Reference-free quantification of EEG spectra: Combining current source density (CSD) and frequency principal components analysis (fPCA)

Abstract

Objective: Definition of appropriate frequency bands and choice of recording reference limit the interpretability of quantitative EEG, which may be further compromised by distorted topographies or inverted hemispheric asymmetries when employing conventional (non-linear) power spectra. In contrast, fPCA factors conform to the spectral structure of empirical data, and a surface Laplacian (2-dimensional CSD) simplifies topographies by minimizing volume-conducted activity. Conciseness and interpretability of EEG and CSD fPCA solutions were compared for three common scaling methods. Methods: Resting EEG and CSD (30 channels, nose reference, eyes open/closed) from 51 healthy and 93 clinically-depressed adults were simplified as power, log power, and amplitude spectra, and summarized using unrestricted, Varimax-rotated, covariance-based fPCA. Results: Multiple alpha factors were separable from artifact and reproducible across subgroups. Power spectra produced numerous, sharplydefined factors emphasizing low frequencies. Log power spectra produced fewer, broader factors emphasizing high frequencies. Solutions for amplitude spectra showed optimal intermediate tuning, particularly when derived from CSD rather than EEG spectra. These solutions were topographically distinct, detecting multiple posterior alpha generators but excluding the dorsal surface of the frontal lobes. Instead a low alpha/theta factor showed a secondary topography along the frontal midline. Conclusions: CSD amplitude spectrum fPCA solutions provide simpler, reference-independent measures that more directly reflect neuronal activity. Significance: A new quantitative EEG approach affording spectral components is developed that closely parallels the concept of an ERP component in the temporal domain.

Electroencephalographic Alpha Measures Predict Therapeutic Response to a Selective Serotonin Reuptake Inhibitor Antidepressant: Pre- and Post-Treatment Findings

“…The increase in occipital power in fluoxetine responders was also found in the theta band, which suggests that this qEEG difference is not specific to alpha, but includes somewhat lower frequencies as well. It is noteworthy that a distinct spectral component with a peak that spans both the alpha and theta bands is identifiable in the resting EEG using a reference-free approach (17).…”

“…Lastly, although differences between responders and nonresponders in alpha power and asymmetry appear to represent stable, state-independent traits, their biological basis is unknown. An ongoing study is using a high-density electrode array and current source density measures (17) to provide spatial resolution to more adequately address the neuroanatomical origin of these differences. Studies are also needed to determine their relation to genetic and neurochemical mechanisms that may underlie responsiveness to antidepressants.…”

“…Knott et al (15) found that depressed patients who responded to imipramine showed a trend for more alpha, but had significantly less theta compared to nonresponders. Cook et al (16) did not find pretreatment differences between fluoxetine responders and nonresponders in theta, but did find group differences in "cordance", a measure based on a form of surface Laplacian (see Tenke and Kayser [17] for a discussion of methodological limitations). We found a difference in alpha asymmetry between fluoxetine responders and nonresponders (8), which was predicted on the basis of dichotic listening findings (18).…”

Electrophysiological predictors of clinical response to antidepressants

“…Cook et al (1999) did not find pretreatment differences in relative theta power between responders and nonresponders to 8 weeks of treatment with fluoxetine, but did find differences in "cordance", a measure that integrates relative and absolute theta power. Although the biophysical basis of cordance is not clear (Tenke & Kayser, 2005), it has been reported to be positively correlated with PET scan measures of cortical perfusion . Based on pretreatment measures of EEG cordance, 24 patients were retrospectively divided into concordant or discordant groups depending on the type of association of absolute and relative power in theta band.…”

“…Second, to improve the spatial resolution of spectral topographies, EEG was recorded from a dense array of recording electrodes (67 channels) and current source density (CSD) measures were used to reduce volume conduction from distal sites, and avoid problems associated with selection of a reference electrode. CSD measures are reference-free and indicate the strength of underlying radial current generators (Nunez & Srinivasan, 2006;Kayser & Tenke, 2006;Tenke & Kayser, 2005). Third, CSD measures were quantified using frequency principal components analysis (fPCA) to derive empirically-based frequency bands.…”