Distortions in EEG interregional phase synchrony by spherical spline interpolation: causes and remedies

Abstract

Measures of the coherence of electroencephalography (EEG) time-series recorded at spatially distant points on the scalp are often used by researchers to characterize the dynamic interactions of brain regions. In dense-array EEG recordings, one or more electrode signals often contain prominent artifact necessitating replacement of the recorded data with an estimated signal using interpolation from data in valid recordings from the surrounding electrodes. Typically the signal estimation is carried out using spherical spline interpolation (SSI; Perrin et al., 1989); however, it is shown that this can introduce an erroneous increase in coherence between signals because the estimated signal is derived from elements in common recordings from other electrode sites. Although SSI performance depends on three SSI parameters, including interpolation order m, the Legendre polynomial order n, and regularization parameter λ, clear guidelines on how to optimally choose parameters have yet to be established for ensuring the temporal features of interpolated signals are accurate.