2.2.1. Experiment 1 (electrophysiological study)
The experiment 1 was conducted in a cross-sectional study design from July 2012 to January 2013. To perform PT measure, in a semi-darkened room, volunteers were instructed to sit comfortably in a shiatsu chair and use a blindfold. A 10 cm circular coil connected to stimulator (Neurosoft, Russia; peak magnetic field = 2.2 T) was placed in a vertical position (its handle pointing upward) on the inion–nasion line, with its inferior limit 1 cm above the inion, as described before [19]. The circular coil may induce a larger electric field increasing chances of evoking phosphenes [20]. After each TMS single-pulse over visual cortex, the volunteers were encouraged to relate any sensory experiences (e.g. visual, smell or taste sensations). The coil position was defined as the site where stimulation resulted consistently in phosphenes (short-lasting flashes or lines in the subject's visual field). Then, the stimulator intensity (initially applied at 60% of stimulator output) was adjusted until the subject reported phosphenes at least five out of ten trials (phosphene threshold) and was expressed as a percentage of maximum stimulator output. To determine interictal excitability of visual cortex in migraine patients, we used two standard deviations from PT average of healthy individuals. If PT was two standard deviations below PT mean of healthy individuals, patients were considered hyperexcitable and when PT was two standard deviations above, patients were considered hypoexcitable. In migraine subjects, all recordings were made in a headache-free interval (interictal period) of at least 48 hours after a migraine attack. As changes in hormone levels can modify the neuronal activity [21], all women performed the experiment in no more than seven days after the first day of menstrual cycle. Furthermore, as 40 min of blindfolding can also change cortical excitability [22], light deprivation was limited to 15 min.