I was involved in a study in the late 1990s that looked at brain activity and stuttering. I volunteered as a control (non-stutterer) subject and got to keep electronic copies of some of the resulting Positron Emission Tomography (PET) scans. A small amount of slightly radioactive water (with a very short half life!) was injected into a vein in my left arm. The whole thing was completely painless, except for the IV because I hate needles! The most active brain cells absorbed the most water and show up against the green background as yellow and red in these images. This scan is a horizontal cross section of the lower mid-part of my brain. The dark "holes" are actually normal ventricles, filled with Cerebrospinal fluid (CSF). Neat.