HD EEG Registered with MRI
Precise registration of the High Density EEG (HD EEG) sensor coordinates with structural MRI, either an atlas or an individual's MRI, provides an accurate head model geometry for electrical source estimation. With 128 or 256 channels, the HD EEG recording with the Geodesic Sensor Net provides sufficient sampling of the head surface potentials to allow estimation of the neural activity that gives rise to the EEG with the GeoSource software.
Most EGI researchers continue to compute conventional referenced (e.g., mastoid referenced) EEG and ERP signals at the scalp so that they can compare results directly with earlier studies. However, the exciting new work is to examine the neural sources of the significant experimental effects.
Statistical analysis with HD EEG source measures provides convergent validation of the reliability of the source measures. Analysis in source space yields a new window on the temporal dynamics of human neurophysiology. See Luu, et al. (2010).
The Geodesic Photogrammetry System allows precise determination of 3D positions of the sensors in the Geodesic Sensor Net so that these can be registered with the MRI images.
EGI's GeoSource software integrates well-known distributed source estimation methods (LORETA, LAURA) with a high-resolution finite-difference method model of the head that is registered with the Montreal Neurological Institute average MRI atlas. Even when structural MRI is not available for the individual participant, the HD EEG (128- or 256-channel) laboratory gains the advantage of MR registration with the atlas-based framework of GeoSource. See Kuo, et al. (2014).
With continued advances in MR-registered EEG, including constraining distributed sources to be normal to the cortical surface, there will be increasing advantages for HD EEG studies to link source analysis to head models built from the participant's own structural MRI.