Continuous and intermittent theta burst stimulation of primary visual cortex do not modulate resting state functional connectivity: A sham-controlled multi-echo fMRI study

Introduction: Theta burst stimulation (TBS) is a type of rTMS protocol which has the advantage of a shorter delivery time over traditional rTMS. When applied to motor cortex, intermittent TBS (iTBS) has been shown to yield excitatory aftereffects, whereas continuous TBS (cTBS) may lead to inhibitory aftereffects, both lasting from minutes to hours. The majority of TBS research has targeted motor, frontal, and parietal regions, and to date very few studies have examined its efficacy at visual areas. We designed a sham-controlled study to investigate the immediate poststimulation and short-term (1 h post-stimulation) effects of iTBS and cTBS to V1.Methods: Using multiecho functional magnetic resonance imaging, we measured the direct and indirect effects of TBS by comparing resting state functional connectivity (FC) before and after stimulation in whole brain networks, and seeds from V1 (stimulation site) and neighboring occipital and parietal visual networks. In addition, we also measured pre- and post-TBS phosphene thresholds (PTs) to examine the modulatory effects of TBS on cortical excitability.Results: We found no changes in FC for iTBS, cTBS or sham stimulation conditions from baseline to poststimulation timepoints. Additionally, cTBS and iTBS had no effect on visual cortical excitability.Conclusions: Our results indicate that unlike our previous low frequency rTMS to V1 study, which resulted in widespread FC changes up to at least 1 h after stimulation, TBS to V1 does not affect FC. Contrary to the studies showing comparable TBS and rTMS aftereffects in motor and frontal regions, our findings suggest that a single session of cTBS or iTBS to V1 at 80% PT using a standard protocol of 600 pulses may not be effective in targeting FC, especially in clinical settings where therapy for pathological networks is the goal.

Automated summary

In this study, the immediate and short-term effects of iTBS and cTBS on V1 were investigated using a sham-controlled design. The results showed that neither iTBS nor cTBS had any effect on functional connectivity or cortical excitability in V1. This suggests that a single session of cTBS or iTBS using a standard protocol may not effectively modulate FC in V1, especially in clinical settings targeting pathological networks.