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The subjective experience of one’s environment is constructed by interactions among sensory, cognitive, and affective processes. For centuries, meditation has been thought to influence such processes by enabling a non-evaluative representation of sensory events. To better understand how meditation influences the sensory experience, we employed arterial spin labeling (ASL) functional magnetic resonance imaging to assess the neural mechanisms by which mindfulness meditation influences pain in healthy human participants. After four-days of mindfulness meditation training, meditating in the presence of noxious stimulation significantly reduced pain-unpleasantness by 57% and pain-intensity ratings by 40% when compared to rest. A two factor repeated measures analysis of variance was used to identify interactions between meditation and pain-related brain activation. Meditation reduced pain-related activation of the contra lateral primary somatosensory cortex. Multiple regression analysis was used to identify brain regions associated with individual differences in the magnitude of meditation-related pain reductions. Meditation-induced reductions in pain intensity ratings were associated with increased activity in the anterior cingulate cortex and anterior insula, areas involved in the cognitive regulation of nociceptive processing. Reductions in pain unpleasantness ratings were associated with orbitofrontal cortex activation, an area implicated in reframing the contextual evaluation of sensory events. Moreover, reductions in pain unpleasantness also were associated with thalamic deactivation, which may reflect a limbic gating mechanism involved in modifying interactions between afferent in put and executive-order brain areas. Taken together, these data indicate that meditation engages multiple brain mechanisms that alter the construction of the subjectively available pain experience from afferent information.
Rather than a passive reflection of afferent information, our perception of the sensory environment is shaped by our own unique past experiences, current cognitive state, and future expectations. Perhaps no better example of this dynamic integrative process exists than the transformation of nociceptive information into the subjective experience of pain. Although the intrinsically salient nature of noxious stimuli initiates a host of physiological and neurophysiological alterations within our bodies, its effects are nevertheless highly malleable and subject to modification. Contemplatives have emphasized that the nature of our mental activity can be manipulated to promote self-regulation through the practice of mindfulness meditation (Nh*âat and Vo-Dinh, 1987). One form of mindfulness meditation is Shamatha (Wallace, 2006) or focused attention (Lutz et al., 2008). Focused attention is the cognitive practice of sustaining attention on the changing sensations of the breath, monitoring discursive events as they arise, disengaging from those events without affective reaction, and redirecting attention back to the breath (Wallace, 2006; Lutz et al., 2008).
Meditation and pain bothalter sensory, cognitive, and affective dimensions of our subjective experience (Koyama et al., 2005; Cahn and Polich, 2006). Given that mindfulness meditation can attenuate the subjective experience of pain (Grant and Rainville, 2009; Brown and Jones, 2010; Zeidan et al., 2010b), brain regions involved in meditation must somehow interact with those involved in nociceptive processing. Yet, the specific brain mechanisms associated with mindfulness meditation remain poorly characterized. In addition, how these brain mechanisms could potentially influence pain-related brain activity remains unknown (Buhle and Wager, 2010).
Employing an emerging functional magnetic resonance imaging (fMRI) technique, pulsed arterial spin labeled MRI (PASL), we investigated how meditation affects pain-related brain processes. PASL MRI is appropriate for imaging steady cognitive states like meditation because it is less susceptible to slow drifts in signal intensity than conventional Blood Oxygen Level Dependent (BOLD) firm(Detre and Wang, 2002; Wang et al., 2003). Moreover, this technique provides a fully quantitative measure of cerebral blood flow (CBF) and allows global CBF changes such as those produced by respiratory confounds to be assessed. In addition, PASL MRI can assess activation in the orbitofrontal cortex (OFC), an area contributing to affective regulation processes, more effectively than BOLD (Wang et al., 2003).
We postulated that, in addition to reducing psychophysical pain ratings, meditation would modulate brain regions associated with constructing the pain experience (Oshiro et al., 2009). We specifically hypothesized that meditation would engage brain regions such as the prefrontal cortex (PFC), anterior cingulate cortex (ACC) and anterior insula (AI), known for their role in attentional control and affective processing (Berthier et al., 1988; Raz and Buhle, 2006; Rolls and Grabenhorst, 2008). Furthermore, we sought to determine if meditation-related activation of these brain regions was directly associated with pain modulation (Wiech et al., 2008).
Eighteen subjects were recruited for participation in this study. During psychophysical training, one subject was dismissed for low sensitivity and one for hypersensitivity to heat stimulation. A third subject was dismissed for falling asleep during the course of the meditation intervention. Fifteen healthy volunteers, six males and nine females (age range: 22–35 years, mean age: 26 years) completed the study. All subjects were right-handed, 13 were White, one Asian, and one Hispanic. Subjects gave written, informed consent recognizing that: 1) they would experience painful, heat stimuli, 2) all methods were clearly explained, and 3) they were free to withdraw from the study without prejudice. The Institutional Review Board of the Wake Forest University School of Medicine approved all procedures.
An outline of experimental procedures is illustrated in Figure 1. Subjects participated in a psychophysical training session to familiarize them with the noxious stimuli and psychophysical rating procedures (see Psychophysical Training). The first MRI session was conducted after psychophysical training but before meditation training (Figure 1). In the first half of MRI session 1, subjects were instructed to close their eyes and reduce movement (rest condition). In the second half of both MRI sessions, subjects were instructed to “meditate by focusing on the changing sensations of the breath.” In MRI session 1, this condition was labeled as “attention to breath”-ATB, thereby providing a control (i.e., divided attention) for comparisons with the meditation condition after training. Subjects then participated in four days of mindfulness meditation training (see Mindfulness-Based Mental Training). After successful completion of meditation training, subjects returned for MRI session 2. Similar to MRI session 1, in the first half of MRI session 2, subjects were instructed to close their eyes and reduce movement (rest condition). Afterwards, subjects were instructed to “begin meditating by focusing on the changing sensations of the breath (“mindfulness-based attention to breath” meditation).
All subjects were initially familiarized with thirty-two, 5s duration stimuli (35–49°C) in order to provide experience with the visual analog scales (VAS). After familiarizing subjects with the heat stimuli, they received five minutes and 55 seconds of stimulation, identical to the paradigm employed in subsequent fMRI experimental sessions. A 15 cm plastic sliding VAS scale was used to quantify pain intensity and unpleasantness [Paresian Novelty (Price et al., 1994)]. The minimum rating was represented as “no pain sensation” or “not at all unpleasant,” whereas the maximum was designated with “most intense imaginable” or “most unpleasant imaginable.”
Subjects were positioned in the MRI scanner, a pulse oximeter was attached to each subject’s left index finger to assess heart rate, and a transducer was placed around the chest to gauge respiration rate. Noxious thermal stimuli were delivered to the posterior aspect of the right calf by a 16 × 16 mm2 TSA II thermal stimulator (MedocRamat Yishai, Israel). The “heat” condition consisted of thermal stimuli that were administered in alternating patterns of heat (49°C) and neutral (35°C) with 12-s durations at each temperature (5 min and 55s total duration per MRI series). The baseline temperature was maintained at 35°C, and stimulus temperatures were delivered with rise and fall rates of 6°C/s. The “neutral” series consisted of only neutral stimuli (35°C for 5 min and 55s). Thermal stimuli were counterbalanced across series. The heat probe was moved to a new location on the right calf after each series to reduce habituation. After each series, subject’s evaluation of pain intensity and unpleasantness were acquired with the VAS scale. They were instructed that their ratings should reflect the overall experience of the entire series.
In MRI session 1, four functional series (2 heat; 2 neutral) were separated by a structural acquisitions can. In the first half of the experiment, subjects were instructed to keep their eyes closed and restrict movement across conditions. After the structural image was obtained, subjects were instructed to “meditate by focusing on the changing sensations of the breath.”
Mindfulness-based mental training was carried out in four separate, 20-minute sessions conducted by a facilitator with over ten years of experience leading similar meditation regimens. Subjects had no prior meditative experience and were informed that such training was secular and taught as the cognitive practice of Shamatha or mindfulness meditation. Each training session was held with one to three participants.
On mindfulness meditation training day 1, subjects were encouraged to sit with a straight posture, eyes closed and to focus on the changing sensations of the breath occurring at the tips of their nostrils. Instructions emphasized acknowledging discursive thoughts and feelings and to return their attention back to the breath sensation without judgment or emotional reaction whenever such discursive events occurred. On training day 2, participants continued to focus on breath-related nostril sensations and were instructed to “follow the breath,” by mentally noting the rise and fall of the chest and abdomen. The last ten minutes were held in silence so subjects could develop their meditative practice. On training day 3, the same basic principles of the previous sessions were reiterated. However, an audio recording of MRI scanner sounds was introduced during the last ten minutes of meditation to familiarize subjects with the sounds of the scanner. On the final training session (day 4), subjects received minimal meditation instruction but were required to lie in the supine position and meditate with the audio recording of the MRI sounds to simulate the scanner environment. Contrary to traditional mindfulness-based training programs, subjects were not required to practice outside of training.
Subjects also completed the Freiburg Mindfulness Inventory short-form (FMI), a 14-item assessment that measures levels of mindfulness, before psychophysical pain training and after MRI session 2. The FMI is a psychometrically validated instrument with high internal consistency (Cronbach alpha = 0.86) (Walach, 2006). Statements such as “I am open to the experience of the present moment” are rated on a 5-point scale from 1 (rarely) to 5 (always). Higher scores indicate more skill with the mindfulness technique.
After successful completion of meditation training, subjects participated in MRI session 2. This session consisted of eight functional series (four heat; four neutral). After completion of the first four “rest” series, subjects were again instructed to “meditate by focusing on the changing sensations of the breath” at which point the anatomical scan was conducted. Subjects were instructed to meditate until the end of the experiment. Four minutes after the anatomical scan, functional acquisition was resumed, and four meditation series were obtained (Figure 1). Evaluation of pain ratings and experimental procedures were identical to MRI session 1.
CBF images were acquired on a 1.5 T General Electric Twin-Speed LX Scanner with an 8-channel head coil (General Electric Medical Systems) pulsed arterial spin labeled MRI technique(PASL, Q2TIPS-FAIR) (Luh et al., 1999). Scan parameters for the PASL Q2TIPS-FAIR acquisition are as follows: TR=2500 ms, TE=17.9 ms, TI=1700 ms, TI1=700 ms, TI2=1200 ms, FOV = 24 cm × 18 cm, matrix size = 64 × 48, slice thickness= 8 mm, slice gap =0 mm, number of slices=13, total number of volumes = 140, frequency direction = A/P, total scan time = 5 minute 55 seconds. A 2D Echo Planar Imaging trajectory with ramp sampling was used for image acquisition. PASL is sensitive to subject motion, which may lead to inaccurate CBF maps. To remove the influence of subject motion on CBF quantification, the Q2TIPS-FAIR time series data (volumes 8 through 140) was filtered to remove individual perfusion weighted images that may corrupt the final CBF map (Tan et al., 2009). The first image in the PASL Q2TIPS-FAIR data was acquired with all inversion and saturation pulses turned off. This image was used as the M0 image to scale raw perfusion weighted images into a quantitative CBF map according to the General Kinetic Model (Buxton et al., 1998). Volumes 2 through 7 were needed to establish a steady state prior and were not included when calculating the CBF maps.
After the rest condition, an accelerated (2x) T1 weighted Inversion Recovery 3D Spoiled Gradient Echo (IR-3DSPGR) structural scan was acquired (scan parameters: flip Angle = 12 degrees, TI= 600 ms, RBW=15.6 kHz, FOV=24×24, matrix size=240×240, slice thickness=1 mm, number of slices = 164, acceleration factor = 2, total scan time=3 minutes 57 seconds).
The functional image analysis package FSL [Functional Magnetic Resonance Imaging of the Brain (FMRIB) Software Library (Center for FMRIB, University of Oxford, Oxford, UK) was used for image processing and analyses. Functional data were movement corrected and spatially smoothed with a 8 mm full-width at half-maximum 3-D isotropic Gaussian kernel. Each functional image was scaled by its mean global intensity (intensity normalization). To minimize confounds arising from global CBF fluctuations, all subject’s functional images were registered to their structural data using a seven-parameter linear 3-D transformation and then transformed into non-linear standard stereotaxic space. High-resolution T-1 weighted images were used to classify brain regions of activation and normalize images to standard space.
Statistical analysis of regional signal changes was performed on each acquisition series (first level analyses) using a fixed effects general linear modeling approach (Wool rich et al., 2001). Random effects analyses assessed activation across individuals. Z (Gaussian zed T/F) statistic images were thresholded using clusters determined by z >2.3 and a corrected cluster significance threshold of p<0.05 (Worsley et al., 1992).
A two-way Repeated Measures Analysis of Variance (RM ANOVA) was conducted on data from MRI session 1 in order to identify the main effects of pain and ATB. A similar RM ANOVA was conducted on data from MRI session 2 in order to identify main effects of pain and meditation, as well as their interaction. Paired comparisons were used to assess differences of main effects of pain and ATB/meditation between MRI sessions. These comparisons allowed us to differentiate brain mechanisms related to meditation (MRI session 2) from those associated with simple attention to breath (MRI session 1). To ensure that there was equal statistical power across sessions, we included only the first two volumes of the rest and meditation conditions in MRI session 2 were included to match the four volumes from MRI session 1.
In MRI session 2, we conducted a multiple regression analysis employing three regressors was conducted to assess the relationship between individual differences in meditation-induced pain reduction and brain activation. The first regressor was the mean effect of meditation vs. restin the presence of heat stimulation. Demeaned percent changes for each subject’s pain intensity ratings were entered as the second regressor, and demeaned percent changes for each subject’s unpleasantness ratings were entered as the third regressor. Intensity and unpleasantness regressors were orthogonal zed to each other in order to identify variability in brain activity uniquely related to each aspect of pain.
A RM ANOVA tested the effects of MRI session (1 and 2) between conditions (rest and ATB/meditation) on pain intensity and unpleasantness ratings (SPSS Inc). We also examined FMI scores before and after mindfulness meditation training.
RM ANOVAs also tested the effects of rest, ATB, and meditation across neutral and heat stimulation condition son heart rate, respiration rate, and global CBF across sessions (SPSS Inc). Due to equipment malfunction, we employed list wise deletion of six subjects for heart rate data and five subjects for respiration data in MRI session 1(Allison, 2002).
When subjects attended to their breathing prior to meditation training, no change in pain intensity [F(1,14)=.92, p=.35] or pain unpleasantness ratings [F(1,14)=4.31, p=.06](Figure 2) was observed when compared to rest.
Prior to meditation training, there was no significantly greater brain activation when subjects were asked to focus on the sensations of the breath as contrasted to the rest condition (Figure 3). However, there was significantly greater default-mode related brain activation (medial PFC) in the rest condition (Raichle et al., 2001) as compared to the ATB condition. These findings suggest that subjects were actively engaged in a cognitive task when instructed to focus on breath sensations than when compared to the rest condition. The thalamus, ACC, and paracingulate gyrus also were more activated in the rest condition.
When compared to neutral stimulation, pain-related brain activity was detected in the ACC, bilateral insula, secondary somatosensory cortex (SII), and SI corresponding to the noxious thermal stimulation site on the leg (Figure 3). These findings are consistent with previous studies using fully quantitative methods to examine CBF during pain (Coghill et al., 1999)and confirm that PASL methodology is sufficient to detect pain-related patterns of brain activation similar to those identified by BOLD fMRI or positron emission tomography (Owen et al., 2008).
The efficacy of meditative training was assessed with the FMI (Walach et al., 2006). Subjects exhibited a 14% increase in mindfulness skills after four days of meditation training (M=51.80, S.E.M=1.95) as compared to pre-training [(M=45.33, S.E.M=2.00), F(1,14)=11.68, p=.004, η2 =.46].
In MRI session 2, meditation produced a 40% reduction in VAS pain intensity ratings as compared to rest (Figure 2). Pain intensity ratings were lower in MRI session 2 as compared to MRI session 1[session main effect, F(1,14)=23.43, p<.001, η2 =.63], and there was a significant main effect of condition [ATB/meditation vs. rest, F(1,14)=20.49, p<.001, η2 =.59]. However, both main effects were driven by the substantial decrease in pain intensity ratings during meditation in MRI session 2 [interaction, F(1,14)=14.23, p<.001, η2 =.50]. Meditation also significantly reduced pain unpleasantness ratings by 57% [F(1,14)=87.99, p<.001, η2 =.86; main effect for session, F(1,14)=32.38, p<.001, η2 =.70, condition, F(1,14)=38.04, p<.001, η2 =.73, and interaction, F(1,14)=18.55, p=.001, η2 =.57](Figure 2).
The form of mindfulness meditation (focused attention/Shamatha) employed relies on sustained attention to an explicit awareness of sensations associated with respiration with an emphasis of acknowledging discursive sensory events without affective reaction. Meditating, after training, produced bilateral activation of SI in regions corresponding to representations of the nose and throat (Penfield & Boldrey, 1937) as well as bilateral activation of the posterior insula and SII (Figure 4). In addition, regions associated with interoceptive attention such as the AI and ACC also were bilaterally activated during meditation (Critchley, 2004). These regions, in addition to the OFC, ventral striatum, ventrolateral prefrontal cortex (VLPFC), and a mygdala also are associated with the cognitive modulation of pain (Rainville, 2002; Baliki et al., 2010). In contrast, significant posterior cingulate cortex and medial PFC deactivation was detected during meditation compared to rest (Figure 4). This pattern of activity further distinguishes meditation from a resting default mode (Raichle et al., 2001; Farb et al., 2007).
When compared to neutral stimulation, pain-related brain activation was identified bilaterally in the AI, posterior insula/SII, ACC, and frontal operculum (BA 6/44) (Figure 4).
The SI and supplementary motor area was sites of a significant interaction between meditation and noxious stimulation conditions. Post-hoc analyses confirmed that in the presence of noxious thermal stimuli, SI activation corresponding to the stimulation site (Penfield & Boldrey, 1937) significantly decreased during meditation when compared to rest (Figure 5).
Diminished psychophysical pain ratings, however, also may have been influenced by meditation-induced activation of other regions important in pain processing. We found considerable overlapping activation in the AI between meditation and pain (Figure 4). This region is activated during expectations for pain, placebo analgesia, and intensity evaluation (Wager et al., 2004; Koyama et al., 2005; Oshiro et al., 2009). Overlap between pain and meditation also was observed in mid-cingulate are as BA 24 and BA 32 (Figure 4), areas associated with the processing of emotional contexts and cognitive control, respectively (Bush et al., 2000; Rainville, 2002; Vogt, 2005).
In order to better characterize brain regions associated with meditation-induced reductions in pain ratings, a regression analysis was conducted on the demeaned percent decrease in VAS pain ratings between meditation and rest in the presence of noxious heat stimuli. In contrast to the previous within-subjects analyses, this analysis focused on identifying inter-individual factors associated with the efficacy of meditation. Individuals with the greatest reductions in pain intensity ratings exhibited the largest meditation-induced activation of the right AI and bilateral ACC (Figure 6). After accounting for the variance explained by brain mechanisms related to pain intensity reductions, individuals with the greatest reductions in pain unpleasantness ratings exhibited the greatest activation of the OFC and the greatest deactivation of the thalamus (Figure 6).
Meditating, after training, activated brain regions related to the self-regulation of pain. Areas involved in cognitive control (mid-ACC)(Vogt, 2005), emotion regulation (bilateral OFC) (Roy et al., 2009; Petrovic et al., 2010) and in teroceptive awareness (right AI)(Critchley et al., 2004)(Figure 7) were more active during meditation (after training) when compared to ATB (before training).
The main effect of pain in MRI session 2 (collapsed across meditation and rest) was compared with the main effect of pain in MRI session 1 (collapsed across ATB and rest). After meditation training, reduced pain-related brain activation was observed in the leg representation of SI corresponding to the stimulation site (Penfield & Boldrey, 1937), SII, and posterior insula (Coghill et al., 1999; Coghill et al., 2003)(Figure 7). After meditation training, brain activity was more pronounced in the frontal pole, thalamus, mPFC and the ACC as compared to before training.
A three way RM ANOVA (session X stimulus X cognitive task) revealed a significant main effect of cognitive task on global CBF. There was a decrease in global CBF when subjects attended to the breath (i.e., ATB/meditation) (M=67.29, S.E.M=3.53) when compared to rest (M=70.26, S.E.M=2.93), F(1, 14)=5.38, p=.04, η2 =.28 (Table 1). However, simple effects tests revealed no differences in global CBF between rest (M=72.85, S.E.M=2.97) and ATB (M=70.09, S.E.M=3.32) in MRI session 1 (p=. 09) or rest (M=67.68, S.E.M=2.88) and meditation (M=65.28, S.E.M=3.73) in MRI session 2(p=. 16). There were no other significant main effects or interactions (ps>.05).
Respiration means (S.E.M) across conditions and sessions are reported in Table 1. Five subjects were not included due to equipment malfunction in MRI session 1. A three way RM ANOVA (session X stimulus X cognitive task) revealed decreased respiration rate in MRI session 2(M=13.72, S.E.M=. 79) as compared to MRI session 1(M=19.73, S.E.M=1.18), F(1,9)=36.81, p<. 001, η2 =.80. Respiration rate was higher during heat (M=17.13, S.E.M=1.01) than neutral stimulation (M=16.30, S.E.M=.82), F(1,9)=44.34, p<.001, η2 =.83, and there were no differences between cognitive tasks (p=.19). Post-hoc analyses revealed that the significant three-way interaction, F(1,9)=F(1,9)=5.4-, p<.05, η2 =.38 was indicative of a significant reduction in respiration rate during meditation in the presence of heat as compared to all other conditions (Table 1). Decreases in respiration rate previously have been reported to predict reductions in pain ratings (Grant and Rainville, 2009; Zautra et al., 2010). In the present data [MRI session 2 (N=14)], no significant relationship between the decreased respiration rates and pain intensity (p=.22, r=−.35), pain unpleasantness (p=.41, r=−.24), or FMI ratings (p=.42, r=.24) was found.
Six subjects were not included due to equipment malfunction in MRI session 1. A three way RM ANOVA(session X stimulus X cognitive task) found that heart rate was higher during heat stimulation (M=75.18, S.E.M=2.56)when compared to neutral stimulation (M=72.45, S.E.M=2.49), F(1,8)=8.87, p=.02, η2 =.53 (Table 1). There were no other significant differences or interactions (ps > .05).
For centuries, contemplatives have depicted meditation as a practice to rid oneself of mentality that “misconceives the inherent nature” of self (Nh*âat and Vo-Dinh, 1987). In the present investigation, meditation reduced all subjects’ pain intensity and unpleasantness ratings with decreases ranging from 11 to 70% and from 20 to 93%, respectively. Moreover, meditation-related pain relief was directly related to brain regions associated with the cognitive modulation of pain. These findings provide unique in sights into the manner that meditation alters the subjective experience of pain.
Meditation likely modulates pain through several mechanisms. First, brain areas not directly related to meditation exhibited altered responses to noxious thermal stimuli. Notably, meditation significantly reduced pain-related afferent processing in SI (Figure 4), a region long associated with sensory-discriminative processing of nociceptive information (Coghill et al., 1999). Executive-level brain regions (ACC, AI, OFC) are thought to influence SI activity via anatomical pathways traversing the SII, insular and posterior parietal cortex (Mufson and Mesulam, 1982; Friedman et al., 1986; Vogt and Pandya, 1987). However, because meditation-induced changes in SI were not specifically correlated with reductions in either pain intensity or unpleasantness, this remote tuning may take place at a processing level before the differentiation of nociceptive information into subjective sensory experience.
Second, the magnitude of decreased pain intensity ratings was associated with ACC and right AI activation (Figure 6). Activation in the mid-cingulate and AI overlapped between meditation and pain indicating a likely substrate for pain modulation. Converging lines of evidence suggest that these regions playa major role in the evaluation of pain intensity and fine-tuning afferent processing in a context-relevant manner (Koyama et al., 2005; Oshiro et al., 2009; Starr et al., 2009). Suchroles are consistent with the aspect of mindfulness meditation that involves reducing appraisals that normally impart significance to salient sensory events.
Third, OFC activation was associated with decreases in pain unpleasantness ratings (Figure 6). The OFC has been implicated in regulating affective responses by manipulating the contextual evaluation of sensory events (Rolls and Grabenhorst, 2008) and processing reward-value in the cognitive modulation of pain (Petrovic and Ingvar, 2002). Meditation directly improves mood (Zeidan et al., 2010a), and positive mood induction reduces pain ratings (Villemure and Bushnell, 2009). Therefore, meditation-related OFC activation may reflect altered executive-level reappraisals to consciously process reward and hedonic experiences (e.g., immediate pain relief, positive mood) (O’Doherty et al., 2001; Baliki et al., 2010; Peters and Buchel, 2010).
Meditation-related activation in these executive-level cortical areas may also influence thalamic nociceptive processing. The correlation between decreased pain unpleasantness ratings and the widespread deactivation of the thalamus during meditation may reflect a filtering mechanism associated with the modulation of ascending sensory information at thalamic levels (Figure 6). The thalamic reticular nucleus (TRN) facilitates such filtering between frontal cortices and the thalamus (Crick, 1984). TRN is a GAB Aergic structure that operatesasa “gatekeeper” between the thalamus and the cortex, in that all feedback connections between the thalamus and cortex travel through the TRN (Crick, 1984; Guillery et al., 1998). Projections from the PFC contact TRNGAB Aergic neurons, which in turn modulate sensory nuclei of the thalamus, and thereby influence the transmission of subsequent sensory information to the cortex in a context dependent manner (Sherman, 2001; Rauschecker et al., 2010). Cortically mediated shifts in executive attention activate TRN (Guillery et al., 1998; Kilmer, 2001), a mechanism fitting for sustaining attention and engaging/disengaging distractors (i.e., focused attention meditation). Thus, meditation-induced activation of this limbic-thalamic (Figure 6) gating mechanism would modulate ascending noxious information prior to its accessing cortical regions implicated in conscious perception. The fact that TRN modulates visual and auditory modalities (Guillery et al., 1998) is consistent with the effects of meditation on binocular rivalry (Carter et al., 2005) and dichotic listening (Lutz et al., 2009). Taken together, these findings indicate that both cortico-cortical and cortico-thalamic interactions provide potential substrates for executive-order processes to alter the elaboration of nociceptive information into a subjectively available pain experience.
Pain-relieving cognitive manipulations such as attentional control, expectation manipulation, and placebo all likely share common modulatory pathways (Wiech et al., 2008). We propose that meditation is another manipulation that also engages this final common pathway for pain modulation. For one, cognitive control is enhanced after training (Zeidan et al., 2010c), allowing subjects to more effectively focus on the breath. This attentional set may reduce the saliency of noxious stimuli (Pessoa et al., 2003). Second, reducing expectations of impending noxious stimuli promotes pain modulation (Koyama et al., 2005). In meditation training, subjects were taught to reduce anticipation of future events, which may have lead subjects to reduce anticipatory responses to noxious stimuli (Brown and Jones, 2010). Finally, the notion that meditation could induce beliefs related to the promotion of pain relief is worth noting. Similar to previous reports examining placebo analgesia (Petrovic et al., 2002; Wager et al., 2004), the present study finds increased OFC and ACC activation related to pain modulation. However, meditation differs from placebo in that meditation is a volitionally induced cognitive state.
The absence of significant differences in pain ratings during rest between MRI session 1 and 2 indicates that order effects related to the presentation of thermal stimuli, VAS usage, experimental instructions, scanner environment, and shifts in rating criteria were negligible. Although the within group design controlled for these factors, the inclusion of a separate sham meditation control group may have provided additional insights into possible interactions between order effects and meditation training. For example, the inclusion of a sham meditation group would have allowed better assessment of demand characteristics related to meditation’s reputation. However, the relationship between executive-level brain regions and pain reductions indicates that neuro-cognitive factors other than report biases were engaged during meditation-induced reduction of pain reports.
One might also argue that meditation’s pain-relieving effects are simply due to divided attention rather than any unique attributes of mindfulness meditation per se. We believe, that this explanation is unlikely, as it fails to incorporate the differences in the mental processes involved between the two. For example, mindfulness meditation requires both the active allocation of attentional processes and the acknowledgment/regulation of responses to discursive stimuli. By contrast, divided attention likely lacks this meta-cognitive component. When compared to the divided attention condition, meditation was more effective at reducing pain ratings and evoking greater brain activation in areas involved in the cognitive/affective modulation of pain. Distraction has been found to reduce pain-related activity in the mid-ACC and AI (Bantick et al., 2002). In contrast, we found that increases in these areas were directly related to pain modulation. Moreover, in studies directly comparing meditation to distraction, meditation reduced pain ratings more than distraction (Sedan et al., 2010b)and activated emotion regulatory brain regions (PFC, ACC) to a greater extent than distraction (Holzel et al., 2007).
Mindfulness meditation is characterized by two distinct cognitive practices. The fundamental practice of mindfulness is called focused attention (Lutz et al., 2008) or Shamatha (Sanskrit translation: calm abiding) (Wallace, 2006). Focused attention promotes a sense of detachment from ongoing affective states and enhances cognitive control (Maclean et al., 2010; Zeidan et al., 2010c). Traditionally, focused attention is cultivated as a prerequisite to another form of mindfulness meditation labeled open monitoring(Lutz et al., 2008)or Vipassana (Sanskrit translation: insight into the nature of reality)(Wallace, 2006). Open monitoring practitioners commonly refer to mindfulness as a moment-to-moment non-evaluative awareness of “whatever arises” (Wallace, 2006). The present findings, therefore, are distinct from open monitoring’s effects on pain. In open monitoring, meditators are taught to fully experience the intensity of a sensory event. Consistent with this, open monitoring has been found to reduce pain unpleasantness, but not pain intensity ratings (Brown and Jones, 2010; Perlman et al., 2010). Thus, focused attention may attenuate pain by altering the elaboration of nociceptive information to pain, whereas open monitoring promotes a non-evaluative stance to a fully experienced sensory event.
Although the benefits of meditation have long been associated with extensive training (Grossman et al., 2004), our findings demonstrate that some of meditation’s beneficial effects can be realized after just four-days (20 min/day) of training, suggesting that it may serve as an effective adjunct therapy in clinical settings. However, it is highly unlikely that the qualitative experience of meditation is similar across expertise level. In fact, studies have shown a positive relationship between meditative experience and pain relief (Grant et al., 2010, Brown & Jones, 2010) suggesting that the benefits of this technique can be further realized after extensive training. Nevertheless, we found that the increased capacity to reliably focus on the breath in a non-evaluative manner following brief training can effectively reduce the subjective experience of pain. Because meditation likely alters pain by enhancing cognitive control and reframing the contextual evaluation of nociceptive information, the constellation of interactions between expectations, emotions, and cognitive appraisals intrinsic to the construction of the sensory experience can be regulated by the meta-cognitive ability to non-judgmentally sustain focus on the present moment.
This study was supported by the Mind and Life Institute Varela Grant, the National Institutes of Health NS39426, and the Center for Biomolecular Imaging of Wake Forest University School of Medicine.
There are no conflicts of interest.