Abstract
Dear Sir In a recent paper Fumal et al. showed that daily sessions of high-frequency repetitive transcranial magnetic stimulation (hf-rTMS) over the visual cortex in migraineurs can restore habituation to prolonged visual pattern stimulation (1). This is an interesting paper which points towards the use of rTMS as a potential therapeutic tool because of its ability to induce long-term plastic effects on cortical excitability in migraine. Indeed, lack of habituation to repeated stimulation has been generally reported in migraine and is considered a neurophysiological marker of this disease. Therefore, the opportunity to restore normal habituation could have positive effects, perhaps reducing susceptibility to migraine attacks in patients.
Based on a previous study (2), Fumal et al. (1) argue that the visual migraine cortex presents a reduced preactivation excitability level that is responsible for the deficient habituation; hf-rTMS, because of its facilitatory effect, is able to restore habituation. Moreover, in the authors' view, the relationship between reduced cortical activation and deficient visual habituation in migraineurs is also confirmed by the observation that the depressive effect of 1 Hz rTMS impairs habituation in normal subjects.
In our opinion, the interpretation of the observed effects could be reversed. In fact, the effects of rTMS depend greatly on the basal excitability state of the stimulated cortex. In a recent study (3) we found that during visual deprivation, a condition known to increase visual cortical excitability in healthy subjects, hf-rTMS exerted effects opposite to those found in the baseline condition (without deprivation) (3). In particular, in this study hf-rTMS was found to normalize visual cortical excitability when applied during light deprivation. Since evidence has been provided that light deprivation induces a down-regulation of GABA inhibitory circuits (4–6), we interpreted the effect of hf-rTMS as due to a recovery of intracortical inhibitory circuits.
If we assume that habituation phenomena depend on the activity of inhibitory circuits, that have been shown to be hypoactive in migraine (7–10), hf-rTMS could restore habituation by simply up-regulating cortical inhibitory circuits.
In conclusion, we agree with Fumal et al. (1) when they speak about cortical hypoexcitability, only if they refer to the inhibitory circuits. However, it's important to note that these conflicting hypotheses both point to the promising potential therapeutic role of hf-rTMS in migraine.