Increases of theta rhythm have been seen in altered neurophysiological states of the brain, involving altered cholinergic functioning, such as Alzheimer’s disease, aging, and the transition from wakefulness to sleep (Niedermayer and Lopes Da Silva, 1999). Slow EEG activity (theta and delta) has been correlated with cholinergic activity and central cholinergic pathways (Steriade, 1990). Elevated tonic theta power in the EEG may reflect a deficiency in information-processing capacity of the CNS (Klimesch et al., 2001). The theta power increase may be an electrophysiological index of the imbalance in the excitation-inhibition homeostasis in the cortex.
