Cortical thickness, mental absorption and meditative practice: Possible implications for disorders of attention
Highlights
► Zen meditators report greater attentional absorption than controls. ► Grey matter thickness predicts subjective experience of attentional absorption. ► Regions associated with thinning in ADHD are thicker in participants with greater absorption. ► Meditators have thicker attention/executive cortices related to their training. ► Meditation may aid the symptoms of ADHD by bolstering cortical thickness.
Introduction
The term meditation refers to a family of mental exercises aimed at enhancing the practitioner's ability to attain and maintain a target state, often attentional or affective in nature (e.g. sustained attention or a state of compassion) (Lutz et al., 2008). Although often viewed as spiritual, many such techniques are, for the most part, completely secular and are gaining recognition as clinically relevant (Chiesa and Serretti, 2010). Functional imaging studies have reported that meditating in an MRI scanner activates attention-related cortices such as the anterior cingulate cortex (ACC) and frontoparietal networks (Brefczynski-Lewis et al., 2007, Manna et al., 2010). A number of studies have also reported regional grey matter differences between individuals who meditate and those who do not (Pagnoni and Cekic, 2007, Grant et al., 2010, Holzel et al., 2008, Holzel et al., 2010, Lazar et al., 2005, Luders et al., 2009, Vestergaard-Poulsen et al., 2009). In all cases meditators have been found to have more grey matter, in specific regions, than non-meditators. Further, several of these effects have involved regions implicated in attention/executive processing (e.g. ACC, superior and middle frontal gyri and orbitofrontal regions) (Grant et al., 2010, Holzel et al., 2008, Lazar et al., 2005, Luders et al., 2009, Vestergaard-Poulsen et al., 2009). Behaviourally, practitioners of meditation have been shown to perform significantly better on attention and executive function tasks such as the Attention Network Task (ANT), the Stroop Task, attentional blink, Symbol Digit Modalities Test, verbal fluency, and the n-back task (Prakash et al., 2010, Zeidan et al., 2010, Tang et al., 2007, van Leeuwen et al., 2009). While it is certainly possible that there are pre-existing differences in meditators, there is now evidence from longitudinal studies that improvement in attention performance (Lutz et al., 2009, Tang et al., 2007, Zeidan et al., 2010) and increases in grey matter (Holzel et al., 2010, Holzel et al., 2011) occur over the course of meditative training. Importantly, preliminary evidence suggests that meditative training may be an effective adjunct treatment for patients suffering from attention-deficit/hyperactivity disorder (ADHD). Following an 8 week meditation program, improvements were observed on the ANT, the Stroop Task and the Trail Making Test, as well as in self-reported ADHD symptoms (Zylowska et al., 2008).
In the literature there are parallels between meditative practice and ADHD at several levels. ADHD is characterized by inattention, impulsiveness and hyperactivity. Neuroimaging studies have strongly implicated the fronto-striato-thalamic circuitry in the pathophysiology of this disorder (Bush et al., 2005, Seidman et al., 2005). Functional MRI studies of ADHD have repeatedly shown hypo-activation of the anterior cingulate, dorsolateral and inferior prefrontal cortices as well as the basal ganglia, thalamus, and parietal cortices (Dickstein et al., 2006). Morphometric brain imaging studies have likewise found structural differences, such as cortical thinning, in many of these same areas, in populations of both adults and children with ADHD (Shaw and Rabin, 2009, Seidman et al., 2005). Furthermore, cortical thinning of a subset of these regions has been associated with poor clinical outcome 5 years later (Shaw et al., 2006). While ADHD and cortical thickness have substantial heritability (Durston et al., 2004, Forero et al., 2009, Rimol et al., 2010), morphometric longitudinal studies suggest regional grey matter may also vary as a function of training and performance (Draganski and May, 2008). One particularly notable study found that training naïve participants to juggle resulted in grey matter density increases, concomitant with performance gains, in brain regions previously implicated in processing visual motion (Draganski et al., 2004). Findings such as these suggest that, despite the high likelihood of a genetic predisposition, it may be possible to combat the functional deficits of a disorder like ADHD by targeting the vulnerable cortices with a suitable training intervention. As discussed above, one potential candidate to bolster both grey matter thickness and attention/executive function is meditative practice.
The present study provides initial evidence that cortical, attention-related brain regions, that appear to be sensitive to thinning in ADHD, are related to an experiential measure of attention and are thicker in practitioners of meditation. The analysis took place over two phases. We were first interested in testing the hypothesis that an experiential measure of attention would (a) differ between meditators and controls and (b) relate to grey matter thickness in attention-related brain regions. With evidence to support these hypotheses a post hoc analysis was performed examining the physical overlap between the present results and a report of grey matter thinning observed in a study of ADHD (Shaw et al., 2006).
Section snippets
Participants
Eighteen Zen meditators (14 male/4 female) were recruited, that had been practicing between 2 and 30 years and had accumulated a minimum of 1000 h of lifetime practice (mean = 6406, SD = 1955, min = 1010, max = 39,439). Eighteen age- and gender-matched control subjects (14 males/4 female) were subsequently recruited. All participants provided informed written consent, approved by a local Ethics Committee (CMER-RNQ 05-06-020). Meditators were recruited from meditation centres in the Montréal area and had
Self report measures
Scores for all questionnaires were normally distributed based on the Shapiro–Wilk test. Meditators scored slightly higher than controls on the TAS (t(34) = 1.91, p < 0.05, d = 0.64) indicating a tendency to be more absorbed in their experience. Meditators also scored higher than controls on three of the subscales of the FFMQ (OBS: t(34) = 3.30, p < 0.01, d = 1.09, NR: t(34) = 3.64, p < 0.001, d = 1.20 and AWARE: t(34) = 2.03, p < 0.05, d = 0.67) indicating a greater tendency to be mindful. Across the entire sample TAS
Discussion
Attentional absorption has been described as one's tendency for “episodes of ‘total’ attention that fully engage one's representational resources” (Tellegen and Atkinson, 1974). Consistent with previous reports (Davidson et al., 1976, Holzel and Ott, 2006), meditators had a greater, albeit modest, tendency to become absorbed in their experiences than control subjects. Further, absorption scores of meditators were related to the frequency of their practice suggesting the trait may be malleable
Limitations
Thus far we have argued that our results combined with the existent literature point to meditation as a plausible causal agent, positively influencing cortical thickness and potentially modulating behaviour in turn. However, such causality cannot be determined from a cross sectional study and indeed a case can be made for alternative interpretations. For example, it may be, and is perhaps even likely, that there is a substantial genetic contribution to one's tendency for absorption (Ott et al.,
Financial disclosures
None of the authors have any financial or other relationship that might lead to a conflict of interest.
Acknowledgments
This research was supported jointly by a Canadian Institutes of Health Research (CIHR) operating grant (P.R.) and a Mind and Life Institute Varela Grant (J.A.G.). J.A.G has also been supported by a fellowship from CIHR. We would like to thank L’Unité de Neuroimagerie Fonctionnelle du Centre de recherche de l’Institut universitaire de gériatrie de Montréal for technical help with scanning and members of the Duncan-Rainville labs.
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