Variable anisotropic brain electrical conductivities in epileptogenic foci

Area of Science:

• Neuroscience
• Biophysics
• Medical Imaging

Background:

• Established source localization models rely on assumed isotropic brain electrical conductivity (approx. 0.33 S/m).
• These assumptions lack ex vivo human validation, particularly concerning anisotropy and patient-specific variations.

Purpose of the Study:

• To determine bidirectional electrical conductivities in human neocortex and white matter ex vivo.
• To compare measured conductivities with clinical variables and MRI findings in pediatric epilepsy patients.
• To assess the impact of tissue anisotropy and patient variability on conductivity values.

Main Methods:

• Utilized the 4-electrode technique to measure electrical conductivities perpendicular and parallel to the pial surface in 15 pediatric epilepsy surgery patients.
• Compared ex vivo measurements with clinical data and magnetic resonance imaging (MRI) findings.

Main Results:

• Mean electrical conductivities were significantly lower (0.10 +/- 0.01 S/m) than assumed values, approximately 30% of the standard.
• Conductivity varied substantially (243%) between patients and exhibited significant anisotropy (45% difference between perpendicular and parallel measures).
• Anisotropic principal axes correlated with MRI findings: perpendicular with normal tissue, parallel or isotropic with epileptogenic lesions; lower conductivity observed in cortical dysplasia.

Conclusions:

• Human brain electrical conductivity is anisotropic, patient-specific, and lower than commonly assumed.
• Anisotropy and patient variability, especially in the presence of lesions, challenge current source localization models.
• Accurate, non-invasive measurement of brain electrical conductivity is crucial for improving EEG source localization in epilepsy surgery patients.