Increase of posterior cerebral artery blood flow velocity during threshold repetitive magnetic stimulation of the human visual cortex: hints for neuronal activation without cortical phosphenes.

To analyze the effects of low intensity repetitive transcranial magnetic stimulation over the occipital cortex on regional cerebral perfusion, bilateral simultaneous monitoring of posterior cerebral artery blood flow velocity was performed using transcranial Doppler ultrasonography in 14 healthy subjects. During 20 s of unilateral magnetic stimulation with 3 Hz and 6 Hz a significant increase of ipsilateral flow velocity was observed during both stimulus conditions (3 Hz, 10.2 +/- 3.7%; 6 Hz, 12.8 +/- 4.7%). A significantly smaller flow velocity increase occurred also in the contralateral posterior cerebral artery (3 Hz, 8.6 +/- 4.0%; 6 Hz, 10.6 +/- 4.1%). Flow velocity increases were similar to values reported in the literature for physiological activation paradigms so that excessive excitation of the visual cortex does not seem to occur during repetitive magnetic stimulation. The largest increase of flow velocity was observed in the ipsilateral posterior cerebral artery of 5 subjects who experienced phosphenes in the contralateral visual half-field during stimulation (14.3 +/- 4.1%: mean of 3 and 6 Hz stimulation). In the other 9 subjects significant velocity increases indicated cortex activation in the absence of cortical phosphenes. The occurrence of maximum velocity responses within 2-3 heart beats following the first cortex stimulus points to a fast adjustment of cerebral perfusion in response to transcranial brain stimulation.