Yoga comprises various domains of practices but practically we can divide yoga into three categories: poses exercise (asana), breathing (pranayama), and meditation (dhyana). Asana is usually considered as a form of exercise through actions of the body. Breathing-based practice involves purposeful inhalations and exhalations that can be designated at a specific speed and intensity. Breathing exercise can be practiced in a seating, standing or lying position (savasana). Previous studies have shown various mental and physical health benefits related to yoga. For example, studies examining mental health outcomes have reported decreased feeling of anxiety and depression after yoga practice . Cognitive benefits of yoga in healthy subjects have also been demonstrated with improved spatial and verbal memory , increased executive functions  and better psychometric abilities .
Mental health and cognitive benefits of yoga are now well accepted by the community, but the neuronal changes in the brain associated with these benefits mostly remain to be investigated. Changes in behavior are often associated with changes in neuronal activity within the brain. Understanding what can elicit brain modulations that lead to improved cognition can give insight into the development of cognitive interventions in both healthy and clinical populations. In this article, we will review studies on neural changes detected by neuroimaging techniques that occur through yoga practice, which may lead to mental health and overall wellbeing in healthy population. For each section, we will first describe the technique and then report findings related to yoga intervention and practice.
PET measures cerebral metabolic rates for glucose or changes in regional cerebral blood flow as an indirect measure of the neural synaptic activity (Figure 1). A PET scan using fluorodeoxyglucose (18F) summates approximately 30 minutes of cerebral glucose metabolism and allows assessment of regional variations. PET is invasive because it involves the injection of positron-emitting radionuclides with short half- lives. As the radioactive compound accumulates in different regions of the brain and positron annihilations occur, the scanner detects the coincident rays produced at all positions outside the head and reconstructs an image that displays the location and concentration of the radioisotope within a plane of the brain. PET is combined with CT imaging to co-register the functional PET image with an anatomic resolution image .
Regarding yoga, decreased blood flow has been measured in the amygdala whereas increased blood flow has been observed in the frontal lobes, after three months of Iyengar yoga training practice in fours subjects  (Figure 2). This result suggests that yoga training may prevent negative feelings as the amygdala is
thought to regulate emotions, and is especially active during
negative emotions. Prolonged focus and attention may have
been developed throughout the training as the frontal cortex is
involved in these processes.
MRI can detect an increase in blood oxygen concentration
that occurs in an area of heightened neuronal activity (Figure
3). The basis for this capacity comes from the way neurons
make use of oxygen. Functionally induced increases in blood
flow are accompanied by alterations in the amount of glucose
the brain consumes but not in the amount of oxygen it uses.
Additional blood to the brain without a concomitant increase
in oxygen consumption leads to a heightened concentration
of oxygen in the small veins draining the active neural centers
. The reason is that supply has increased, but the demand
has not. Therefore, the extra oxygen delivered to the active
part of brain simply returns to the general circulation by way
of the draining veins. The commonest form of functional MRI
(fMRI) is blood oxygenation level dependent (BOLD) imaging
. The BOLD signal depends on the ratio of oxygenated to
deoxygenated hemoglobin. In regions of neuronal activity this
ratio changes as increased flow of oxygenated blood temporarily
surpasses consumption, decreasing the level of paramagnetic
These localized changes cause increases in magnetic
resonance signal, which are used as markers of functional
activation. So far, only a few MRI studies have investigated the
benefit of yoga on brain structure and function. Froeliger et al.
 showed that while viewing negative emotional images and
when presented with distracters, seven practitioners of hatha
yoga and meditation presented less activation in the dorsolateral
prefrontal cortex as compared to seven non-practitioners.
This finding suggests that yoga training may decrease negative
emotional stimuli from distracting working memory, and thus
improve negative emotional response to incoming sensory
information. The authors also reported a correlation between
amygdala activation and decay of positive affect in nonpractitioners,
which was not the case for the yoga trainees.
More specifically, non-practitioners showed a decreased
positive affect that could be predicted by the magnitude of the
activation of the amygdala. Additionally, a positive correlation
was found between gray matter volume in the frontal lobe and
yoga experience, which suggests that long-term practice of yoga
increases gray matter volume. Another study on fourteen yoga
practitioners found that asana-based yoga practitioners with
six to eleven years of experience (doing four to ten practices
a week) also presented increased gray matter, mostly in the
insular cortex, area involved in the pain matrix . This increase
in gray matter volume correlated with higher pain thresholds
during thermal and pain tasks. Gray matter volume in the insula
was also increased with yoga experience in this study.
Another study measured the volume of the hippocampus
in seven healthy elderly adults before and after an extensive
6-month yoga intervention with asana-based and breathingbased
training (including “OM” chanting) . The hippocampus
(brain region involved in memory formation and vulnerable
to loss of grey matter with aging) was found to increase
substantially after the yoga intervention. The increase in gray
matter was observed in bilateral hippocampus (Figure 4), which
suggests that yoga may have the potential to reduce neuronal
senescence. Note, that the sample size of this study (as well as
all the studies mentioned so far) is small and there is no control
group for comparison, so the results should be taken with caution.
However, the increase of gray matter volume was recently
confirmed in the left hippocampus of 13 other experienced yoga practitioners, compared to 13 controls . The authors
also found less activation in the dorsolateral prefrontal cortex
in these yoga practitioners during a working memory task
(compared to controls), which may indicate increased efficiency
. Similarly, increased cortical thickness was measured in the
left prefrontal region of 21 older woman practicing hatha yoga
for more than 8 years compared to 21 women naive to yoga ,
as previously observed with PET .
Finally, in another fMRI study of 12 subjects (four with
formal training in yoga including meditation, and eight naïve),
the audible “OM” chanting, which produces a sensation of
vibration, induced in both groups deactivation in the amygdala,
anterior cingulate gyrus, hippocampus, insula, orbitofrontal
cortex, parahippocampal gyrus and thalamus, when compared
to the brain at rest  (Figure 5). These brain regions are
part of the limbic system, which controls emotions and drives.
In contrast, deactivation did not occur in these brain regions
during a control condition (i.e., enunciating “sss”, which does not
have the vibratory property). This study shows the direct effect
of “OM” chanting on brain modulation. But this is however not
strictly related to yoga practice as naïve subjects were included,
and it rather shows the brain experience of audible “OM”.
EEG is a non-invasive technique that allows the detection of
spontaneous brain electrical activity from the scalp. It provides
temporal resolution in the millisecond range. Traditional
EEG technology provides insufficient spatial detail to identify
relationships between brain electrical events and structures and
functions visualized by fMRI. Recent advances help to overcome
this problem by recording EEG from more electrodes, by
registering EEG data with anatomical images, such as structural
MRI, and by reducing the distortion caused by volume conduction
of EEG signals through the skull and scalp. In addition, statistical
measurements of sub-second interdependences between
EEG time-series recorded from different locations can help
to generate hypotheses about the instantaneous functional
networks that form between different cortical regions during
mental processing .
Desai & colleagues  recently reviewed the current
literature on EEG and yoga. Based on 15 articles, they concluded
that yoga increased overall brain electrical activity, which
include increases in alpha, beta and gamma band activity. Alpha
band neural oscillations are in the frequency range of 8-12Hz
and are predominant in the occipital brain areas during periods
of eyes closed. In a study from 1992 on eight healthy subjects,
an increase in alpha activity was measured over 30 consecutive
days of breathing and relaxation yoga training of 50 minutes .
More specifically, increases in alpha wave activity were observed
in occipital and prefrontal cortices. Another study reported
increased alpha band and beta band (12-30Hz) activation during
pranayama practice (i.e., alternate nostril breathing) in eight
long-term practitioners . On the other hand, a recent study
that looked at the effects of yoga asana training, pranayama
training and a combination of both on brain activity in 80 healthy
subjects, found a significant increase in alpha wave activity
in asana group and in the combination group, but not in the
pranayama group . Another study reported that Kriya yoga
and pranayama yoga practitioners presented larger beta waves
and spurts of alpha waves (compared to controls), indicating
relaxation with the co-occurrence of alertness . Kriya-based yoga practice in eight subjects has also been shown to elicit theta
activity (slower activity than alpha and beta), with an increase
of 40%, mostly in parietal regions . This finding suggests
that the brain was more deeply relaxed after a session of yoga
with reduced feelings of anxiety and better emotional control.
Additionally, the effects on gamma (>35Hz) and beta (15-35Hz)
wave activity were also investigated after a pranayama practice
(i.e., producing a vibrating sound while exhaling through the
nose) and showed again an increase in both frequency bands
during pranayama in eight healthy subjects .
In summary, the results from the reviewed articles were all
in favor of yoga practice. Among yoga practitioners and after
a yoga intervention, the three subdivisions of yoga (i.e., asana,
meditation and breathing) produced changes in behavior, in
cognition and in the brain. Indeed, yoga training has been
correlated with decreased amygdala activation and decreased
negative emotion in response to emotional distracter images.
Yoga also seems to have a constructive effect on the anatomy of
the brain, with an increase of grey matter in the hippocampus and
frontal regions in elderly subjects. It also seems to have positive
effects on brainwave activity in terms of increasing alpha, beta,
and theta electrical activity, which have been associated with
improvements in cognition, memory, mood, and anxiety.
We are still however at the beginning of yoga research and
most findings come from small sample size, uncontrolled and
non-randomized studies. Comparing different groups are crucial
for accurate testing and interpretation of yoga intervention
experiments. Selecting appropriate comparison groups can
however be difficult because yoga includes a heterogeneous set
of practices. To isolate the specific mechanisms of yoga’s effects,
using an active control group seems to be the best option, for
example a group doing physical exercise other than yoga .
Yoga intervention is another good option and in an ideal world,
one wants to combine both: group comparison (between
experienced and naïve subjects) and yoga intervention (before
and after, within subject). Further research should examine the
effects of yoga training into the clinical treatment of neurological
and psychosocial disorders . Because of the extensive variety
of yoga practices, more studies should also directly compare
the different styles of yoga and evaluate the extent of their
neurocognitive and neuro-anatomical properties on healthy and
This article was supported by the University and University
Hospital of Liege, the Belgian National Funds for Scientific
Research (FRS-FNRS), the European Union’s Horizon 2020
Framework Programme for Research and Innovation (Grant
Agreement No. 785907), Luminous project (EU-H2020-fetopenga686764),
the James McDonnell Foundation, Mind Science
Foundation, and the BIAL Foundation. OG is post-doctoral
researcher at FRS-FNRS. I thank Marit Sathrum and Joan
Bliefernincht for introducing me to yoga practice and teaching.