Abstract: Apparatus and method for biofeedback of human central nervous system activity using radiation detection. This invention uses radiation from the brain resulting either from an ingested or injected radioactive material or radio frequency excitation or light from an external source impinging on the brain. The radiation is measured by suitable means and is made available to the subject on which the measurement is being made for his voluntary control. The measurement may be metabolic products of brain activity or some quality of the blood, such as its oxygen content. One such system utilizes red and infrared light to illuminate the brain through the translucent skull and scalp. Absorption and scattering of incident radiation depends on the degree of oxygen saturation of the blood in the illuminated tissue.