Effect of Meditation on Cortical Thickness of the Brain

Scientific evidence suggests that meditation alters the functional and structural plasticity of the brain.  In the milestone study, Sara Lazar, et al. in 2005, demonstrated that meditation increases the cortical thickness of the brain. Since then a large number of studies, mostly with varying sample sizes are conducted to identify the relationship between meditation techniques and development of cortical thickness in different brain areas. The results have been remarkably consistent. There is increasing evidence that  the insula undergoes structural refinement through meditation. Meditation stops insular cortical thinning process.  Insular cortical thinning occurs due to age and depression.   In this article we focus on the impact of meditation on overall cortical thickness of the brain. Advancements in developmental biology, neuroscience, and medical imaging have brought us closer than ever to understand the effect of meditation on brain.

There are several meditation methods, such as Yoga, Om meditation, Zen, mindfulness, breathing awareness, and Samatha. Specially, Om chanting and meditation  has several benefits. The chanting of Om improves the somatic relaxation and the meditation on the meaning of Om increase cognitive relaxation. Special cognitive relaxation of  Om meditation and somatic relaxation of Om chanting improves the cortical characteristics of the brain. Moreover, the practice of Om improve immune competence, especially in older populations who often experience a loss of immune function. Om mantra stabilizes the fight-and-flight response of the amygdala. Which improves the gyrification and cerebral characteristics.

Buddha

Buddha

To study the effect of meditation on brain structure there are three measures; cortical thickness, gray matter volume and gyrification. Gyrification is the measure of the pattern and degree of cortical folding. It is an important cerebral characteristic related to the geometry of the brain’s surface. In this article we will focus on cortical thickness.

Cortical Thickness of Human Brain

Cortical thickness is the measure of the thickness of the cerebral cortex. It varies considerably between cortical areas.The cerebral cortex is the brain’s outer layer of neural tissue in humans and other mammals. The cerebral cortex plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. In human the cerebral cortex is folded, giving a much greater surface area in the confined volume of the skull. With a faster growing outer surface layer and slower growing inner core – drives the formation of cortical folds. Normally, as people age, they have fewer folds in their brains. A fold or ridge in the cortex is termed a gyrus (plural gyri) and a groove or fissure is termed a sulcus (plural sulci). In the human brain more than two-thirds of the cerebral cortex is buried in the sulci.

Cortical ThicknessThe neurons of the cerebral cortex are grouped into six main layers, from outside (pial gray matter surface) to inside (white matter). The cortical layers are not simply stacked one over the other; there exists characteristic connections between different layers and neuronal types, which span all the thickness of the cortex. Typical values of cortical thickness in adult humans are between 1.5 and 3 mm and during aging, a decrease on the order of about 10 μm per year can be observed. After the age of forty, the human brain decreases by approximately five percent every decade.

Developments in computational neuroanatomy have now made possible MRI-based quantification of cortical thickness. MRI-based cortical thickness quantification follows cortical folding patterns and captures the distance between white matter surface and pial gray matter surface.

Gray Matter Volume verses Cortical Thickness

Among all MRI analysis techniques detecting brain morphometric changes, grey matter volume and cortical thickness are the two most widely used measures. Magnetic resonance imaging (MRI) of the brain makes it possible to get a measure for the thickness of the human cerebral cortex. The thickness of different cortical areas varies but in general, sensory cortex is thinner than motor cortex.  Grey matter volume was thought as a combination of several measures such as cortical thickness, surface area, gyrification and intensity contrast. The method most commonly used to measure the volume of the gray matters is known as voxel-based morphometry (VBM). In contrast to VBM, MRI-based cortical thickness quantification follows cortical folding patterns and captures the distance between white matter surface and pial gray matter surface.

Meditation and Cortical Thickness Recent Studies

However, all these meditation styles can be grouped into two broad categories; focused attention (FA) meditation and open monitoring meditation. The open monitoring meditation reflects non-reactive monitoring of the content of experience moment-by-moment. This includes no explicit focus on objects, non-reactive meta-cognitive monitoring, and non-reactive awareness of automatic cognitive and emotional interpretations of sensory, perceptual, and endogenous stimuli. On the other hand, FA meditation entailed the voluntary focusing of attention on a chosen object. The characteristics of this type involve directing and sustaining attention on a selected object (e.g., breath sensation), detecting mind wandering and thoughts, disengagement of attention from distractors and shifting attention back to the selected object.

Effect-of-Meditation-on-Brain

Lazar et al. (2005) compared cortical thicknesses between 20 participants with extensive training in insight meditation experience, which involves focused attention to internal experiences and 15 control participants. They observed that the right anterior insula (AI), right PFC, left STG, and left SI were significantly thicker in meditators than in controls.

Kang et al. (2013) employed a whole-brain cortical thickness analysis in the brains of 46 experienced meditators and 46 matched control volunteers. They observed  Cortical thickness was significantly greater in meditators than in controls in the anterior regions of the brain, located in frontal and temporal areas, including the medial prefrontal cortex, superior frontal cortex, temporal pole and the middle and interior temporal cortices.

Grant et al. (2010) investigated cortical thicknesses in 19 Zen meditators and 20 controls. Meditators were found to have a thicker cortex in the bilateral ACC, bilateral SII, bilateral insula, bilateral parahippocampal gyrus, bilateral SI, and bilateral PFC. Furthermore, when assessed in all subjects, lower pain sensitivity was associated with a thicker cortex in affective, pain-related regions including the ACC, bilateral parahippocampal gyrus, and AI.

These studies indicated that long-term meditation increased cortical thickness in several brain regions, such as insular cortex , prefrontal cortex (PFC), anterior cingulate cortex (ACC), posterior cingulate cortex (PCC). Several studies reported that default mode network was changed with long-term meditation training.  Meditation training also improves emotional regulation, involving the amygdala. Taken together, these data indicate that meditation engages multiple brain mechanisms that alter the construction of the brain.

Summary:

Yet, despite significant progress throughout the past decade, the underlying mechanisms of brain development during different types of meditation remain largely unknown. The number of years of experience of meditation, type of meditation and duration of meditation has different impact on brain.  Normally, the higher benefits of meditation is associated with extensive training, but many studies demonstrated that benefits can be realized after just four-days (20 min/day) of meditation.

Sources:

1. Meditation: The Science of Health and Wellbeing by Banani Ray and Amit Ray
2. Meditation experience is associated with increased cortical thickness
3.  The effect of meditation on brain structure: cortical thickness mapping and diffusion tensor imaging
4. Global and Regional Alterations of Hippocampal Anatomy in Long-Term Meditation Practitioners

 

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