Our brains rely on constant communication between the left and right hemispheres to make sense of the world. For decades, neuroscientists have proposed that attention works through a balance between the hemispheres, where each side inhibits the other to maintain stable perception. This balance helps us attend evenly to both sides of space. When it breaks down, as seen in conditions like spatial neglect after stroke, awareness of one side of the environment can disappear.
A new study using transcranial magnetic stimulation combined with electroencephalography offers fresh insight into this model. The findings suggest that state dependent interhemispheric attention is not a fixed feature of the brain. Instead, it emerges specifically when the attentional system is actively engaged.
Why The Posterior Parietal Cortex Matters For Attention
The posterior parietal cortex plays a central role in directing visual attention. Damage to the right posterior parietal cortex often leads to left sided neglect, highlighting how asymmetries between hemispheres shape perception. Prior research showed that inhibiting one parietal cortex could induce neglect like behavior in healthy individuals. However, brain imaging at rest did not consistently show the expected push pull relationship between hemispheres.
This raised an important question. Does interhemispheric rivalry only appear when attention is actually being used?
How TMS EEG Captured Attention In Action
To answer this, researchers studied healthy adults using single pulse TMS delivered to either the left or right posterior parietal cortex while recording EEG. Participants completed a visual task designed to create two distinct brain states. In one condition, they actively searched for specific visual targets, demanding focused attention. In the other, they performed a simple detection task with minimal attentional load.
This design allowed the researchers to directly compare how the brain responded to stimulation during high versus low attentional engagement.
State Dependent Interhemispheric Attention Emerges During Focus
The results were striking. Differences between left and right hemisphere responses appeared only during the attention demanding task. When participants were actively engaged, stimulation of the left posterior parietal cortex produced stronger brain responses than stimulation of the right. These effects occurred within milliseconds and were detected over frontal brain regions connected to attention control.
In contrast, no hemispheric differences were seen during the low attention condition. This indicates that state dependent interhemispheric attention is activated when the brain is truly focused, not when it is idling.
What This Means For Brain Networks And Asymmetry
Interestingly, the observed effects were not limited to direct parietal to parietal communication. Instead, they involved fronto parietal interactions across hemispheres. This suggests that attention relies on distributed networks rather than simple one to one inhibition.
The findings also support the idea that the right hemisphere exerts stronger inhibitory control over the left during attention. When TMS briefly disrupts the left posterior parietal cortex, this balance shifts, allowing right hemisphere driven networks to become more active and rebalance attention.
Clinical And Translational Implications
These insights have important implications for interventional psychiatry and neuromodulation. If interhemispheric dynamics depend on cognitive state, then brain stimulation therapies may be more effective when paired with task engagement. This could inform rehabilitation strategies for stroke related neglect and guide precision targeting in TMS protocols for attentional and cognitive disorders.
Understanding state dependent interhemispheric attention moves the field closer to personalized neuromodulation that adapts not just to brain anatomy, but to brain state.
Citations
- Brancaccio A, Lanza C M, Schintu S. Investigating the attentional state dependency of interhemispheric interactions: a TMS EEG study. Brain Stimulation. 2025. https://doi.org/10.1016/j.brs.2025.103011
- Kinsbourne M. Hemi neglect and hemisphere rivalry. Advances in Neurology. 1977;18:41–49. https://pubmed.ncbi.nlm.nih.gov/867980