Human waking life contains many moments in which the mind is not engaged by external goals or tasks, but is instead absorbed in a state of stimulus-independent thought (SIT) (Christoff et al., 2004; Mason et al., 2007), commonly referred to as “mind wandering” and “resting state activity”. Experience sampling studies suggest that SIT is often self-reflective and focuses on the evaluation of past experiences and the simulation of future events (Diaz et al., 2013; Fox et al., 2015). The content and dynamics of SIT have been shown to vary considerably between individuals (Andrews-Hanna et al., 2013; Smallwood & Schooler, 2015), with important implications for mental health. While SIT appears to serve multiple adaptive functions, including self-regulation (Uchicha et al., 2015; Morawetz et al., 2016), memory consolidation (Censor et al., 2014; de Voogd et al., 2017), and problem solving (Hearne et al., 2017; Ito et al., 2017), the resting mind can paradoxically become restless in nature when thoughts are negatively-oriented, repetitive, or intrusive (Smallwood & Schooler, 2006; Nolen-Hoeksema et al. (2008) Andrews-Hanna et al., 2014). Such perseverative cognitions reinforce maladaptive coping strategies endemic to affective disorders (e.g., anxiety and depressive conditions) (Watkins, 2008; Kaplan et al., 2018; Spinhoven et al., 2019; Watkins & Roberts, 2020).
The disruption of repetitive negative thinking is notoriously challenging (Mennin & Fresco, 2013; Watkins & Roberts, 2020); however, research suggests that cognitive training in mindful awareness may alter the resting mind so as to function more adaptively (Hasenkamp & Barsalou, 2012; Vago & Zeidan, 2016). The effects of mindfulness training for the treatment of affective disorders and concomitant rumination are well-documented (Piet et al., 2011; Vøllestad et al., 2012; Gu et al., 2015; Perestelo-Perez et al., 2017); however, the mechanistic involvement of resting state activity is less well-understood. Prevailing theory posits that mindfulness training may alter such activity via attentional mechanisms, ostensibly reflected in the reorganization of neural circuitry (Hölzel et al., 2011; Tang et al., 2015; Vago & Zeidan, 2016). Accordingly, recent research has begun to investigate resting state neural circuitry as an outcome of mindfulness training (e.g., Kilpatrick et al., 2011; Kral et al., 2019) and as a characteristic of dispositional mindfulness (e.g., Doll et al., 2015; Bilevicius et al., 2018; Harrison et al., 2019; Parkinson et al., 2019). Such studies support the involvement of candidate interacting brain networks--including the default mode network (DMN), salience network (SN), and the frontoparietal control network (FPCN)--through which mindfulness may facilitate the flexible allocation of attentional resources between introspective and perceptual processes (Dixon et al., 2018). However, there remains little consensus about how mindfulness alters functional connectivity within and between these networks. The current meta-analysis sought to update brain-based models of mindfulness by comprehensively examining mindfulness-driven resting state functional connectivity (rsFC) outcomes.
The Neurocognitive Features of Stimulus Independent Thought (SIT)
When measured as functional connectivity, changes in neural circuitry reflect strengthened or weakened coordination between regions and, at a larger scale, between networks of interest (Van Dijk et al., 2010; Buckner et al., 2013). Analogous to the spontaneous flow of thought characteristic of resting states, the brain at rest spontaneously emits blood oxygen level dependent (BOLD) fluctuations that self-organize into cohesive neural networks (Raichle et al., 2001; Fox & Raichle, 2007) that are commonly termed ‘resting-state’ or ‘intrinsic’ functional networks (Snyder & Raichle, 2012). Although there is little consistency in the taxonomy of intrinsic functional networks (for review see Uddin et al., 2019), it is generally accepted that a minimum of 10 networks are observable during the resting state (Damoiseaux et al., 2006).
Among recognized large-scale networks, the DMN, SN, and FPCN have been frequently implicated in explanatory frameworks of SIT. The DMN has received the most attention historically for its role in internally-directed mentation (Andrews-Hanna et al., 2014; Raichle, 2015). Indeed, prevailing evidence suggests that the primary function of the DMN may be the generation and maintenance of internal mental processes (Andrews-Hanna et al., 2010a; Axelrod et al., 2017). The DMN may be further parcelled into three subnetworks, the midline-located core DMN, the medial temporal lobe subsystem (DMN-MTL), and the dorsomedial prefrontal cortex subsystem (DMN-dmPFC) (Andrews-Hanna et al., 2010b). While each subnetwork supports different functions of internally oriented mentation, the core DMN--encompassing the posterior cingulate cortex (PCC) and ventromedial prefrontal cortex (vmPFC)--is most robustly associated with self-referential thought (Legrand & Ruby, 2009; Andrews-Hanna et al., 2010b; van Buuren et al., 2010; Denny et al., 2012). Hyperconnectivity within the core DMN has been reliably observed in psychopathologies characterized by self-focused perseverative cognition (i.e., depression) (Kaiser et al., 2015; Coutinho et al., 2016; Lois & Wessa, 2016); however, the role of the DMN in maladaptive SIT is likely more complex. In support of this viewpoint, neural models of SIT suggest that the regulation (or dysregulation) of internal mental states relies on inter-network coordination between the DMN and networks implicated in attention, namely the SN and FPCN (Sridharan et al., 2008; Leech & Sharp, 2014; Dixon et al., 2018).
According to the dynamic framework of mind-wandering (Christoff et al., 2016), internal experiences--maintained by the DMN--may be deliberately or automatically constrained via coordination with the FPCN and SN, respectively. The FPCN, characterized by dorsolateral PFC and anterior inferior parietal structures (Uddin et al., 2019), is well-known for its integral role in cognitive control processes (Zanto & Gazzaley, 2013). Studies combining experience sampling with neuroimaging suggest that the FPCN flexibly couples with the DMN to deliberately regulate internal mentation (Cole et al., 2013), and that such coordination is used to inhibit internal thought when external attention is required (Spreng et al., 2010; Gao & Lin, 2012).
In contrast, the SN has been theorized to support automatic constraints on internal experiences through cognitively efficient cross-network signaling (Christoff et al., 2016). Key nodes of the SN include the midcingulate cortex (MCC) [also known as the dorsal anterior cingulate cortex (dACC) (Allman et al., 2001)] and anterior insula (Uddin et al., 2019), which jointly operate within a cortico-striato-thalamo-cortical loop to detect motivationally important stimuli, either internal or external (Peters et al., 2016). Through its extensive connectivity with the DMN and FPCN, the SN facilitates “set-shifting” by automatically and flexibly directing attention to and from internal and external cues (Seeley et al., 2007; Sridharan et al., 2008). Interestingly, SN engagement has also been suggested as a putative mechanism of perseverative cognition (Andrews-Hanna et al., 2019). According to this viewpoint, aberrant SN connectivity exacerbates interoceptive awareness of unpleasant sensations, and due to the high motivational value of ruminative thoughts, draws attentional resources to internally-directed mentation (Andrews-Hanna et al., 2019). While this theory is indirectly supported by evidence of SN hyperconnectivity in high-rumination individuals (Kühn et al., 2012; Kühn et al., 2014), it fails to explain why therapies aiming to enhance awareness of internal experiences (i.e., mindfulness) would ameliorate ruminative thought. Thus further research is warranted to elucidate how cognitive training, like mindfulness, alters the dynamics of internal experiences at both a subjective and neurobiological level.
Multi-method investigations suggest that variability in functional connectivity within and between networks likely reflects the content and dynamics of internal mentation (Doucet et al., 2012; Diaz et al., 2013; Marchetti et al., 2015), and resting state functional connectivity (rsFC) has been associated with individual differences in symptoms of anxiety (Liao et al., 2010), depression (Kaiser et al., 2015; Drysdale et al., 2017), and trait rumination (Rosenbaum et al., 2017; Satyshur et al., 2018). Thus neurocircuitry reorganization via rsFC has been suggested as a key mechanism of treatment for psychopathologies featuring repetitive negative thought (Broyd et al., 2009; Dichter et al., 2015; Klumpp & Fitzgerald, 2018; Xu et al., 2019; Feurer et al., 2021). Although the research is nascent, there is initial evidence that mindfulness training may improve regulation of mental states, and that rsFC indices may be used to better understand the mechanisms through which mindfulness extends its therapeutic effects (e.g., Creswell et al., 2016).
Mechanisms of Mindfulness: Theory and Empirical Support
Mindfulness, commonly defined as the act of attending to present-moment thoughts, emotions, and sensations without judgment or appraisal (Brown & Ryan, 2003), is relatively unique as a treatment of maladaptive SIT. Unlike popular cognitive behavioral therapies (CBTs), which enable the individual to challenge dysfunctional thoughts (Beck, 2012), mindfulness instead targets one’s relationship to such thoughts (Kabat-Zinn, 1994; Segal et al., 2018). This non-judgmental stance towards internal experiences may be promoted through a combination of neurocognitive mechanisms (for review see Hölzel et al. 2011; Vago & Silbersweig, 2012; Tang et al., 2015; Vago & Zeidan, 2016; Fox et al., 2014; Fox & Cahn, 2018; Young et al., 2018). Mounting evidence indicates that mindfulness practice enhances attentional control, which may potentially support the deliberate constraint of maladaptive SIT (Andrews-Hanna et al., 2020). Foundational to the practice of mindfulness is the development of attentional control through a meditative technique called Focused Attention (FA). FA meditation trains the practitioner to focus on and maintain attention to a neutral sensory object (e.g., the breath), and direct attention back to that object when the mind begins to wander (Lutz et al., 2008). This recursive process of shifting and sustaining attention has previously been linked to enhanced functional connectivity within the FPCN of experienced meditators (Hasenkamp & Barsalou, 2012), suggesting that FA improves top-down cognitive control needed to disengage from distracting thoughts and emotions.
Alternative models of mindfulness posit that mindfulness may indirectly regulate SIT by promoting awareness of internal experiences (Vago & Silbersweig, 2012). According to this framework, the sustained concentration conferred by mindfulness facilitates awareness (or mindful meta-awareness), defined as the capacity to observe one’s mental patterns with a sense of equanimity and psychological distance (Lutz et al., 2008; Dunne et al., 2019). This internal awareness thereby supports recognition of thoughts and feelings as discrete mental states, and in turn, improves flexible, adaptive responding (Lutz et al., 2008). The cultivation of mindful awareness has been theoretically attributed to enhanced functional cohesion of networks linked to self-awareness (e.g., default mode network) and attention monitoring (e.g., salience networks) (Hasenkamp & Barsalou, 2012; Tang et al., 2015). However, support for this neural model draws largely from cross-sectional research (e.g., Brewer et al., 2011; Hasenkamp & Barsalou, 2012; Farb et al., 2013), as well as correlational evidence (Doll et al., 2015; Bilevicius et al., 2018; Harrison et al., 2019; Parkinson et al., 2019). Moreover, this neural model of mindful awareness does not fully account for the role of non-judgment, or acceptance, which may operate in tandem with awareness to reduce “experiential fusion” with one’s thoughts (Bernstein et al., 2019; King & Fresco, 2019). It has been speculated that mindfulness may dampen experiential fusion through DMN downregulation (King & Fresco, 2019; Vago & Silbersweig, 2012); however, it is unclear how neural substrates of attention and awareness may mediate such effects.
Building on previous models of mindfulness meditation (Hölzel et al. 2011, Vago & Silbersweig, 2012; Tang et al., 2015; Vago & Zeidan, 2016), it is plausible that mindfulness training (MT) alters the resting state via reorganization of neural circuitry (i.e., intrinsic functional connectivity). Specifically, we posit that focused attention training recruits the FPCN, implicated in cognitive control and executive function. Given its documented role as a ‘hub’ of functional connectivity (Cole et al., 2013), the FPCN may plausibly facilitate functional connections between other resting state networks--particularly between networks related to mind-wandering (i.e., default mode network) and internal awareness (i.e., salience network)--thus enabling the flexible regulation of mental states (Vago & Silbersweig, 2012). Coupled with improvements in executive functioning, such functional coordination between DMN and SN may likewise support meta-awareness skills needed to identify and disengage from maladaptive cognitive patterns. To extend this theoretical framework, the present study posed the question: Does mindfulness training facilitate flexible switching between DMN, salience, and FPCN during rest?
Present Study
The aim of this study was to determine if mindfulness training modifies intrinsic functional connectivity (IFC) observed during resting states. Specifically, this study sought to examine connectivity within and between the frontoparietal control network (FPCN), the default mode network (DMN), and salience network (SN). To date, several studies have investigated the impact of mindfulness training on resting state functional connectivity (rsFC) using controlled, experimental designs (Brewer et al., 2011; Froeliger et al., 2012; Creswell et al., 2016; King et al., 2016). Although insightful, such studies typically suffer from low statistical power, a limitation endemic to research relying on high-cost neuroimaging modalities such as fMRI. Addressing such concerns, meta-analytic approaches may be used to pool information from well-controlled studies while modeling convergence of effects across pooled samples. Thus, we conducted a systematic review and meta-analysis of rsFC outcomes of mindfulness training relative to structurally-equivalent programs (i.e., active controls). To test the neuroplastic changes associated with mindfulness skills--namely, executive functioning and meta-awareness--we hypothesized that 1) mindfulness training would enhance rsFC between the FPCN and DMN as an indicator of enhanced cognitive control; and 2) mindfulness training would enhance rsFC between the DMN and SN, reflective of meta-awareness.