Variation in cortical responses to neuromodulation: Motor thresholds vs. Visual phosphenes

Theta burst stimulation (TBS) is a repetitive transcranial magnetic stimulation (rTMS) protocol with the added benefit of shorter stimulation time (compared to traditional rTMS protocols). This increases efficiency and compliance in both research and clinical settings. Even though TBS applications in the primary motor cortex (M1) have been well-explored, scant attention has been paid to its impact on the primary visual cortex (V1). TBS includes two variants: Intermittent (iTBS; excitatory) and continuous (cTBS; inhibitory). In our lab, previous neuroimaging studies including magnetic resonance spectroscopy and resting state functional magnetic resonance imaging did not lead to notable alterations in GABA concentration in V1 or functional connectivity when V1 was stimulated with either cTBS or iTBS. However, we previously found significant effects when applying 1Hz rTMS to V1. This emphasises the need to understand TBS effects on visual brain areas and optimise TBS protocols accordingly. In this study, we explored cTBS' influence on M1 and V1 utilising stereotactic neuronavigation. We investigated the disparities in biophysical parameters such as scalp-to-cortex distance (SCD), electric fields at hotspots and stimulation intensities between M1 and V1. We used motor thresholds (MTs) for M1 stimulation and phosphene thresholds (PTs) for V1 stimulation as markers. Our preliminary results suggest that PTs are significantly higher than MTs despite longer SCD at V1, and shorter SCD at M1. In addition, we found that post-cTBS PTs increased in comparison to sham stimulation, hinting at an inhibitory aftereffect. However, both active and sham cTBS in M1 yielded to increased MTs. A profound understanding of the varied stimulation parameter effects and their location-specific variability is crucial for effective and efficient TMS protocol application in research and therapeutic contexts. These insights will facilitate tailoring TMS approaches to each target region's unique requirements, enhancing the overall effectiveness of TBS in vision research and clinical settings.