Berger in 1929 discovered what is now known as the a rhythm of the brain. This consists of oscillations of electrical potential at a frequency of 8½ to 12 per second and is best developed in the parieto-occipital region. This rhythm is best demonstrated with the eyes closed, and may be blocked by the subject's attention to vision with the eyes opened or closed.
By flickering lights the a rhythm may be affected in one of three ways. First, a blocking may occur, in which case the a rhythm disappears. Second, photic driving may occur in which case the a waves of 8½ to 12 per second are replaced by waves of the same frequency as that of the flashing light. Third, in some normal and some abnormal subjects nothing happens.
Fig 1 demonstrates α blocking and photic driving in a normal patient. With the onset of photic stimulation at
GRADER J, HELLER GL. Photic Stimulation and the EEG in Macular Disease. Arch Ophthalmol. 1964;72(6):763-768. doi:10.1001/archopht.1964.00970020765004