A Quantitative Aproach to EEG Photic Flash Response

Abstract A frequency-domain technique was devised for analyzing the clinical electroencephalogram (EEG) recorded during photic stimulation. Each of four channels of EEG was digitized, and an estimate of its power spectrum was computed during presentation of each stimulus frequency. An interpolation method was used to estimate the response to stimulus, the “excess power” in the spectrum at the stimulus frequency or a harmonic. This method for determining the response to photic stimulation is innocuous to the clinical environment and augments routine EEG studies. It offers advantages over current practice in being quantitative and more highly sensitive. Patients with cerebrovascular disease have been studied using stimulus frequencies from 2 to 18 Hz. Frequency-domain analysis of these data revealed qualitative and quantitative changes in response to stimulus which correlated with the extent and nature of cerebrovascular accident (CVA). In numerous cases, the clinical neurological diagnosis of a unihemispheric CVA corresponded to a quantitative asymmetry in the photic flash response.