No causal effect of left hemisphere hyperactivity in the genesis of neglect-like behavior.

Spatial neglect is traditionally explained as an imbalance of the interhemispheric reciprocal inhibition exerted by the two hemispheres: after a right lesion, the contralesional hemisphere becomes disinhibited and its enhanced activity suppresses the activity in the lesioned one. Even though the hyperexcitability of the left hemisphere is the theoretical framework of several rehabilitation interventions using non-invasive brain stimulation protocols in neglect, no study has yet investigated directly the actual state of cortical excitability of the contralesional hemisphere immediately after the brain lesion. The present study represents the first attempt to directly assess the interhemispheric rivalry model adopting a novel approach based on the induction of neglect-like biases in healthy participants. Applying repetitive transcranial magnetic stimulation (rTMS) over the right posterior parietal cortex while concurrently recording the EEG activity allows to measure specific neurophysiological markers of cortical activity (i.e. TMS-evoked potentials, TEPs) both over the stimulated right hemisphere and over the contralateral homologous area. Besides the effectiveness of the protocol used in modulating behavior, our results show an inhibition of the cortical excitability of the directly stimulated parietal cortex (right hemisphere) and, most importantly, a comparable reduction of cortical excitability of the homologous contralateral (left) area. TEPs and additional electrophysiological measures reliably provide strong evidence for a bilateral hypo-activation following TMS induction of neglect-like biases. These results suggest that the parietal imbalance typically found in neglect patients could reflect a long-term maladaptive plastic reorganization that follows a brain lesion.