In vivo assessment of human visual system connectivity with transcranial electrical stimulation during functional magnetic resonance imaging

Functional magnetic resonance imaging (MRI) was used to investigate local a nd distant cerebral activation induced by transcranial electrical stimulati on in order to noninvasively map functional connectivity in the human visua l system. Stimulation with lateromedially directed currents and the anode 4 .5 cm dorsally to the inion over the right visual cortex induced phosphenes extending into the contralateral lower quadrant of the visual field. fMRI showed a focal hemodynamic response underneath the anode in extrastriate co rtex and distant coactivation in subcortical (lateral geniculate nucleus), cortical visual (striate and extrastriate), and visuomotor areas (frontal a nd supplementary eye fields). This pattern of activation resembles a networ k of presumably interconnected visual and visuomotor areas. Analysis of act ivation sites supplies new information about cerebral correlates of phosphe nes and shows that the cortical region underneath the cranial stimulation s ite is not necessarily the origin of behavioral and/or perceptual effects o f transcranial stimulation. We conclude that combining transcranial electri cal stimulation of neural tissue with simultaneous fMRI offers the possibil ity to study noninvasively cerebral connectivity in the human brain. (C) 20 01 Academic Press.