During the interictal phase, the cerebral cortex is characterised by a hyperresponsiveness to repeated sensory stimuli, manifested by a lack of habituation or adaptation of cortical responses. Lack of habituation has been shown in the visual cortex in studies of visual evoked potentials (VEP) during the interictal period and it is possibly explained by a reduction in the cortical pre-activation level due to thalamo-cortical dysrhythmia. Just before and during the migraine attack, cortical reactivity changes drastically: habituation is restored and the amplitude increases. In chronic migraine (headache occurring on 15 or more days per month for more than 3 months with features of migraine headache on at least 8 days per month), VEPs habituate normally like those recorded during attacks of episodic migraine, but have in addition an increased amplitude in the 1st block of responses. Chronic migraine was therefore compared to a "never ending migraine attack" accompanied by cortical hypersensitivity.
In this study the investigators aim to demonstrate that cathodal tDCS over the visual cortex with simultaneous anodal tDCS over the left dorsolateral prefrontal cortex is able: 1) to reduce cortical hypersensitivity and habituation as assessed by VEPs and contact heat evoked nociceptive potentials (CHEPS), as well as to decrease pain perception assessed by quantitative sensory testing (QST) and the nociceptive blink reflex (nBR); 2) to decrease headache and migraine frequency.
