Is the contralesional hemisphere hyperactivated in neglect?

The dysfunction underlying neglect syndrome is traditionally explained on the basis of “rivalry models” which postulate a pathological hyperactivation of the unaffected hemisphere due to the reduced inhibitory influences from the lesioned hemisphere. The aim of the present study is to test these models analyzing the effect that repetitive Trancranial Magnetic Stimulation (rTMS) exerts on the stimulated and contralateral hemispheres in the processing of visual stimuli. Specifically we aim at assessing the contribution of left and right parietal cortices in an impaired neglect-like functioning induced rTMS in healthy participants. 14 healthy volunteers performed a Line Bisection task and a simple detection task of unilateral checkerboards stimuli. Both tasks were performed either before and after 30 minutes of rTMS (1 Hz) over the right posterior parietal cortex. The EEG signal was continuously recorded throughout the experiment. The efficacy of rTMS in inducing neglect-like phenomena was confirmed by the Line Bisection task where participants showed a rightward deviation after rTMS, a performance comparable to that of neglect patients. Detection task results showed that the effect of rTMS was a lengthening of reaction times for both left and right visual stimuli and a reduction of the amplitude of P200 component registered both on left and right parietal sites. Therefore, our results did not show an hyperactivation of the left hemisphere due to the inhibition of the right hemisphere (as theorized by “rivalry models”). Conversely, the inhibition of the right parietal cortex induced a spreading of the inhibition to the homologous left hemisphere.