For thousands of years, meditation has been used as a tool to train the mind and achieve harmony between the physical, mental, intellectual, and spiritual personalities of humans (Fox et al.,
2016). Learning meditation or meditation intervention has been associated with morphometric changes in the brain when compared to controls and is observed both in cross-sectional and longitudinal imaging studies (Fox et al.,
2012; Hölzel et al.,
2011; Luders et al.,
2011). Studies provide evidence that short- and long-term meditation practice brings about changes in GM of the brain, specifically increasing density in the right inferior frontal region, with larger volumes in the orbitofrontal cortex and right hippocampus. These regions involve in emotional regulation and response control (Dodich et al.,
2019; Luders et al.,
2009). Meditation training induces neural plasticity in the brain similar to other forms of skill acquisition. Lazar et al. (
2005) provided structural evidence for experience-dependent plasticity in the cortical regions in Buddhist insight meditators. Studies on different types of meditation provide evidence that the enhanced neural plasticity of GM is based on regularity and the type of meditation technique practiced (Engen et al.,
2018; Hernández et al.,
2016; Hölzel et al.,
2008; Luders et al.,
2009).
The effects of meditation practice on the brain were observed in different brain regions, possibly because of considerable heterogeneity among meditation techniques practiced. Studies show ample evidence that meditation practice causes structural changes in specific brain regions. These studies include findings of structural changes in the medial prefrontal cortex (PFC) that permits understating of self and others (Frith & Frith,
1999; McAlonan et al.,
2005); superior frontal gyrus (SFG), ventromedial and ventrolateral orbitofrontal cortices along with inferior parietal, and superior temporal cortices which involve attentional control and introspective perception (Hernández et al.,
2016; Hopfinger et al.,
2000); precuneus/posterior cingulate cortex (PCC) along with middle temporal gyrus (MTG) and middle frontal gyrus that enables better control over mind wandering and mediates happiness (Panda et al.,
2016; Sato et al.,
2015); hippocampus that maintains emotional response (Davidson et al.,
2000; Luders et al.,
2013); and occipital regions such as cuneus, superior, middle, and inferior occipital cortices that are involved in visual attention (Corbetta et al.,
1998; Hopfinger et al.,
2000).
Most of the meditation practices that focus on their thoughts or breathing are closed-eye meditation techniques. However, Rajyoga meditation (RM) is an open-eye meditation technique taught by the Brahma Kumaris’s organization (Brahmakumaris,
2020; Ramesh et al.,
2013). The details of RM practice are described in various papers and also posted in the Brahma Kumaris’s organization web page. Briefly, during the practice of RM, an individual is instructed to realize himself/herself as an eternal form of self/soul and to gaze on the point of light, which is considered as Supreme Soul/God Father. Also, individuals are instructed to think positively during meditation. Thinking and feeling good/positive about the self by pondering on virtues/positive thoughts (PT) including peace, love, happiness, and bliss that reside in an individual are described as positive thinking in the context of RM practice (Gupta et al.,
2011; Ramsay et al.,
2010; Telles et al.,
1993). Incorporation of PT may reduce preservative behavior, modulate cognitive control, and provide flexible updates of working memory (Dreisbach,
2006; Sharma et al.,
2018). Despite altered circumstances, whether it is positive or negative, the propensity of positive thinking can influence a person’s feelings of well-being (Diener et al.,
2009). Over time, RM practice harmonizes spiritual, mental, and physical energy to raise up the inner strength and enhanced cognitive control in order to be more focused to lead a stress-free, healthy, satisfied, and happy life (Gupta et al.,
2011; Ramesh et al.,
2013).
Currently, neurocognitive research on RM is gaining more attention, and a few studies have been conducted using electrophysiological and imaging modalities. A study by Sharma et al. (
2018) in long-term RM practitioners was observed to show alpha and theta wave pattern in frontal and parietal areas of the brain while maintaining meditative experience. This suggests that RM practice has impact on the brain regions involved in the management of selective and sustained attention while dealing with the cognitive and emotional aspects. Studies on both the short- and long-term meditation practitioners proved their ability to attain a stable state in meditation in a few minutes. However, long-term practitioners, under varying external conditions, can attain this state within a minute. Interestingly, these achievements were made with open-eye during meditation which has potential applications in daily life (Nair et al.,
2017). Diffusion tensor imaging study on long-term RM practitioners has reported an increased white matter (WM) integrity in corpus callosum which is essential for the functional integrity between hemispheres at the time of high attention demand (Sharma et al.,
2018). A study in RM practitioners by Panda et al. (
2016) have observed changes in the default mode network’s spatial extents and temporal dynamics during meditation. RM practice has an impact on GMV of reward processing regions of the brain and helps to achieve higher happiness levels (Babu et al.,
2020). Although these studies provide evidence of the influence of RM practice on the brain, there are no structural MRI studies elucidating the influence of RM practice on PT and GMV changes in specific brain regions of RM practitioners.
The hypotheses of the present study are that RM practice would cause structural alterations (GMV changes) in brain regions associated with meditation experience and positive thinking. The objective of this study was to elucidate GMV changes in different regions of the brain from structural MRI of RM practitioners, analyze using voxel-based morphometric (VBM) technique, and compare the same with age, gender, and handedness matched non-meditators (NM). Additionally, these specific brain regions were correlated to hours of meditation experience and PT scores within the group of RM practitioners.
Discussion
The present study provides initial evidence on the association between PT score and morphometric changes analyzed by VBM method in various brain regions of RM practitioners. The mean TIV is found to be increased in the RM practitioners group, although it is not significant in this study. A significant increase in the mean global GMV was found in the RM practitioners group when compared to that of NM. Other meditation studies report a significant change in GMV of specific regions of the brain in meditators when compared with control. However, very few studies report a significant increase in global GMV. Hernández et al. (
2016) found a larger global GMV in Sahaja Yoga meditation practitioners compared with that of NM, and it was predominantly on the right hemispheric brain areas. On comparing the hemispheric differences between meditators and control, Kurth et al. (
2015) reported that the cluster-specific GMV was significantly more in the left hemisphere than the right. Although cluster-specific increased GMV on the left hemisphere in RM practitioners was identified, the specific hemispheric global GMV changes were not analyzed in the present study. In this study, a voxel-wise comparison using the SVC method was applied first, as shown in Fig.
2A, B and Table
1. Accordingly, using this method, a significantly higher GMV was observed in the brain regions including the right SFG, left inferior OFC, left IPG, bilateral precuneus, bilateral cuneus, and left posterior cerebellum, as well as a significant increase in GMV of left MTG by voxel-wise comparison of whole-brain was observed.
As mentioned, increased GMV observed in the right SFG, a part of the PFC, is a region of the brain typically involved in monitoring attention and self-regulation (Tang et al.,
2015). Additionally, PFC has been implicated in several studies, as an important area for sustained and selective attention as well as maintaining meditation (Cabeza & Nyberg,
2000; Hernández et al.,
2016; Lutz et al.,
2008; Newberg & Iversen,
2003). A study on Brain Wave Vibration meditators showed structural differences in both gray and white matters, particularly in the frontal cortex that is necessary for attention and emotional regulation (Kang et al.,
2013). The superior frontal cortex, along with the inferior parietal and superior temporal cortices, is also involved in involuntary attentional control (Hopfinger et al.,
2000). PFC activation is individually distributed and very much dependent on both the experience and the practice of Silent Mantra meditation (Engstrom et al.,
2010). In a functional MRI study of experienced Adept Meditators, greater activation was observed in the medial PFC along with anterior cingulate cortex that is involved in emotional regulation in presence of distracting events (Hölzel et al.,
2007). The medial PFC, along with the superior temporal sulcus, is associated with the theory of mind (Den et al., 2005). Generally, the neural circuit of various prefrontal regions and subcortical limbic structures implies emotional self-regulation. Thinking positively and experiencing happiness support the development of the brain by generating and reinforcing new synapses, mainly in PFC which serves as the integration center for mind and brain functions (Susan,
2011). Findings in the current study appear in agreement with prior evidence indicating that RM practice may help in enhancing voluntary regulation of attention, behavior, and emotional self-regulation.
Further, in RM practitioners, a significantly higher GMV was observed in the left inferior OFC when compared with NM. This region involves in emotional regulation and is associated with reward and punishment. OFC, along with the hippocampus and ventral tegmental areas, involves in the reward and motivational process. Learning and adjusting one’s behavior in response to a task mainly depends on the function of OFC (Quirk & Beer,
2006; Wikenheiser & Schoenbaum,
2016). Increased GMV in OFC was observed in a meditation study suggested that meditators can disengage automatically generated thoughts and habits by enhancing emotional self-regulation (Luders et al.,
2009). Thus, practicing RM may help motivate the learners to overcome the automatic thought process and maintain emotional self-regulation.
Moreover, a significant increase in the bilateral precuneus was also observed in RM practitioners. Functionally precuneus plays a vital role in self-awareness and attention (Kurth et al.,
2014). In a functional study on a group of moderately experienced Silent mantra meditators, increased activation on the right precuneus along with other regions was found during meditation (Engstrom et al.,
2010). A functional MRI study by Hernández et al. (
2018) observed an increased activity in the bilateral precuneus regions [after removal of covariates] while maintaining mental silence. Yang et al. (
2016) reported increased activity of the right precuneus during mindfulness meditation. A questionnaire-based study revealed a positive association between the subjective happiness score and GMV in the right precuneus, which indicates that this region plays a role in mediating subjective happiness by integrating the emotional and cognitive components of happiness (Sato et al.,
2015). The striking finding in this study of a significant increase in GMV of the bilateral precuneus in RM practitioners suggests that RM practice may help to enhance self-awareness, attention, and happiness.
Additionally, a significant increase in the left IPG in RM practitioners was observed when compared with NM. A significant positive association of the left SPG and IPG with hours of meditation experience was also observed in the RM group. These regions play an essential role in the perception of emotion and interpretation of sensory information. Also, these regions help to maintain body image and spatiality with respect to the environment (Bermudez et al.,
2017; Singh-Curry & Husain,
2009). Studies show that the IPG, along with the SPG and STG, involves in voluntary attentional control that is selectively activated by cues (Ding et al.,
2015; Hopfinger et al.,
2000). Increased GMV in these regions suggests that these regions help to regulate voluntary attention control while maintaining environmental awareness in RM practitioners.
GMV of left MTG showed a significant increase in RM practitioners when compared to the same in NM, indicating that RM practice helps in gaining attentional control. Similar findings, with higher GMV in the left middle and inferior temporal gyrus, were found by Leung et al. (
2013) in loving-kindness meditation experts. A study on Brain Wave Vibration practitioners with average of 41 months of meditation experience showed that they have significantly increased cortical thickness in the bilateral middle and inferior temporal gyrus (Kang et al.,
2013) and a VBM study on Sahaja Yoga meditation reported a larger GM volume in bilateral inferior temporal gyrus (Hernández et al.,
2016). Panda et al. (
2016) have reported a higher activity in left MTG along with right MFG in RM practitioners during meditation. These regions typically involve in greater attentional control.
A significant increase in GMV in the bilateral cuneus (a part of the occipital region) was also observed in RM practitioners. Other mediation studies have also identified GM changes in the respective cuneal areas that are responsible for the primary visual perception (Luders et al.,
2012; Pickut et al.,
2013). Luders et al. (
2012) have reported an increased gyrification on the left parietooccipital and right cuneus in meditators. Study on Brain Wave Vibration practitioners also showed a significantly increased cortical thickness and fractional anisotropic values in the cuneus in comparison with controls (Kang et al.,
2013). The cuneus involves shifting and maintaining visual attention (Corbetta et al.,
1998; Hopfinger et al.,
2000; Luders et al.,
2012). Since RM is an open-eye meditation practice in which meditators focus their visual attention on the point of light, the observation of a significantly higher GMV in the bilateral cuneus is similar to previous studies as explained.
Additionally, a significantly higher GMV was observed in the left posterior cerebellum when compared with that of the control. An in-depth analysis of the cerebellum by recent research studies indicate its role in cognition and behavior, in addition to traditionally known functions of the cerebellum in the coordination of voluntary movements, gait, posture, speech, and motor functions. Particularly the posterior lobe of the cerebellum subserves the functions of cognition and emotion. A lesion in the posterior lobe causes cerebellar cognitive-affective syndrome (Rapoport et al.,
2000; Schmahmann,
2019). The study on mindfulness-based stress reduction has identified a significant increase in GM concentration in the cerebellum along with the PCC and temporoparietal junction which are involved in learning, memory processes, emotion regulation, self-referential processing, and perspective-taking (understanding the concept from an alternate point of view) (Hölzel et al.,
2011). Thus, the findings on RM practitioners in this study indicate that the posterior cerebellum is also associated with cognitive learning, thinking positively during RM practice. But, on comparing the GMV changes in cerebellum of RM practitioners based on their duration of Rajyoga meditation experience, a significant negative correlation of the left posterior cerebellum was observed. This negative correlation was particularly higher in short-term RM practitioners when compared to long-term RM practitioners. This may be due to an increase in cerebellar neural plasticity during short-term periods of RM training. But in long-term RM training, there is a decline in the neural plasticity of the left cerebellum. It is possibly similar to the changes observed in studies reported on juggler learners where a reduction in the regional GMV after an initial increase in the same region was observed (Boyke et al.,
2008; Driemeyer et al.,
2008). Thus, RM practice may also regulate the cognitive process via left posterior cerebellum.
A significant positive correlation between PT score and hours of meditation experience was observed in this study, with a higher mean score in long-term than short-term RM practitioners. It has been observed that mindfulness training influences the positive affect of individuals (Jung et al.,
2010; Patel et al.,
2018). Additionally, a significant positive association between PT scores and GMV of the right STG in RM practitioners was also observed. Similarly, Lazer et al. (2005) have observed an increased cortical thickness in the left STG in meditators. The studies on patients with schizophrenia, characterized by auditory hallucination and thought-related disorders, have identified a significant decrease in the bilateral posterior STG and a positive correlation between thought disorder and decreased GMV in the right anterior STG (Holinger et al.,
1999). This indicates that the decreased GMV changes in right STG correlate to thought disorders, whereas an increase in GMV in the same region in RM practitioners is observed to be correlated to higher PT scores. A similar finding in the temporal region indicates that a long-term practice of RM enhances positive thought process and may help to overcome thought related disorders such as schizophrenia and schizoaffective disorders.
Thus, this study provides the initial evidence of the influence of RM practice on GMV changes in specific brain regions that are associated with PT processes. The overall increase in the global GMV in RM provides evidence of meditation-induced neural plasticity in regions that process regulation of self-awareness, behavior, automatic thought process, and visual attention. Thus, the practice of RM enhances GMV changes and PT process, a prerequisite for enhancing happiness and well-being.
Limitations and Future Research
Among the regional GMV extracted using AAL atlas, shown in Table
S1 (Supplementary Materials), statistically significant (
p < 0.05) increase in GMV was observed in the right superior and middle OFC, bilateral recti, right Heschl gyrus, and bilateral inferior occipital gyri in RM practitioners when compared with NM. However, there were no significant differences in these regions either with the voxel-wise comparison with SVC or with whole-brain analysis between RM practitioners and NM. Though a significant difference was not observed in the right SFG in the extracted regional GMV, as shown in Table
S1 (Supplementary Materials), a significant difference was found in VBM analysis between RM practitioners and NM. A similar trend was observed in correlation and comparative analysis from the extracted regional GMV among meditators, as shown in Figs.
3b, c and
5b, c. This discrepancy may be due to the size of the cluster or height of the voxels in these regions which is possibly not sufficient to show a significant difference. This discrepancy may be avoided by increasing the sample size of the study. Additionally, a significant increase (
p < 0.05) in extracted GMV in the left anterior cingulate gyrus was observed in NM, but not with the voxel-wise analysis of both the groups. Though the education levels of the participants were added as a covariate, the hippocampus and amygdala regions did not show any significant differences in both the groups. The levels of education of the participants have a direct or indirect influence on these regions (Kang et al.,
2018; Piras et al.,
2011; Tang et al.,
2017). Thus, the levels of education have a strong confounding effect in brain image analysis. Although both the groups are matched for age, gender, and handedness, the education levels were not matched in the present study. More cross-sectional studies with greater sample size and matching for education levels of participants along with other covariables may help to provide stronger evidence of the neural plasticity observed in specific brain regions, particularly associated with PT processes in RM practitioners. Additionally, interventional or follow-up studies with a large sample size may provide stronger confirmative evidence and could also overcome the confounding effect of education level on these findings.
Publisher’s Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.