“Crime investigators always have their ears open for information only a perpetrator could know-where a gun used in a murder was stashed, perhaps, or what wounds a stabbing inflicted. So imagine a detective asking a suspect about a killing, describing the crime scene to get the suspect to visualize the attack. The detective is careful not to mention the murder weapon. Once the suspect has conjured up the scene, the detective asks him to envision the weapon. Pay dirt: his pattern of brain activity screams “hammer” as loud and clear as if he had blurted it out.
To detect patterns of brain activity, a subject must agree to lie still in a neuroimaging device such as a functional magnetic resonance imaging (fMRI) tube, but in an age when many jurisdictions compel not only convicts but also suspects to provide a DNA sample, that isn’t difficult to imagine. Now, neither is the prospect of reading thoughts by decoding brain-activity patterns. Just a year ago, neuroscientists couldn’t do much better than distinguish thoughts of faces from thoughts of places (the brain has distinct regions that process images of each). “All we could do was tell which brain region was active,” says neuroscientistof the Max Planck Institute for Human Cognitive and Brain Sciences in Leipzig, Germany. “There were real limits on our ability to read the content of that activity.” No longer. “The new realization is that every thought is associated with a pattern of brain activity,” says Haynes, “and you can train a computer to recognize the pattern associated with a particular thought.”