The Effects of Theta Burst Stimulation to the Occipital Cortex on Brain Biomarkers Measured by Magnetic Resonance Spectroscopy

Theta burst stimulation (TBS), a type of transcranial magnetic stimulation (TMS), uses repeated high-frequency bursts to induce neural noise in the cortex. An intermittent TBS (iTBS) protocol is generally considered excitatory, while continuous TBS (cTBS) is generally considered inhibitory. TBS effects are highly variable and have been primarily studied in the primary motor cortex (M1). We investigated the effects of iTBS and cTBS to the primary visual cortex (V1) on occipital Glx (glutamate and glutamine composite) and GABA+ (gamma-aminobutyric acid and macromolecules composite) concentrations compared to sham stimulation. Thirty participants received a single session of individual stereotaxically-guided TBS to the V1. Participants received either cTBS, iTBS or sham TBS. GABA+ and Glx were measured at the stimulation site using magnetic resonance spectroscopy. Baseline pre-TBS GABA+ and Glx levels were compared to those immediately post- and one hour post-TBS. The results show a trend for a decrease in GABA+ immediately following cTBS compared to one hour post-TBS and a trend for an increase in Glx immediately following iTBS compared to one hour-post TBS. There was also an increase in GABA+ from baseline to one hour-post TBS and a trend for an increase in GABA+ and Glx composite ratio levels following sham. Since there was a lack of systematic trends in the data, we suspect there is no relevant change from a single session of TBS to neurotransmitter levels in the V1. Perhaps, only repeated application of TBS may lead to substantial benefits. This indicates that a single session of TBS can be used safely in the laboratory without effects on neurotransmitter levels in the V1.