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Certified Iyengar yoga instructors taught the yoga interventions, which were monitored by the Principal Investigator to ensure consistency in presentation of weekly posture sequences. Written lists of the weekly sequences and pictures of the postures were given to the subjects. After 4 weeks of instruction, subjects were encouraged to practice at home. The intrascan yoga sequence was taught in class and monitored by research staff during Imaging Session II. The structure of the walking intervention was designed to be similar to that of the yoga intervention, with weekly group sessions in which subjects walked around the gym perimeter at 2.5mph for 60 minutes. The intrascan walking session was done on a treadmill set to 2.5mph with 0 incline. This design controlled for group effects and interaction time with research staff.

Imaging Subjects were scanned on a 4-Tesla full-body MR scanner (Varian/UnityInova, Varian Inc., Palo Alto, CA) at Mclean Hospital in Belmont, MA. Scout images confirmed optimal positioning. After global shimming on unsuppressed water, T1-weighted anatomical images were taken in sagittal and axial planes [echo time (TE)/repetition time (TR)ј6.2 seconds/ 11.4 milliseconds, field-of-viewј22_22_8 cm(sagittal) and 22_22_16cm (axial), readout durationј4ms, receive bandwidthј_32 kHz, in-plane matrix sizeј128_256_16 (sagittal) and 256_256_64 (axial), in-plane resolutionј 0.94_1.9mm (sagittal) and 0.94_0.94mm (axial), readout pointsј512, slice-thicknessј2.5mm, flip-angleј118]. In our previous study, a post-hoc regional analysis that used multivoxel spectroscopic imaging showed that the greatest increase in GABA levels after the yoga intervention was in the thalamus.8 The selection of the left thalamus was based on evidence that the left side has greater parasympathetic innervations and that GABA levels are lower in the left thalamus in post-traumatic stress disorder subjects. For this study, an algorithm was developed to position a 2_2_3- cm voxel over the left thalamus. Proton spectroscopy implemented a MEGAPRESS [MEscher-GArwood Point- Resolved Echo Spectroscopy Sequence] difference-editing sequence specifically tuned for GABA.24 Manual voxel shimming yielded global water-line widths ranging from 8 to 15Hz. The MEGAPRESS sequence collected 68-millisecond echo-time spectra in an interleaved fashion where the GABA editing pulse was applied on every second transient. Additional MEGAPRESS acquisition parameters were: TRј2 seconds, spectral-bandwidthј2 kHz, readout-durationј512 milliseconds, Number of Excitations (NEX)ј384, and total scan durationј13 minutes.

In order to quantify GABA, the difference-edited spectra were processed and then fitted with LCModel using basis sets acquired at 4 T. A separate LCModel template was used to fit the unedited 68-milisecond subspectrum to obtain creatine (Cr). All fitted metabolite areas were normalized to the fitted Cr resonance from the 68-millisecond subspectrum. One (1) spectrum from the MEGAPRESS acquisition in the thalamus was excluded from analysis due to low signal-to-noise. GABA/Cr ratios are referred to as GABA levels. In order to ascertain the gray and white matter contribution to each voxel, the axial T1-weighted images were segmented into gray matter, white matter, and cerebrospinal fluid compartments using the commercial software package FSL 4.1 (FMRIB Software Library; Analysis Group, FMRIB; Oxford, UK).