The main result of the present study is that MSPA is not reduced in episodic migraine without aura. This confirms data from a previous study that showed reduced MSPA in migraine with aura, but not in migraine without aura in a smaller and less severely affected cohort (14 patients with migraine without aura, 1.5 ± 1.1 migraine attacks/month, [
7]). It has been suggested that MSPA is the result of direct activation of inhibitory interneurons in the occipital cortex by the TMS pulse, and that reduced MSPA indicates impairment of occipital inhibitory neural networks, resulting in functional hyperexcitability [
13,
6]. Occipital hyperexcitability may favour cortical spreading depression, which is thought to initiate migraine attacks with visual aura [
14]. In contrast, occipital spreading depression does not seem to play a major role in migraine without aura [
15]. Reduced MSPA in migraine with aura but not in migraine without aura is consistent with these pathophysiological ideas. However, previous studies have also shown reduced MSPA in chronic migraine [
7,
8], which in most cases develops from migraine without aura. The present results, with no correlation between MSPA and number of headache days, do not directly support the concept of MSPA decreasing with migraine frequency, reaching minimal values in chronic migraine [
7,
8]. A large proportion of the chronic migraine patients tested previously overused acute headache medication [
7,
8]. In addition, it is difficult or even impossible to test chronic migraine patients interictally. This is important since cortical excitability changes rapidly in the peri-ictal period [
16]. Maybe medication overuse and/or peri-ictal testing may have contributed to the MSPA reduction reported in chronic migraine [
7,
8]. Further studies will be necessary to clarify this point.
A previous study has shown good reproducibility of MSPA group mean values at test and retest after 2 weeks in 10 healthy volunteers [
12]. Our results confirm these results in a larger group (n = 33) also including migraine patients and complete these findings by also assessing the test-retest correlation, which was good. Unfortunately, only 9 migraine patients could be tested interictally during test and retest, precluding a separate analysis of test-retest reliability in controls and migraine patients.