Generative mechanisms and scaling laws of EEG suggest an alternative physiological interpretation of ICA

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Abstract
In this study, we subject the conventional physiological interpretation of independent component analysis (ICA) applied to EEG, the small-patch model, to systematic falsification, and propose an alternative large-patch model. The small-patch model assumes that ICs correspond to localized cortical patches with < 1 cm2. However, this assumption has remained unvalidated. The small-patch model predicts that approximately 70% of sources are localized within sulci up to 15 mm deep, with rapidly changing dipole orientations across the cortex. In contrast, the large-patch model (>6-10 26 cm2) predicts relatively stable radial orientations accompanied by physiologically implausible source depths due to depth bias. First, we conducted a stimulation study using a forward-inverse modeling framework with a four-layer head conductor model. We confirmed that depth bias emerges when a single equivalent dipole is fitted to a potential field generated by a broad array of parallel dipoles. This observation led to the key hypothesis that the presence of depth bias in empirical data would favor the large-patch model. Second, we analyzed resting-state EEG from two European open datasets comprising 820 recordings (62-64 channels), yielding dipole depth and orientation distributions for nearly 15,000 qualified brain ICs. Results showed that more than 80% of ICs were localized at physiologically implausible depths (19-26 mm), favoring the large patch model. A novel dipole-orientation analysis revealed broad, low-spatial-frequency structure in dipole orientations, further supporting the large-patch model. We conclude that the revised physiological interpretation of ICA aligns with electrophysiological literature and computational insights into EEG-specific spatial scaling laws.

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