The last refuge of secrets and lies - the brain - may be about to reveal all.

Scientists are finding ways to use the brain's activity to expose truths a person may try to hide. The techniques could revolutionize police work, improve national security, and threaten personal privacy.

"It's the scariest thing around," said physicist Robert Park, an outspoken critic of old-fashioned, unreliable polygraph machines. "The only thing worse than a lie detector that doesn't work is one that does."

Ruben Gur, a neuropsychologist at the University of Pennsylvania, says new kinds of brain scans can reveal when a person recognizes a familiar face, no matter how hard he or she tries to conceal it.

The scanning machine, called a functional MRI, takes pictures that highlight specific parts of the brain activated during certain tasks. Telltale parts of your brain "light up," he said, when you are presented with a face you have seen before.