Our data indicate that regular practice of meditation is associated with increased thickness in a subset of cortical regions related to somatosensory, auditory, visual and interoceptive processing. Further, regular meditation practice may slow age-related thinning of the frontal cortex. Previous studies of cortical plasticity in animals and humans have shown that when a task requires that attention be consistently directed towards a behaviorally relevant sensory stimulus (e.g. a somatosensory [9
] or auditory stimulus [10
]) over repeated practice sessions [11
], robust changes in sensory cortical maps result ([12
] and Kerr CE, Wasserman RH and Moore CI. Cortical plasticity as a therapeutic mechanism for touch healing, under reveiw). Additional studies suggest that relaxation facilitates the learning-based process that underlies such cortical plasticity [13
]. It may be useful to conceptualize meditation practice as engaging in an analogous set of cortical remodeling processes: namely, directing attention towards behaviorally relevant sensory stimuli within a relaxing setting over repeated practice sessions [2
]. Increased cortical thickness could be due to greater arborization per neuron, increased glial volume or increased regional vasculature. The methods employed do not distinguish between these possibilities; however, each of these mechanisms is supportive of increased neural function.
We hypothesized that meditation practice should promote neural plasticity in regions that are routinely engaged during formal practice. Many factors including age, sex, genetics, neuropathology and psychopathology [4
], however, influence the thickness of cortex nonspecifically, confounding these analyses. Perhaps the largest of these confounds is the effect of age. The rate of age-dependent thinning is highly variable across the cortical surface [5
]. Meditation-related effects on thickness may have been counterbalanced by the effects of age on cortical thinning, thereby minimizing our ability to detect significant correlations. Thinning is most pronounced in the frontal lobe, and indeed there were many regions in the parietal, temporal and occipital lobe where there was little if any difference in the average thickness in our older and younger participants (data not shown). Such age-related effects may account for the fact that the strongest correlation with experience was found in the occipitotemporal region, while other regions of interest, which all lie in frontal regions, had only low correlation with experience. Interestingly, despite the effects of aging on the prefrontal cortex, in one focal region of BA 9/10 the average cortical thickness of the 40-50-year-old meditation participants was similar to the average thickness of the 20-30-year-old meditators and controls, suggesting that regular practice of meditation may slow the rate of neural degeneration at this specific locus. Future longitudinal studies will be required to verify this finding.
Another factor possibly confounding our ability to detect correlations between thickness and experience is heterogeneity in the specific mental exercises that Insight practitioners engage in over time. Beginners are taught to maintain focused awareness on interoceptive stimuli and then are gradually taught to expand their awareness to focus on thoughts, emotions and external stimuli such as sounds, although there is no prescribed schedule or order in which these practices are taught. Correspondingly, the insula, an area associated with the interoceptive processes and breath awareness techniques common to beginning and experienced meditators, had the largest and most significant between-group difference, while unimodal sensory areas, which may be associated with more advanced and heterogeneous practices, had less significant differences.
As a result of the cross-sectional nature of the study, the findings are necessarily correlational, and a causal relationship between cortical thickness and meditation cannot be inferred. For example, it is possible that people with thicker sensory cortex are for some reason drawn to meditation. Several factors, however, suggest that these findings relate to the meditative practice itself. First, although there were significant ‘regional’ differences in thickness between groups, there was no between-group difference in ‘global’ mean cortical thickness, indicating that these findings are unlikely to be due to spurious between-group differences that might impact cortical structure nonspecifically. Second, the regions of cortical thickening correspond well to the specific activities that practitioners of Insight repeatedly engage in over time - paying attention to breathing sensations and sensory stimuli. It is unlikely that nonspecific lifestyle effects such as diet would be associated with the specific pattern of differences found. The most plausible explanation for the specific pattern observed is experience-dependent cortical plasticity.
Finally, both years of practice and change in respiration rate (a physiological measure of cumulative meditation experience) were correlated with cortical thickness in two regions, the inferior occipitotemporal visual cortex and right anterior insula. These findings are consistent with other cross-sectional reports of experience-dependent differences in neural volume [16
]. In addition, a longitudinal study [18
] has demonstrated that learning to juggle is associated with increases in visual motion cortical areas. Our finding of a correlation between the thickness in two regions and amount of experience lends support to the hypothesis that the observed differences are acquired through extensive practice of meditation, and are not simply due to preexisting or incidental between-group differences.
Most of the regions identified in this study were found in the right hemisphere. The right hemisphere is essential for sustaining attention [19
], which is a central practice of Insight meditation. The largest between-group difference was in the thickness of right anterior insula. Functional imaging and electrophysiological studies in humans and monkeys have implicated the right anterior insula in tasks related to bodily attention and increased visceral awareness [20
]. Structural measures of gray matter volume of the right anterior insula predict accuracy of objective measures of interoceptive performance, as well as subjective ratings of global visceral awareness [21
]. The differential thickness between groups in this region is consistent with increased capacity for awareness of internal states by meditators, particularly awareness of breathing sensations. Right BA 9/10 has been shown to be involved in the integration of emotion and cognition [22
]. It has been hypothesized that by becoming increasingly more aware of sensory stimuli during formal practice, the meditation practitioner is gradually able to use this self-awareness to more successfully navigate through potentially stressful encounters that arise throughout the day [2
]. This eastern philosophy of emotion dovetails with Damasio’s theory that connections between sensory cortices and emotion cortices play a crucial role in processing of emotionally salient material and adaptive decision making [24
Other forms of yoga and meditation will likely have a similar impact on cortical structure, although each tradition would be expected to have a slightly different pattern of cortical thickening based on the specific mental exercises involved [7
]. Although numerous studies have shown that indices of cortical size can decrease as a result of aging and pathology (e.g. [4
]), there are limited data indicating mechanisms that promote cortical thickening [16
]. Our findings suggest that cortical plasticity can occur, in adults, in areas important for cognitive and emotional processing.