A method of measuring human brain activity includes the steps of simultaneously measuring pulses at two locations on a human subject that each receives blood from a different carotid artery that feeds a respective one of left and right hemispheres of the brain of the human subject, determining pulse characteristics from the measured pulses, and evaluating relative left and right hemisphere activity of the brain of the human subject based on the determined pulse characteristics. The method may use dual photoplethysmograhic blood pulse sensors that measure left and right hemisphere activity by determining pulse amplitude difference and time or phase differences between the earlobes while the subject carries out various mental functions. The data from the sensors are processed to provide a measure of brain function and the mental activity of the subject.