The fMRI BOLD signal tracks electrophysiological spectral perturbations, not event-related potentials.
Functional magnetic resonance imaging (fMRI) and electroencephalography (EEG) are primary tools of the psychological neurosciences. It is therefore important to understand the relationship between hemodynamic and electrophysiological responses. An early study by Huettel and colleagues found that the coupling of fMRI blood-oxygen-level-dependent signal (BOLD) and subdurally-recorded signal-averaged event-related potentials (ERPs) was not consistent across brain regions. Instead, a growing body of evidence now indicates that hemodynamic changes measured by fMRI reflect non-phase-locked changes in high frequency power rather than the phase-locked ERP. Here, we revisit the data from Huettel and colleagues and measure event-related spectral perturbations (ERSPs) to examine the time course of frequency changes. We found that, unlike the ERP, γ-ERSP power was consistently coupled with the hemodynamic response across three visual cortical regions. Stimulus duration modulated the BOLD signal and the γ-ERSP in the peri-calcarine and fusiform cortices, whereas there was no such modulation of either physiological signal in the lateral temporal-occipital cortex. This finding reconciles the original report with the more recent literature and demonstrates that the ERP and ERSP reflect dissociable aspects of neural activity.
Engell, AD; Huettel, S; McCarthy, G
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