transcraneal Stimulation

Understanding Non-Invasive Brain Stimulation (NIBS)

Non-invasive brain stimulation (NIBS) techniques, such as transcranial electrical stimulation (tES), provide a means to modulate brain activity using weak electrical currents applied to the scalp. Among these, transcranial alternating current stimulation (tACS) and transcranial direct current stimulation (tDCS) are widely studied for their ability to influence neuronal excitability and network function. These techniques hold potential for both fundamental neuroscience and clinical interventions, but their effects must be carefully modeled to maximize efficacy and specificity.

Modeling Current Propagation: From Electrodes to the Brain

To understand and optimize the effects of transcranial stimulation, it is crucial to model how electrical currents propagate through biological tissues. This involves simulating how electrical fields travel through the skin, skull, cerebrospinal fluid, and brain tissue, taking into account the varying conductivity of each layer. Using MRI-based reconstructions of individual head anatomy, realistic models can predict the spatial distribution of electric fields and their interaction with cortical and subcortical structures. These simulations provide insight into the effectiveness and precision of stimulation protocols and help refine experimental and clinical applications.

Taking Cortical Geometry Into Account

The brain’s folded structure significantly influences how electric fields interact with neuronal populations. Advanced modeling techniques integrate realistic cortical geometry to:

Identify areas where stimulation is most effective.
Assess differences in electric field strength across gyral and sulcal surfaces.
Improve accuracy in predicting neuronal responses to stimulation.

Related citations (Cabrera-Álvarez et al., 2023)

Optimizing Electrode Montages for Targeted Stimulation

The placement of stimulation electrodes is a critical factor in determining both the focality and effectiveness of neuromodulation. Computational modeling enables the optimization of electrode configurations by identifying montages that maximize the desired effect while minimizing unintended stimulation of surrounding regions. Subject-specific models further enhance this approach, allowing stimulation parameters to be tailored to individual anatomical differences. This personalized strategy is particularly relevant for clinical applications where precision is key.

Linking Electric Field Effects to Brain Activity

While current propagation models provide a detailed view of where stimulation reaches, understanding how it interacts with brain activity requires linking these models to neural dynamics. Electric fields influence neuronal excitability, but their effects are state-dependent, meaning they interact with ongoing brain activity in complex ways. This research aims to explore how externally applied stimulation modulates endogenous oscillations, alters functional connectivity, and reshapes large-scale network activity. Integrating electric field modeling with computational brain activity simulations provides a more complete framework for predicting stimulation outcomes.

Towards Functional and Clinical Applications

Beyond understanding the mechanisms of tES, a key goal is to leverage stimulation to target specific network disruptions and cognitive processes. Future research directions include:

Restoring Network Function: Using stimulation to counteract maladaptive network activity in neurological and psychiatric conditions.
Enhancing Cognition: Exploring the use of tACS and tDCS to modulate cognitive functions such as attention, memory, and learning.
Closed-Loop Stimulation: Developing adaptive stimulation protocols that respond dynamically to brain activity in real time.

This research line aims to bridge biophysical modeling, neurophysiology, and cognitive neuroscience to refine and optimize the use of transcranial stimulation as both an investigative and therapeutic tool.

The alpha rhythm is often associated with relaxed wakefulness or idling and is altered by various factors. Abnormalities in the alpha rhythm have been linked to several neurological and psychiatric disorders, including Alzheimer’s disease. Transcranial alternating current stimulation (tACS) has been proposed as a potential tool to restore a disrupted alpha rhythm in the brain by stimulating at the individual alpha frequency (IAF), although some research has produced contradictory results. In this study, we applied an IAF-tACS protocol over parieto-occipital areas to a sample of healthy subjects and measured its effects over the power spectra. Additionally, we used computational models to get a deeper understanding of the results observed in the experiment. Both experimental and numerical results showed an increase in alpha power of 8.02% with respect to the sham condition in a widespread set of regions in the cortex, excluding some expected parietal regions. This result could be partially explained by taking into account the orientation of the electric field with respect to the columnar structures of the cortex, showing that the gyrification in parietal regions could generate effects in opposite directions (hyper-/depolarization) at the same time in specific brain regions. Additionally, we used a network model of spiking neuronal populations to explore the effects that these opposite polarities could have on neural activity, and we found that the best predictor of alpha power was the average of the normal components of the electric field. To sum up, our study sheds light on the mechanisms underlying tACS brain activity modulation, using both empirical and computational approaches. Non-invasive brain stimulation techniques hold promise for treating brain disorders, but further research is needed to fully understand and control their effects on brain dynamics and cognition. Our findings contribute to this growing body of research and provide a foundation for future studies aimed at optimizing the use of non-invasive brain stimulation in clinical settings.

Understanding Non-Invasive Brain Stimulation (NIBS)

Modeling Current Propagation: From Electrodes to the Brain

Taking Cortical Geometry Into Account

Optimizing Electrode Montages for Targeted Stimulation

Linking Electric Field Effects to Brain Activity

Towards Functional and Clinical Applications

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References

2023