Neural Connectivity Changes to tACS: A Preliminary eLORETA EEG Study

Investigating how tACS influences brain connectivity

What happens when specific brain regions receive non-invasive electrical stimulation? In a new EEG study using our Starstim 8 system, researchers explored how transcranial alternating current stimulation (tACS) at 40 Hz affects interactions between the frontal and parietal lobes.

Using eLORETA (an algorithm applied to EEG data, a head model, and mathematical inversion to map current densities across brain voxels), researchers assessed changes in both cortical power and functional connectivity, focusing on current source density (CSD) and phase synchronization. The study involved individuals with mild cognitive impairment (MCI), ADHD, and learning disabilities, and aimed to evaluate how 40-Hz tACS over the left frontoparietal network influences neural communication in brain regions tied to executive functions.

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Transcranial alternating current stimulation (tACS) delivers oscillatory electrical currents to the brain, allowing researchers to investigate how externally applied frequencies interact with intrinsic neural oscillations.

When applied at specific frequencies, such as in the gamma range, tACS can influence brain rhythmicity and the timing of neuronal activity across regions. Since many neurocognitive disorders involve disruptions in communication between cortical areas, modulating functional connectivity—particularly between the frontal and parietal lobes—has become a promising target. Tools like EEG and eLORETA help capture these neural dynamics by mapping oscillatory activity and assessing phase synchronization across the cortex.

How was the study conducted using Starstim 8?

This preliminary EEG study evaluated the effects of tACS over the left frontal and parietal cortices across 12 treatment sessions, delivered by our Starstim 8 system. Participants underwent EEG scans before and after the stimulation course, allowing researchers to quantify changes in brain dynamics using eLORETA.

Participants were instructed to continue their normal medication regimens and engage in computerized cognitive training games during stimulation. EEG data were recorded with a 19-channel Brainmaster device and processed using EEGLAB and artifact correction tools before being analyzed through eLORETA

• Participants: 34 individuals with cognitive impairments (MCI, ADHD, LD, TBI)

• Montage: Electrodes at F3 and P3, following the 10–20 EEG system

• Stimulation Parameters:

  • Frequency: 40 Hz sine wave

  • Intensity: Up to 2.0 mA

  • Duration: 25 minutes per session over 3 weeks

Fig.1. An illustration of the F3 and P3 positions on the scalp.

What neural changes were observed after 40-Hz tACS?

Following treatment, researchers found distinct changes in cortical power and functional connectivity, particularly in the gamma and beta frequency ranges. eLORETA revealed a reduction in CSD power in stimulated regions and an increase in contralateral hemispheric activity—a pattern linked to inter-hemispheric rivalry. Additionally, gamma-band phase synchronization between frontal and parietal areas significantly improved.

• Decreased CSD power in the left frontoparietal cortex, most notably in gamma and beta bands

• Increased CSD power in the right hemisphere, reflecting inter-hemispheric compensatory activity

• Enhanced phase synchronization at gamma frequency between Brodmann areas 8/9 and 39/40

• No significant changes observed in coherence or non-gamma frequency connectivity

  
  

These findings indicate that timing coordination between regions improved, even though local signal strength decreased in the targeted area.

What do these findings reveal about the impact of tACS?

The study challenges the simplistic view that tACS simply synchronizes neural activity. Instead, the data suggest a more nuanced mechanism involving plasticity, phase-based entrainment, and network rebalancing. Power reductions in stimulated areas alongside increased inter-regional synchronization point to a shift in how neural networks communicate, rather than just boosting local excitability.

This aligns with other research indicating that tACS influences connectivity rather than power, especially when measured post-treatment. These effects may underlie observed improvements in executive functioning tasks, such as Trail Making and Verbal Fluency.

Conclusion

This study demonstrates that 40-Hz tACS can significantly alter neural dynamics, not by increasing local power, but by enhancing inter-regional connectivity. As interest in personalized and non-invasive brain therapies continues to grow, studies like this help pave the way for more refined, connectivity-based approaches.

Start your journey today by discovering how our Starstim 8 tES-EEG system can support your neuroscience research.

Key Takeaways

What are the neural effects of tACS?

tACS can alter local brain power and modulate timing-based connectivity between regions. In this study, it decreased power at the stimulation site and increased synchronization between frontal and parietal regions.

How does gamma tACS affect the brain?

Gamma (40-Hz) tACS may reduce local synchronization while improving inter-regional timing coordination, especially in networks tied to executive function.

Is Starstim 8 suitable for EEG and tACS studies?

Yes. Starstim 8 offers integrated EEG and tACS capabilities, making it an effective tool for neuromodulation research and real-time brain monitoring.

Does tACS synchronize brain activity?

Not necessarily. This study found no clear evidence of power-based synchronization, but it did find improvements in phase synchronization, indicating more precise timing between brain regions.

References:

• Lee, Tien-Wen, and Gerald Tramontano. "Neural Changes to Transcranial Alternating Current Stimulation in the Gamma Range Over the Left Frontoparietal Network: A Preliminary eLORETA EEG Study." medRxiv (2025): 2025-01. https://doi.org/10.1101/2025.01.24.25321077