Purpose: The EEG rhythms demonstrate changes in frequency and power with spontaneous changes in behavioral state that do not have well-understood metabolic correlates within the brain. To investigate this question and compare the temporal lobe theta and delta rhythms, resting-state functional MRI was obtained with simultaneous EEG.
Methods: Simultaneous EEG-functional MRI was recorded from 14 healthy sleep-deprived subjects in awake and drowsy states. Scalp electrodes corresponding to bilateral temporal lobes were used to calculate delta and theta band power. The resulting time series was used as input in a general linear model, and the final power curves were convolved with the standard hemodynamic response function. Resulting images were thresholded at Z > 2.0.
Results: Positive and negative correlations for unilateral theta and delta rhythms were present bilaterally in different structures and with differing correlation signs. Theta rhythm positive correlation was present in hindbrain, peri-opercular, and frontoparietal regions and subcortical gray structures, whereas negative correlation was present in parietooccipital cortex. Delta rhythm positive correlation was present in parietooccipital cortex, and negative correlation roughly resembled positive correlations for the theta rhythm.
Conclusions: Temporal lobe theta and delta rhythms are correlated with functional MRI signal in an almost mutually exclusive distribution. The different distributions indicate different corresponding networks. These normal findings supplement the understanding of theta and delta rhythm significance.