The purpose of this study is to investigate the independent and interactive effects of 40Hz auditory, visual, and external trigeminal nerve stimulation (eTNS) on brain state dynamics and functional connectivity. While 40Hz visual and auditory sensory stimulation have been explored for potential cognitive and therapeutic benefits, the underlying mechanisms on large-scale brain networks and the potential effects of combining multiple modalities remain unclear.

This study aims to determine whether multimodal co-stimulation genuinely compounds network-level effects or if targeted dual-pairings are more efficient for brain network reorganization. In this within-subject study, 23 participants will receive all eight possible combinations of 40Hz auditory, visual, and eTNS (targeting the maxillary branch) in a randomized order. Each stimulation condition will last for 5 minutes, separated by 1-minute inter-stimulus intervals. Continuous electroencephalography (EEG) will be recorded throughout the procedures to evaluate brain responses. The collected EEG data will be analyzed using microstate analysis, functional connectivity analysis, and graph-theoretic network analysis. Ultimately, the study seeks to understand how these non-invasive stimulation techniques, independently and together, drive large-scale network reorganization and optimize the balance between local specialization and global integration in the brain.

Physical neuromodulation at 40 Hz has emerged as a promising therapeutic approach, with visual and auditory stimulation demonstrating the capacity to entrain gamma oscillations and confer cognitive benefits. Analogous effects have also been observed with electrical approaches, including external trigeminal nerve stimulation (eTNS). While the prevailing assumption in the field suggests that combining multiple sensory modalities will more comprehensively recruit distributed brain circuitry and yield greater therapeutic effects, the mechanistic foundations of this additive hypothesis remain contested. Furthermore, recent evidence indicates that network-level reorganization, rather than localized gamma entrainment, may represent a more causally relevant substrate for these cognitive improvements.

This study is designed to systematically characterize the independent and interactive effects of 40Hz auditory, visual, and eTNS stimulation on brain state dynamics, functional connectivity, and graph-theoretic network properties. The trial employs a fully crossed 2×2×2 within-subject design. A total of 23 participants will undergo all eight possible combinations of 40Hz stimulation modalities: sham (no stimulation), auditory only, visual only, eTNS only, auditory + visual, auditory + eTNS, visual + eTNS, and the three-way combination of auditory + visual + eTNS.

During the experimental session, the eTNS will specifically target the maxillary branch of the trigeminal nerve. The eight stimulation conditions will be administered in a randomized order for each participant. Each stimulation block will last for 5 minutes, separated by 1-minute inter-stimulus intervals to prevent carry-over effects.

Continuous electroencephalography (EEG) will be recorded throughout the session using a 32-channel eego™ mylab system (ANT Neuro, Netherlands) to capture real-time cortical responses. Following data acquisition, standard preprocessing pipelines will be applied, including notch filtering, 0.5-45Hz bandpass filtering, average re-referencing, ICA-based artifact rejection, and bad channel interpolation.

Inclusion Criteria:

• Healthy adults aged ≥18 years old.
• Normal or corrected-to-normal vision and hearing (essential for 40Hz visual and auditory stimulation).
• Right-handedness.
• Willing to participate in the study and capable of providing written informed consent.

Exclusion Criteria:

• Personal or family history of epilepsy or seizures (crucial safety criteria for 40Hz flicker and electrical stimulation).
• History of any neurological or psychiatric disorders.
• Presence of metallic implants in the head, cardiac pacemakers, or other implanted electronic medical devices.
• Skin lesions, infections, or hypersensitivity at the eTNS stimulation site (maxillary branch of the trigeminal nerve).
• Current use of psychoactive medications or substances that may significantly alter EEG signals or brain state dynamics.
• Pregnant or lactating individuals.
• Poor sleep quality or irregular sleep-wake cycles prior to the experiment.

Individual participant data (IPD), particularly raw continuous electrophysiological recordings (EEG), contains sensitive and potentially identifiable biometric information. In accordance with institutional data security policies and ethical guidelines regarding participant privacy, raw IPD will not be made publicly available. However, aggregated statistical data and summary results will be published in peer-reviewed journals. De-identified, summary-level data or analytical scripts may be shared by the corresponding author upon reasonable academic request, subject to formal institutional review and approval.