Investigating the State Dependency of Interhemispheric Interactions – a TMS-EEG Study

We fully perceive our surroundings due to the balance between the two cerebral hemispheres, each directing attention toward the contralateral side of space. This balance has been proposed to be maintained by reciprocal interhemispheric inhibition in a push-pull dynamic (Kinsbourne, 1977). Supporting evidence come from neglect patients; however, no push-pull pattern has emerged in healthy individuals under resting-state conditions (Schintu et al., 2020, 2021). We thus hypothesized that the dynamic of interhemispheric interactions may depend on the state of the system, namely on the level of engagement of the right dominant hemisphere in attentional processing. Twenty-two healthy participants underwent a single session of TMS-EEG co-registration, preceded by the Landmark task to quantify the visuo-spatial bias. TMS-evoked activity was measured following single-pulse TMS stimulation of the Right and Left PPC (IPS1; Schintu et al., 2020, 2021) while participants were engaged in a visual task demanding two different levels of attentional deployment (Attention vs Detection). The results revealed that only in the Attention condition, TMS over Left PPC elicited stronger TEPs in contralateral frontal areas compared to Right PPC stimulation. As expected, this dynamic did not emerge in the Detection condition. This pattern may reflect a mechanism in which, when the attentional system is engaged, TMS-induced activation of the left PPC temporarily disrupts right-to-left interhemispheric inhibition, resulting in stronger activation of contralateral frontal areas. ISP analyses revealed no significant difference between conditions, suggesting a more complex mechanism than direct parieto-parietal interaction. Overall, these findings provide preliminary evidence for an asymmetric, state-dependent pattern of interhemispheric interactions in intact brains.