Abstract: A method of evaluating the effects of administering an external stimuli or treatment such as a psychoactive compound, a drug, or an environmental influence like temperature, noise, vibration, light and similar sensory-perceived influences, on a subject's brain using imaging techniques with positron emission tomography (PET). The method measures cerebral metabolism before and after administering the external stimuli or treatment, and employs a behavioral clamp to control behavioral influences on the subject's brain after administration of the external stimuli or treatment.