According to New ‘Brain Fingerprinting’ Could Help Solve Crimes, scientists can measure involuntary brain activity that registers recognition — in some cases, of things only a criminal would know about a crime:
Brain fingerprinting works by measuring and analyzing split-second spikes in electrical activity in the brain when it responds to something it recognizes.For example, if a suspected murderer was shown a detail of the crime scene that only he would know, his brain would involuntarily register that knowledge. Under Farwell’s system, that brain activity is picked up through electrodes attached to the suspect’s scalp and measured by an electroencephalograph (EEG) as a waveform.
A person who had never seen that crime scene would show no reaction.