Brain mapping scans. Screen grab of magnetic resonance imaging (MRI) scans of a healthy human brain used for image-guided transcranial magnetic stimulation (TMS) brain mapping. During TMS a coil held next to the subject's brain has an electric current passed through it, producing a magnetic field that induces an electric current within a discrete area of the subject's brain. The subject experiences different physiological reactions depending upon where the electrical current is aimed. For example, when placed over areas of the primary motor cortex it elicits movements in specific parts of the body. The area targeted is mapped to these scans (green lines) via trackers on the subject's head and the coil.