Electrophysiological hemispheric asymmetries induced by parietal stimulation eliciting visual percepts

Objective: We aimed to establish if the electrophysiological activity resulting from the direct stimulation of the intraparietal sulcus and eliciting visual percepts is hemispheric-specific. Methods: We tested nineteen participants. Each received 360 TMS pulses at phosphene threshold intensity over right and left IPS while recording EEG. After each pulse, participants had to report if they had seen a phosphene. Results: Parietal phosphene perception is associated with hemispheric-specific activations: phosphenes elicited by left TMS involve central and frontal electrodes at about 30 ms, and frontal, central and parieto-occipital electrodes from 120 to 250 ms; phosphenes elicited by right parietal TMS involve parietal and centro-parietal electrodes at about 60 ms, and frontal, central and parietal electrodes from 150 to 250 ms. Correlated Component Analysis shows that primary visual areas are not activated when phosphenes are produced by TMS over IPS. Conclusions: Our results show that direct stimulation of IPS gives rise to sustained patterns of activity specific to the stimulated hemisphere. Moreover, elicited parietal phosphenes are associated with evoked activity specific to the stimulated hemisphere and located outside early visual processing areas. Significance: This study highlights hemispheric differences in the electrophysiological dynamics related to parietal phosphenes, and shows that the dorsal pathway can give rise to visual conscious percepts.