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Abstract: EEG tracings of 106 mentally defective patients at various age levels were analyzed. Records were obtained in bipolar fashion on an "Ediswan" 16-channal EEG. "Evoked" potentials were initiated by "threshold" single and repititive flashes of light:1. Different degrees of alpha rhythm were observed for 59 out of 106 patients (55.7%), but the alpha frequency is lower than that of normals at the same age level.2. It has been suggested that theta rhythm is a common finding among mental defectives. Our results show that it seldom to display complete theta rhythm from occipital region above age 9, from temporal region above age 11 and from parietal region above age 13. The most commonly observed EEG abnormality consists of a(θ) or θ(α) patterns.3. Low percentage and short duration of "blocking" responses, occasional display and narrow range of "driving" frequency as well as absence of after-dischange are traits of the "threshold" evoked potentials in mentally defective patients.4. The incidence of EEG abnormality is highest in cases of idiots and imbeciles.

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