A functional magnetic resonance imaging navigated repetitive transcranial magnetic stimulation study of the posterior parietal cortex in normal pain and hyperalgesia

Noxious stimuli activate a complex cerebral network. During central sensitization to pain, activity in most of these areas is changed. One of these areas is the posterior parietal cortex (PPC). The role of the PPC during processing of acute pain as well as hyperalgesia and tactile allodynia remains elusive. Therefore, we performed a functional magnetic resonance imaging (fMRI) based, neuro-navigated, repetitive transcranial magnetic stimulation (rTMS) study in 10 healthy volunteers. Firstly, pin-prick hyperalgesia was provoked on the right volar forearm, using the model of electrically-induced secondary mechanical hyperalgesia. fMRI was performed during pin-prick stimulation inside and outside the hyperalgesic areas. Secondly, on four different experimental sessions, the left and right individual intraparietal BOLD peak-activations were used as targets for a sham-controlled 1 Hz rTMS paradigm of 10 min duration. We measured psychophysically the (i) electrical pain stimulus intensity on an 11-point numeric pain rating scale (NRS, 0-10), the (ii) area of hyperalgesia, and the (iii) area of dynamic mechanical allodynia. Sham stimulation or rTMS was performed 16 min after induction of pin-prick hyperalgesia and tactile allodynia. Compared to sham stimulation, no significant effect of rTMS was observed on pain stimulus intensity and the area of allodynia. However, a reduction of the hyperalgesic area was observed for rTMS of the left PPC (P<0.05). We discuss the role of the PPC in central sensitization to pain, in spatial discrimination of pain stimuli and in spatial-attention to pain stimuli.