Here's how it works: Farwell fits a suspect with a sensor-filled headband. By flashing a series of pictures on a screen, he can read the subject's involuntary reactions to them. When there's something familiar about an image, it triggers an electrical response that begins between 300 and 800 milliseconds after the stimulus. Scientists have studied these "p300 bumps" for years. Farwell believes that, combined with other measures--he has patented which ones he looks at--he can determine if a subject is familiar with anything from a phone number to an al-Qaeda code word.
Indeed, the CIA has funded his research with more than $1 million, and a former FBI point man for biological and chemical weapons has joined Farwell's firm. Critics say that p300-type testing needs a lot of refinement before it's a perfect polygraph, but such criticism doesn't deter Farwell. "The fundamental task in law enforcement and espionage and counterespionage is to determine the truth," he says. "My philosophy is that there is a tremendous cost in failing to apply the technology."