Frontal midline theta rhythm: differentiating the physiological theta rhythm from the abnormal discharge

The theta rhythm that appears in the frontal midline was studied morphologically and clinically in EEGs performed on 788 patients and 161 normal subjects in their late teens or older. This theta rhythm was classified into 2 types. The Type 1 theta rhythm appeared in short bursts maximally in the Fz lead and spread mainly to the anterior region. It had a high frequency, low voltage and regular waveform. The Type 2 theta rhythm appeared in long runs maximally in the Fz lead and spread mainly to the posterior areas. It had a relatively low frequency, high voltage and irregular waveform. Type 1 was observed in a younger age group (mean age: 20.3 years) that was comprised of healthy subjects and one mentally retarded patient. In no case was the Type 1 theta rhythm accompanied by a paroxysmal EEG abnormality. We therefore believe that Type 1 is a physiological EEG activity (Fm theta). On the other hand, Type 2 was observed in a relatively older age group (mean age: 30.7 years) comprised of patients diagnosed as having epilepsy or intractable headaches. Five of 9 cases showed epileptic EEG abnormalities. We therefore believe that Type 2 might have clinical significance in diagnosis of epilepsy and other cerebral dysfunctions. The EEG pattern that we call the frontal mid-line theta rhythm may be either a physiological theta rhythm or an abnormal discharge. It is possible to determine whether the theta rhythm is the former or the latter, and we believe this differentiation to be clinically important.