Does habituation depend on cortical inhibition? Results of an rTMS study in healthy subjects.

Habituation, i.e. the decremental response to repeated sensorial stimulation, is studied in humans through evoked potential stimulation. Mechanisms underlying habituation are not yet cleared, even if inhibitory circuits are supposed to play an important role. Light deprivation (LD) increases visual cortical excitability likely through down-regulation of GABA circuits. We previously found that high-frequency repetitive transcranial magnetic stimulation (hf-rTMS) can revert these facilitatory effects likely restoring the activity of inhibitory circuits. Here, we studied the effects of LD and rTMS on habituation of visual evoked potentials (VEPs). The hypothesis was that if the inhibitory circuits have a role in habituation, then LD that downregulates GABA circuits, should impair habituation that in turn should be restored by hf-rTMS. Fifteen healthy subjects underwent VEPs recording in baseline (without LD), in LD alone (without rTMS), in LD and 1 Hz rTMS and in LD and 10 Hz rTMS. Habituation observed in baseline (without LD) was significantly impaired after LD; 10 Hz but not 1 Hz rTMS was able to restore normal habituation phenomena. VEPs habituation is impaired by LD but it could be restored if hf-rTMS is given during LD. As LD acts reducing GABA circuits activity and hf-rTMS likely upregulates such circuits, these data give support to the hypothesis that cortical inhibition can play a relevant role in mechanisms underlying habituation.