While some indications of the neural circuits involved in the Conditioned Pain Modulation (CPM) process are now available, there is still need to clarify what parts of the brain are essential for this process, whether the spino-brainstem loop is largely sufficient to explain CPM or whether other cerebral and spinal regions such as frontal, somatosensory and other cortical regions contribute substantially. Whereas mere observation of correlation between these circuits while activated by brain imaging is still of considerable interest, direct experimental manipulations by repetitive transcranial magnetic stimulation (rTMS) could even establish insights into causal relationships.

rTMS of different intensities, frequencies and location will be applied during CPM to evaluate the central mechanisms of pain modulation, their location and role in pain reduction through enhancement or suppression of activity in the relevant brain regions. In other words, cortical regions that may be implicated in CPM will be determined by augmenting or interrupting their activity via rTMSapplied to the areas under investigation. The regions will be the pain network sites, which are assumed to control the top-down influence on CPM and are superficial enough to be stimulated by the magnetic coil. These include primarily DLPFC (dorsolateral prefrontal cortex) and OFC (orbitofrontal cortex), with possible later addition of other relevant sites such as ACC (anterior cortex cinguli), insula and somatosensory cortices, etc. Since rTMS may be administered in a manner that either reduces or enhances the excitability of the stimulated cortical region, it should be possible to clarify the inhibitory or excitatory role of these regions in the CPM process. In summary, the planned studies should allow for identifying the cortical regions of the descending pain system, which are critical as starting points for the top-down modulation of CPM.

Inclusion Criteria:

• age 18-70
• males and females
• right handed

Exclusion Criteria:

• metal in brain/skull
• cardiac pacemaker
• cohlear implants
• history of head trauma
• history of epilepsy or seizures
• pregnancy