Interictal EEG abnormality Increased theta frequency activity in EEG 2017-09-24T01:03:08Z Increased frequency of theta wave activity in the electroencephalogram. Theta waves have a frequency of 3.5-7.5 Hertz, and are present in very small amounts in healthy waking adult EEGs. Theta activity is normal in small very amounts in the healthy waking adult EEG in a symmetrical distribution.