Increases in whole brain grey matter associated with long-term Sahaja Yoga Meditation: a detailed area by area description

Objectives Our previous study showed that long-term practitioners of Sahaja Yoga Meditation (SYM) had around 7% larger grey matter volume (GMV) in the whole brain compared with healthy controls; however, when testing individual regions, only 5 small brain areas were statistically different between groups. Under the hypothesis that those results were statistically conservative, with the same dataset, we investigated in more detail the regional differences in GMV associated with the practice of SYM, with a different statistical approach. Design Twenty-three experienced practitioners of SYM and 23 healthy non-meditators matched on age, gender and education level, were scanned using structural Magnetic Resonance Imaging. Their GMV were extracted and compared using Voxel-Based Morphometry. Using a novel ad-hoc GLM model, statistical comparisons were made to observe if the GMV differences between meditators and controls were statistically significant. Results In the 16 lobe area subdivisions, GMV was statistically significantly different in 4 out of 16 areas: Right hemispheric temporal and frontal lobes, left frontal lobe and brainstem. In the 116 AAL area subdivisions, GMV difference was statistically significant in 11 areas. The GMV differences were statistically more significant in right hemispheric brain areas. Conclusions The study shows that long-term practice of SYM is associated with larger GMV overall, and with significant differences mainly in temporal and frontal areas of the right hemisphere and the brainstem. These neuroplastic changes may reflect emotional and attentional control mechanisms developed with SYM. On the other hand, our statistical ad-hoc method shows that there were more brain areas with statistical significance compared to the traditional methodology which we think is susceptible to conservative Type II errors.


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Introduction 49 Meditation is a general term that includes a large variety of practices that mainly focus on the inner 50 observation of the body and the mind. The western goal of most meditation techniques is to achieve an 51 improved control of attention and emotions in order to live a more balanced, stress-free and healthier life.

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On the other hand, yoga includes many different techniques among which meditation (dhayana in classical 53 yoga) has a main role. If we travel back to the origins of yoga, the first known treaty "The yoga sutras of 54 Patanjali" mentions that "Yoga is the suppression of the modifications of the mind" [1,2]. In ancient yoga, a 55 higher state of consciousness called Nirvichara Samadhi was described, in today's words Nirvichara could 56 be translated as "mental silence" or "thoughtless awareness". In this state, the mind has none thoughts and 57 there is inner calm in a state of inner pure joy and the attention is focused on each present moment. Sahaja 58 Yoga Meditation (SYM) shares the goals of Patanjali's Yoga Sutras to achieve the state of Nirvichara or 59 mental silence.
60 SYM, presumably through the regular achievement of the state of mental silence, has shown health 61 benefits in disorders that are often associated with recurrent or repetitive negative thoughts, such as: 62 depression, stress, anxiety, and attention-deficit/hyperactivity disorder [2][3][4][5][6][7]. Other studies on SYM has 63 shown beneficial effects in treating physiological and neurological diseases such as asthma [8], high blood 64 pressure [6], menopause [9] and epilepsy [10][11][12], for a meta-analysis see [8]. Furthermore, the frequency 65 with which the practitioners perceive the state of mental silence has been shown to be associated with 66 better physical and mental health [13].

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Neuroplasticity is one of the most commonly used terms in today's neuroscience to express the capacity of 68 our human brain to change permanently. One of the key insights over the past 2 decades of neuroimaging 69 research has been that the human brain, even in adulthood, is not static, but on the contrary is a dynamic 70 system that has the ability to shape itself. One of the key fascinating questions that researchers try to 71 answer is hence: how can we improve our brain structure and function? One potential non-pharmacological 72 way to shape our brain could be through meditation [14].
73 Neuroplasticity can be measured by changes in grey matter volume (GMV). Many studies have shown that 74 brain areas that are more utilized through practice of a particular skill for example, in music [15] , or high 75 performance sports [16,17], can become enlarged. It has even been shown that relatively short periods of 76 training of a particular skill, such as 3 months of training to juggle or 3 months of studying for an exam in 77 students can lead to transient changes in the relevant brain areas such as visual-spatial perception regions 78 for juggling [18,19]

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Considering the concern of incurring in Type II errors (false negatives or conservative assumptions), the 95 aim of our study was to analyze in more detail how the GMV differences are distributed across the whole 96 brain. This new study is based in two key issues: 1) The development of an ad-hoc statistical GLM method 97 that adapts itself on each brain area depending on the significance of covariates of that particular area; and 98 2) The parcellation of the human brain using 2 different methods i. Based on the human brain lobes: frontal, 99 temporal, etc…. that gives rise to 16 different brain areas and ii. Using the more specific automated 00 anatomical labelling (AAL) of 116 brain areas [29,30]. The key question for this analysis was whether there 01 were any areas that differed between long-term meditators and healthy controls which were overlooked in 02 our previous paper [28] due to type II error correction effect.

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Participants 06 Forty-six white Caucasian, right-handed, healthy volunteers, between 21 and 63 years participated in this 07 study. Twenty-three of them were long-term expert practitioners of SYM (17 females and 6 males) while the 08 other 23 (also 17 females and 6 males) were non-meditators matched on gender, education degree, body 09 mass index and age (see Table 1). All volunteers informed that they had no physical or mental illness, no 10 history of neurological disorders, and no addiction to alcohol, nicotine or drugs. 88 (2), to the zone 3D where all covariates and interactions were significant represented by the full model eq.

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(1). 90 91 Gender was not included into the AH-GLM because one of the conditions to be able to carry out an ANCOVA 92 is that there is no effect of the factors on the covariates that are included in the model.  Table 3 and Fig 1).
34 Table 3. Statistics of GMV differences between groups in the different lobes (16 areas).  In the two hemispheres GMV was statistically significantly (FDR q < 0.05) larger in meditators relative to non-44 meditators, see Table 4.   45 46 Table 4. Statistics of GMV differences between groups in the hemispheres and whole brain.

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The relative GMV difference between meditators and controls showed both extreme cases at brainstem in 49 meditators. On average, the difference in GMV considering all lobes areas was 6.8 ± 3.8 % larger in 50 meditators. A similar difference was shown for both hemispheres where the relative difference was always 51 larger GMV for meditators: 7,03% in the right hemisphere and 6,72% in the left hemisphere (Table 4). In the 52 whole brain the difference was 6.93 %, which was already shown on our previous paper [28].

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If we consider the reported GMV differences at lobes from  The GMV difference between meditators and controls ranged from +15.3% larger GMV at Right 78 Parahippocampal gyrus to 0.0%, almost equal, at Right Lenticular nucleus -Pallidum. On average the 79 difference in GMV considering all AAL areas was a 6.7 ± 3.0 % larger in Meditators.

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If we consider the 11 AAL areas with significant GMV differences, similar to the calculation for the lobe areas, 81 the summation of the difference in GMV between groups on those 11 areas was 6,25 mL which represents 82 a 14.8 % of the total GMV difference at the whole brain.

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Discussion of the ad-hoc statistical method 85 As previously mentioned in the results section, the GMV differences between groups in the 5 clusters 86 reported in our previous paper represent only 1 % of the total significant GMV difference at the whole brain 87 (see Table 2). Out of the 5 reported clusters, the most significant one, in right insula-vmOFC had a 88 corrected p-value of 0.027 while the whole brain p-value was 0.002, which is ten times more significant (no 89 need of correction at the whole brain analysis because it was a single comparison).

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The analysis conducted in this study shows that 11 out of the 116 AAL areas were significantly larger in 91 meditators which represents a 14.8% of the total GMV difference at the whole brain (see Table 5). Five out 92 of 16 lobes areas were statistically different in GMV between meditators and non-meditators and represent 93 a 20.4% of the GMV differences reported at the whole brain; the left and right hemisphere GMV differences 94 previously reported represent, respectively, 43.9% and 47,3 % of the GMV difference reported at the whole 95 brain.

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What these data seem to show is that the larger the number of area subdivisions tested the smaller the 97 amount of GMV with statistical significance between groups. A possible explanation is the dilution of 98 significant differences at the whole brain with subsequent brain partitions, presumably due to Type II error 99 due to conservative assumptions.

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This conservative bias may occur in other cross-sectional between-group studies where the whole brain 01 GMV is significantly different between groups, in which case the use of an ad-hoc GLM method like the one 02 here presented could be a possible solution to deal with the Type II error that the standard VBM statistical 03 method seems to produce in these situations.

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Based on our ad-hoc GLM method we present here a more sensitive and detailed examination that reveals 05 significantly different areas that were not detected with the statistical VBM standard procedure. The 06 acknowledgment of these areas will allow to better understand the neuroplastic mechanisms associated 07 with the practice of SYM and its inherent consciousness state of mental silence, discussed in the next 08 section.

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The 3 lobe areas with the largest significant GMV differences were in the right hemisphere: R. temporal, R 13 frontal and R brainstem. Furthermore, the 6 AAL areas with the largest significant GMV differences were also 14 in the right hemisphere: in mid and inferior temporal lobe, in inferior and orbital frontal cortices, and in para-15 and precentral lobes (Tables 3 and 5

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The enlargement in the temporal lobe is also interesting. The middle and inferior temporal lobes are closely 39 connected to the limbic system and form crucial part of the emotion control network [53][54][55].

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The enlargement in the brainstem is of particular interest, as previous studies have found increased GMV in 41 long-term meditators relative to controls in the brainstem [56,57]; in a longitudinal study of mindfulness 42 meditation this increase of GMV in the brainstem in the meditators was associated with better well-being [58].

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The brainstem contains several production areas of several modulatory neurotransmitter pathways, such as 44 those arising from the raphe nuclei (serotonergic; associated with modulation of mood and cognitve functions), 45 ventral tegmental area (dopaminergic; associated with motivation and attention) and locus coeruleus 46 (noradrenergic; associated with arousal and attention) [58,59]. The state of mental silence has been described 47 subjectively in meditation scriptures as a state of enhanced alertness, attention and arousal [1,2].

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The autonomic nervous system, brainstem and cortical systems are closely interconnected in their mediation of 49 the regulation of behaviour and cognition [60]. The enlargement of the brainstem in long-term Meditators is 50 therefore potentially a consequence of the long-term practice of achieving the state of thoughtless awareness 51 which leads to enhanced alertness and arousal. It may also be related to the activation of the autonomic nervous 52 system during meditation [61] that is closely interconnected with brainstem regions. Given that the brainstem is 53 closely interconnected with frontal regions. It is also of note that brainstem and the two frontal lobes were 54 increased in GMV in long-term Meditators.

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The 6,9% larger GMV in meditators at the whole brain with a p-value of 0.002 constitutes as far as we know 56 the largest difference in GMV between healthy groups of similar age and conditions. No other meditation 57 technique or practice has shown such a large statistical difference in GMV at the whole brain. One of the 58 assumptions of SYM is the spontaneous (Sahaja = spontaneous) awakening of the Kundalini energy [62] during 59 the meditation which allows the practitioners to perceive the achievement of yoga ( yoga=union) and the state 60 of mental silence, which is felt like a cool breeze of energy on top of the head. It is possible that this experience, 61 which is specific to SYM, may be related to the enlargement of VBM and this needs to be further tested.

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In our previous paper where we used the standard statistical model for VBM, only 5 relatively small brain 65 areas were statistically different in GMV between groups. These 5 areas represented only around 1% of the 66 total 6.9% larger GMV difference shown at the whole brain in meditators compared with non-meditators. statistical method, we have shown in more detail how this 6,9 % larger GMV in meditators, the largest GMV 69 difference in healthy groups of similar age and conditions in the literature so far, is distributed in the 70 meditator's brain subregions. The larger GMV in meditators is focused in particular in the right hemisphere 71 in frontal and temporal brain areas related with attention and emotional control.