Inconsistencies in mapping current distribution in transcranial direct current stimulation

Area of Science:

• Neuroscience
• Medical Physics
• Biomedical Engineering

Background:

• Transcranial direct current stimulation (tDCS) is a non-invasive neuromodulation technique used for treating neuropsychiatric diseases and enhancing cognition.
• Recent meta-analyses highlight significant inconsistencies in tDCS study outcomes.
• A deeper understanding of current flow in the brain is crucial for resolving these discrepancies.

Purpose of the Study:

• To empirically investigate and map the distribution of electrical current induced by tDCS in the human brain.
• To identify potential reasons for the variability and inconsistencies observed in tDCS research.

Main Methods:

• Utilized a phantom model and in vivo human subjects for tDCS current distribution analysis.
• Employed magnetic resonance (MR) phase imaging during stimulation to reconstruct the magnetic field.
• Applied Ampere's law to derive current distribution maps from the magnetic field data.

Main Results:

• In phantoms, over 75% of injected current followed a clear path between electrodes.
• In humans, only about 25% of the current reached the cortex, with significant bypass external to the brain.
• Observed substantial inter-subject and intra-subject variability in human current distribution, unlike in phantom scans.

Conclusions:

• The developed in vivo current mapping technique shows significant current deviation in the human brain during tDCS.
• A substantial portion of tDCS current bypasses the target brain regions, potentially due to tissue conductivity variations.
• These findings offer critical insights into tDCS efficacy and may explain observed inconsistencies in clinical and cognitive studies.