Fig. 2. Effect of EEG spatial transformations on EEG functional connectivity.

Spatial transformations of raw data (a single subject) were obtained using different electrode combinations for re-referencing: Cz, linked mastoids (lm) and average reference of the 64 electrodes (ave). Furthermore, the current source density (csd) was computed as an estimate of the surface Laplacian based on the EEG voltage values across electrodes. Spatial transformations are displayed at the top of the figure. All the results presented in each column were obtained using the same transformation. A) Power spectrum of the 40-ASSR in the electrode Fz are presented in upper panels. The spectral peak corresponding to the 40-Hz frequency bin is denoted by an asterisk (*). The scalp distributions of the ASSR are presented in the lower panels. B) Coherence- related measures were computed in a narrow-band around 40 Hz (38–42 Hz) between all channel pairs. Methods of coherence-related measures were magnitude-squared coherence (msc), phase of cross- spectrum (pcs), and the imaginary part of coherency (ipc). The different coherence-related measures are presented in the box on the left, while contrast between measures derived from different spatial transformations (ave vs. Cz) are presented in the right-hand box. C) Upper panels show neural generators of the 40-Hz ASSR, estimated with BMA. Lower panels display representations of the 40-Hz functional connectivity, estimated in a 20-nodes network including temporal and frontal cortical regions of both hemispheres. Functional connectivity measures were total coherence (totalCoh) and total phase synchronization (totalPhase), as computed by Loreta Key software (Pascual-Marqui et al., 1994) and described in Pascual-Marqui (2007). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)