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Discussion
In this study, the yoga intervention was associated with greater improvements in mood and decreases in anxiety in the tonic, acute and intervention analyses compared to the metabolically matched walking intervention, suggesting that the effect of yoga on mood and anxiety is not solely due to the metabolic demands of the activity. In our prior study, significant acute increases in brain GABA levels immediately after a yoga session were recorded. The current study found near-significant acute increases, but stable tonic levels during the 12-week yoga intervention. These observations are consistent with a time-limited effect of the yoga intervention on thalamic GABA levels. The lack of tonic changes in this study, and the lack of baseline differences between the experienced yoga practitioners and controls in the prior study, suggest that tonic GABA levels are stable in subjects screened to exclude low GABA states. The whole group analysis of the correlations between mood and anxiety scales taken before each scan and the GABA levels obtained from those scans demonstrated significant positive correlations with the positive scales of Tranquility and Revitalization and a negative correlation with the STAI-State. To our knowledge, this is the first study to report a positive correlation between thalamic GABA levels and improved mood or decreased anxiety.

The correlations between group changes in mood and anxiety scores and changes in GABA levels in the tonic and acute conditions suggest that increases in thalamic GABA levels are associated with improved mood and decreased anxiety. This is the first study to show that a behavioral intervention (i.e., yoga) is associated with a positive correlation between changes in thalamic GABA levels and improvements in mood and anxiety. The use of METs controlled for metabolic demands of each intervention and allowed the activity level of the 2 groups (outside of the intervention) to be compared. The significantly greater level of physical activity outside the intervention in the walking group compared to the yoga group was not expected. The higher level of outside activity in the walking group could have contributed to the finding of smaller changes in mood and anxiety in the walking group, as the intervention may not have been a great enough physical challenge given the greater level of outside activity. The small sample size in the yoga group where only a nearly significant increase in thalamic GABA levels was detected was a limitation, which was offset by the findings of significant correlations between GABA levels and mood and anxiety scores. In our previous study, the acute increase in whole-slab GABA levels in experienced yoga practitioners was 27% (published) and a 26% increase in thalamic GABA levels (unpublished), compared to a 13% thalamic increase seen in the current study of yoga-naı¨ve subjects who were trained for 12 weeks. This suggests that while subjects can be trained to practice yoga in a relatively short time with a measurable effect, the associated change in GABA levels may increase with experience.

The effect of the yoga intervention on GABA levels may be due to the ability of yoga practices to increase parasympathetic nervous system (PNS) activity. In a study comparing Iyengar yoga to a walking control, the Iyengar yoga group showed greater increases in PNS activity. Yoga techniques and vagal nerve stimulation (VNS) increase PNS tone by stimulating vagal afferents. Studies suggest that the antiepileptic effects of VNS are largely mediated by widespread release of GABA. Accordingly, the practice of yoga through stimulation of vagal afferents may result in the increase of brain GABA levels as seen in the yoga group. Conclusions The 12-week yoga intervention was associated with greater improvements in mood and anxiety than a metabolically matched walking exercise. This is the first study to demonstrate that increased thalamic GABA levels are associated improved mood and decreased anxiety. It is also the first time that yoga postures have been associated with a positive correlation between acute increases in thalamic GABA levels and improvements in mean scores on mood and anxiety scales. Given that pharmacologic agents that increase the activity of the GABA system are prescribed to improve mood and decrease anxiety, the reported correlations are in the expected direction. The possible role of GABA in mediating the beneficial effects of yoga on mood and anxiety warrants further study.