. . . injected custom-made, 20-nanometer iron oxide particles into a region of the rodents' brains called the ventral tegmental area (VTA), a well-studied deep brain structure essential to the experience of reward, which plays a central role in disorders such as addiction and depression in people
Think of Larry Niven's "wire head' addicts and the Puppeteer's Tasp in Ring World. There are potentials for both private and governmental abuse.