Defective alpha reactivity with mental concentration.

We have studied 120 patients with EEGs that show defective reactivity of alpha activity in which one side fails to attenuate during the performance of mental arithmetic. The side of defective reactivity was the right side in 74 patients and the left in 46. A similar asymmetric reactivity of alpha activity to eye opening (Bancaud phenomenon) was seen in 32 patients. Also, 113 patients had lateralized EEG abnormalities on the side of defective reactivity and on the side in which cerebral lesions or disorders of cerebral function were present. The unilateral failure of alpha attenuation with mental concentration represents a subtle lateralized electrographic manifestation of cerebral dysfunction on the side of defective reactivity.