptive awareness. These results suggest that cultivating non-reactivity through formal meditation practice or other mindfulness training techniques could offer protection from depression. Thus, current or new interventions may benefit from adding or increasing components that foster non-reactivity through mindfulness practices.