Artifactually high coherences result from using spherical spline computation of scalp current density

Area of Science:

• Neuroscience
• Biomedical Engineering
• Signal Processing

Background:

• Common reference montages in electroencephalography (EEG) analysis confound true coherence with electrode power and phase.
• Reference-free EEG data, such as scalp current densities (SCDs), are needed for direct coherence measurement.
• SCDs estimate the potential gradient perpendicular to the scalp.

Purpose of the Study:

• To investigate the properties of spherical spline interpolation for computing SCDs.
• To identify the source of high inter-electrode coherences observed in spherical spline derived SCD data.
• To determine if the spherical spline procedure introduces artifacts into coherence computations.

Main Methods:

• Computed SCDs using spherical spline interpolation (Perrin et al., 1989) and Hjorth methods.
• Calculated inter-electrode coherences from both common reference EEG data and derived SCD data.
• Utilized simulated data to analyze the effects of the spherical spline procedure on coherence.
• Examined the spline weighting matrix for artifact identification.

Main Results:

• Spherical spline derived SCDs exhibited significantly higher inter-electrode coherences compared to common reference data.
• Simulations demonstrated that the spherical spline procedure introduces a major artifact.
• The artifact originates from the spline weighting matrix, leading to inflated coherences in SCD transformed data.

Conclusions:

• The spherical spline interpolation method for deriving SCDs introduces a significant artifact.
• This artifact inflates inter-electrode coherence, potentially leading to misinterpretations of neural connectivity.
• Researchers should be cautious when interpreting coherence values derived from spherical spline transformed EEG data.