1 Introduction
In the aging society we live in, by the year 2050, one in six people in the world will be over the age of 65. And with that it almost doubled one person in eleven people being older than 65 from the year 2019 (United Nations, Department of International Economic and Social Affairs, n.d.). The World Population Prospects 2022 points out that the population above 65 years is increasing at a faster rate than the population below 65 years (United Nations, Department of Economic and Social Affairs, Population Division, 2022). With this higher amount of older people worldwide, diseases of cognitive decline like Alzheimer's disease, dementia and Parkinson's disease, for instance, are more common (Mercadante & Tadi, 2024). In fact, age is the biggest risk factor for Alzheimer´s disease and the most common cause of dementia in people over 65 (Gothe et al., 2019). Yoga has gained more popularity in the West in the 20th century and therefore experienced an exponential growth in the research area of health and wellbeing (Gothe et al., 2019). Various scientists have been researching effects that yoga may have on positively influencing and even reversing age-related neurodegenerative diseases as well as neuropsychological diseases by promoting the functionality and connectivity of the brain (Gothe et al., 2019; Kora et al., 2021; Krause-Sorio et al., 2022; Merlet et al., 2024; Nourollahimoghadam et al., 2021; Rivest-Gadbois & Boudrias, 2019, Verzili et al., 2023). Understanding the promising effects that a regular yoga practice may have on the brain activity, helps to build the scientific foundation for implementing yoga in treatments for various neuropsychological conditions (Gothe et al., 2019; Nourollahimoghadam et al., 2021; Krause-Sorio et al., 2022; Merlet et al., 2019). The aim of this paper is to summarize the state of the art of studies on the effect yoga has on brain activity.
1.1 Yoga
Yoga has its origins in India with its first mentioning dating back to 2500-1000 before Christ in the Vedas, the oldest collection of scriptures in the Hindu culture (Broad, 2013). The practice of yoga has been changing over decades and can be classified into different phases (Broad, 2023). Yoga's origin consisted of religious spirituality with prayers, meditations and sacrifices (Vedic phase until 200 BCE) and later evolved to a focus on spiritual and mental health through meditation, known as the classical phase (Broad, 2013). Around 500 CE during the tantric phase of yoga conscious breath control (named pranayama in Sanskrit) was added to the practice (Broad, 2013). In the modern phase of yoga, starting around 1930, there is a strong emphasis on physical exercises (called asana in Sanskrit). This shift in yoga practice from esotericism to a focus on science and hygiene in the 1930s in India, has been strongly influenced by the Indian independence movement and NeoHinduism. With the goal to revive and modernize Hinduism as the basis for Indian national identity, yoga, with its ancient roots, was seen as a significant element in this effort (Broad, 2013). By now yoga has established itself in popular culture and is practiced globally for fitness, health and spiritual reasons. It can involve different methods of meditation, breathing exercises, specific movement exercises, chanting sounds (mantras) and kriya (sequences of postures, breathwork and sounds) (Gothe et al., 2018). During the modern phase of yoga, many different yoga styles, lineages and schools have evolved, each placing an individual emphasis on different parts of the practice.
1.2 Brain activity
Brain activity is defined by electrical impulses, chemical signalling and metabolic processes within the brain that enable communication among neurons and the nervous system and is therefore responsible for cognitive functions (including perception, thought, emotion and behaviour) and bodily functions (Trökes & Knothe, 2014). Brain activity can be assessed with different measurement devices, each offering different types of information and varying in their applications. In the research field of yoga's effects on the brain, the most used measurement devices are magnetic resonance imaging (MRI), functional magnetic resonance imaging (fMRI) and electroencephalography (EEG).
2 Effect of yoga on brain activity
Reviewing the research on yoga's effect on brain activity, a heterogeneity of definitions of yoga and yoga practitioners, study designs, control stimuli, as well as measurement tools can be identified (Gothe et al., 2019; Merlet et al., 2024) and will be exemplarily presented in this paper. Aware that the scope of this paper cannot do justice to the full complexity of this topic, the aim of this work is to provide a brief overview of recent research on the effects of different practices of yoga on brain activity. For clarity, the studies presented here will be subdivided into the chapters of brain structure, functional brain activation and connectivity and brain frequencies.
2.1 Brain structure
A study from 2019 by Garner and colleagues tested changes in grey matter (GM) density with longitudinal magnetic resonance therapy (MRI) in the form of voxel-based-morphometric (VBM) analysis before and after a 10- week intervention of 75 minutes weekly practice of Hatha Yoga. This practice included body movements, meditation and breathing exercises in yoga-naive subjects (Garner et al., 2019). Their study results showed a significant increase in grey matter density in the right hippocampus over time in the yoga group compared to a decreased GM density over time in their sport control group. An increase in grey matter density refers to a higher concentration of neuronal cell bodies, dendrites and unmyelinated axons (measured by VBM), which implies a greater number of synaptic connections between neurons and therefore enhanced neural processing capacity in a certain area of the brain (Mercadante & Tadi, 2024). The increase of GM density in the right hippocampus specifically is associated with an improved adult neurogenesis (Garner et al., 2019), emotion regulation, learning, spatial orientation, memory consolidation and stress regulation (Garner et al., 2019; Gothe et al., 2018). The findings of this study are in alignment with a mindfulness study that showed an increase in brain grey matter density in the hippocampus due to a mindfulnessbased stress reduction intervention (Hölzel et al., 2011).
In addition to grey matter density the grey matter volume, which is defined as the total amount of grey matter of a specific brain area, can be measured (Gothe et al., 2018). An increase in volume in a brain area indicates the related growth in the neural or glial cells. A greater grey matter volume and functioning therefore improves brain functioning and health, as the plaque that can build up in the grey matter can lead to severe memory loss known as dementia or Alzheimer's disease, and the decline of neurons in grey matter leads to the control of fine motor skills as well as the shaking in Parkinson's disease (Mercadante & Tadi, 2024). Gothe and colleagues measured the differences of the grey matter volume of the hippocampus, thalamus and caudate nucleus in experienced yoga practitioners (n=13) (with more than three years of regular yoga practice) and controls (n=13) with fMRI (Gothe et al., 2018). Their results showed a greater grey matter volume in the left hippocampus in experienced yoga practitioners compared to their control group and no differences in the thalamus and caudate nucleus (Gothe et al., 2018). A greater GM volume in the hippocampus indicates a more enhanced neural connectivity in the brain area which is involved in learning, memory processes and adult neurogenesis (Gothe et. Al., 2018). These findings suggest as well that a regular yoga practice improves brain activity and health and has neuroprotective effects for age- related cognitive declines and neurodegenerative diseases (such as dementia and Alzheimer's disease). Kause Sorio and colleagues' study findings suggest that a 12- week intervention of Kundalini Yoga and Kriya offers better neuroprotective effects for women at risk for Alzheimer's disease compared to a 12-week intervention of memory enhancement training (Krause Sorio et al., 2022). A greater decrease in GM volume was seen in MRI in the control group of memory training than in the yoga group, which showed a preserved grey matter volume across the brain due to the yoga intervention (Krause Sorio et al., 2022).
2.2 Functional brain activation and connectivity
Gothe and colleagues investigated the brain activation during the Sternberg memory task in experienced yoga practitioners compared to their control group in the same study from 2018 (Gothe et al., 2018). The Sternberg memory task is a cognitive task designed to study the processes of memory (Gothe et al., 2018). Through fMRI scans recorded during the task, the encoding, maintenance and retrieval processes of the working memory with their specific brain activation could be measured (Gothe et al., 2018). The fMRI scans showed less activation in the dorsolateral prefrontal cortex in the experienced yoga practitioners compared to the control group during the encoding phase of the Sternberg memory task (Gothe et al., 2018). The dorsolateral prefrontal cortex is activated when executive functions (especially in the encoding phase of verbal working memory tasks) are in use and is engaged more with an increasing task load (Gothe et al, 2018). Following it is to assume that experienced yoga practitioners in this study show less need to use the dorsolateral prefrontal cortex to perform the Sternberg working memory task indicating a higher efficiency in solving the task (Gothe et al., 2018).
Furthermore, a cross-sectional trial investigated the functional connectivity in resting state with fMRI of Yoga practitioners (women above 60, practicing yoga, including pranayama, asana and meditation) minimum twice a week for at least 9 years) compared to matched yoga- naive control group (Santella et al., 2019). A higher correlation between the medial prefrontal cortex and the right angular gyrus of the yoga practitioners was found, compared to their control group (Santella et al., 2019). This greater intra-network anteroposterior brain functional connectivity of the default mode network (DMN) signifies enhanced cognitive integration and a higher baseline functional connectivity and efficiency in the DMN. While with aging often a decrease in resting state functional connectivity (and therefore less neuronal communication and decreased cognitive flexibility and ability) goes along, these study results suggests that yoga can even reverse this age-related impact on the brain (Gothe et al., 2019).
2.3 Brain frequencies
Another way to measure the activity of the brain is to measure brain frequencies with the electroencephalogram (EEG). EEG is a non-invasive method that uses a cap with electrodes on the scalp of the proband to record voltage fluctuations resulting from ionic current flows within the neurons of the brain. This transmits the summed electrical potentials of neurons that fire together (Kora et al., 2021). Different brain frequencies can be distinguished through the recordings and after noise removal and feature extraction of the data (Kora et al., 2021). The different frequency bands are delta (1-4 Hz), theta (4-8 Hz), alpha (8-12 Hz), beta (12-30 Hz), and gamma (> 30 Hz) frequencies (Kora et al., 2021).
Krishna and colleagues investigated the effect of heartfulness meditation on frontal electrical activities of the brain during baseline (5 min), mediation (10 min), transmission (10 min) and post meditation (5 min) of 4 meditation sessions with EEG power spectral density in 28 meditators and 33 nonmeditators as a control group (Krishna et al., 2022). The results showed highly statistically significant higher theta band power, very significant higher alpha band power, highly significant lower beta band power and significant lower delta band power in heartfulness meditators compared to the control group (Krishna et al., 2022). Higher theta band power is associated with light sleep, relaxation, memory building, meditative states and reduced anxiety (Desai et al., 2015; Kora et al., 2021). Higher alpha band power occurs during moderate levels of brain activity, a state of relaxed wakefulness and is correlated with a decreased degree of pain and discomfort and increased perception of calmness (Desai et al., 2015; Kora et al., 2021). Moreover, fast and accurate memory performance as well as word recognition in older adults and a better working memory are correlated with a higher alpha band activity (Desai et al., 2015; Kora et al., 2021). Additionally, a higher activity in theta and alpha frequency bands form the foundation for the brain to experience learning and cognitive performance as they play a vital role in the networking between the neurons (Desai et al., 2015; Kora et al., 2021). Lower beta frequency band power is related to less active thinking, alternes and active concentration (Desai et al., 2015; Kora et al., 2021). Delta band power are the slowest brain waves with a high amplitude in the slowest wave and are observed during deep, dreamless sleep or minimum self-conscious brain involvement (Kora et al., 2021). Sometimes this frequency can be observed in very deep meditative states but in the case of this study, a lower power in delta frequency was observed, which shows an awake awareness (Krishna et al., 2022). Their study findings indicate that the heartfulness meditation practice can result in a wakeful relaxation and can positively influence cognitive enhancement and therefore brain functioning (Krishna et al., 2022).
Although this study shows a positive effect of the meditative aspects of the yoga practice on brain health, it must be considered that the different brain frequency activation measured during meditation or at resting state is highly dependent on the meditation style practiced in different studies (Stapelton et al., 2020) as well as on the meditation experience of the probands (Merlet et al., 2024). A recent review highlights data showing a consistent increase in theta and gamma brain wave activity in experienced meditators during meditation (Merlet et al., 2024). Gamma frequencies are involved in conscious attention and synchronization of neuronal activity and associated with an expanded consciousness, insight and a peak focus (Kora et al., 2021). In meditation novices the trend of studies shows an increase of alpha, theta and to some extant beta power, which aligns with the findings of Krishna and colleagues (Merlet et al., 2024).
3 Discussion
The results of the studies shown in this paper about yoga's effect on brain activity suggest that a regular yoga practice can have a positive influence on brain structure (grey matter density and grey matter volume, hippocampal volume), brain connectivity and on the effectiveness of brain activation. In summary, the present studies indicate a protective effect on cognition and age-related diseases due to a regular yoga practice. Moreover, some studies suggest that yoga can even have a reversing effect on neurodegenerative declines (e.g. Santella et al., 2019).
Reviewing the current research, it is important to consider the heterogeneity of the studies. Depending on the yoga practice being researched, differences in the definition of yoga and yoga practitioners, study design, control stimuli, and measurement tools can be identified. Some studies analyse the effects of a yoga practice that includes breathing exercises, physical exercises, and meditative aspects (e.g., Garner et al.), and consequently, they may not be able to specify which component of the practice is particularly responsible for the observed results. Since physical exercising alone as well as mindfulness exercises alone have been proven to positively influence brain health as well, it is of importance to distinguish which parts of the yoga practice specifically have which effects on the brain (Gothe et al., 2019). Therefore, studies investigating the effect of single aspects of the yoga practice (such as heartfulness meditation done by Krishna and colleagues) contribute to a more specific understanding of the different brain processes occurring as state or trait effects due to a specific aspect of a regular yoga practice.
Future studies in this research area could control biases by implementing well-matched control groups, not only in terms of age, sex and physical fitness, but also on educational levels and intelligence. Additionally, the phase of the menstrual cycle in female participants should be taken into consideration, as it has been shown to influence the hippocampal volume and functional connectivity of the brain (Lisofsky et al., 2015). Moreover, biases in interventional studies could be reduced by randomly allocating participants to the groups instead of allowing them to choose their group by preference (as in Garner and colleagues’ study). Investigating the full spectrum of the individual effects of yoga on brain activity is challenging to investigate in group comparisons though, which is why, there is an urgent need for single case studies and longitudinal studies that are investigating specific effects of yoga on individuals over time.
Research on yoga's effect on brain activity is crucial for designing specific yoga programs to support individuals in preserving and improving their cognitive abilities, brain health as well as overall wellbeing. Moreover, previous and future studies in this area help to build the scientific foundation for implementing yoga as a therapeutic tool in clinical settings to accompany treatments and rehabilitation for (age-related) cognitive impairments and neuropsychological disorders (Merlet et al., 2024; Nourollahimoghadam et al., 2021)
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