Non-invasive neuromodulation approaches (Table 3) can also be used to target SP circuitry. Open-label studies have shown efficacy in reducing OCD and tic symptom severity using low frequency (inhibitory) rTMS to the SMA [110–111 but see 112], though these studies did not examine effects on SP specifically. Based on findings from rTMS and DBS studies in OCD, Senço et al. [113] proposed using tDCS with the cathode over pre-SMA and anode over an extra-cephalic region, a montage that has shown some efficacy in reducing OCD symptom severity [114]. One limitation of standard rTMS approaches is their relatively low depth, which corresponds to targeting mostly cortical structures. To overcome this issue, novel rTMS coils (H-coils) have been designed [115], which generate deeper electromagnetic fields at the same motor threshold intensity [116]. A study in non-psychiatric volunteers showed that H-coil TMS targeting the insula significantly modulated metabolic activity in connection regions, including the sensorimotor striatum [117]. Interestingly, the insula is associated with addiction behavior and clinical trials have shown that H-coil TMS of the insula in substance use disorders alleviated symptoms [118].
