P236 The effect of intermittent theta burst stimulation over the prefrontal cortex on resting-state fronto-striatal connectivity

Fronto-striatal loops play a role in various cognitive and emotional processes. Intermittent theta burst stimulation (iTBS) has been shown to increase regional excitability and functional connectivity of connected areas ( Huang et al., 2005 ). The choice of an optimal TMS intensity, which is interlinked with ethical and practical issues such as pain and muscle twitches, has not been explored in detail. Also, the role of white matter tracts in functional connectivity has often not been taken into account. We therefore applied iTBS with different intensities and methods of determining the exact stimulation sites to systematically test optimal parameters combining strong fronto-striatal activity with minimal adverse effects. Healthy subjects underwent excitatory iTBS at different intensities (90% and 120% of individual resting motor threshold). The protocol consisted of 600 pulses (3.33 min) and was applied over the left prefrontal cortex (lPFC). Directly before and 7 min after stimulation a task-free resting state functional MRI scan was conducted. The stimulation site was determined anatomically, taking into account white matter tracts. This was done by finding individual tracts connecting the left caudate with the lPFC using DSI studio ( Yeh et al., 2013 ). Resting state data was denoised using FSL’s MELODIC ( Jenkinson et al., 2012 ) and analysed using DPABI ( Chao-Gan, 2014 ); with the caudate and the stimulation sites as seed regions. Our findings suggest that a sub-threshold stimulation intensity of 90% is sufficient to induce changes in functional connectivity of the fronto-striatal network, when stimulation is applied over the lPFC. Furthermore, stimulation effects might be increased by determining an individual stimulation site based on DTI-tracts, rather than using only macro-anatomical landmarks. By using sub-threshold intensities adverse effects such as pain and head movements due to muscle twitches can be reduced or avoided.