Forget the elevated pulses, sweaty palms, and respiration changes scrutinized by conventional lie detector tests. There's a more direct and, in theory at least, accurate way to measure deceit: track the flow of blood where the lie is born. Lying requires an extra bit of thought, which pulls more blood into a swath of the brain just beneath the forehead. These flows can be tracked optically. And biophysicist Britton Chance-who 60 years ago was part of the wartime research team that developed radar at MIT's Radiation Laboratory-is pioneering technology that can literally see a lie as you speak it. He believes the method is better than conventional tests because the flows can't be suppressed and are less likely to have been caused by the stress of test-taking.

Chance's device uses infrared light, which penetrates tissue. Some of the light is reflected, but since blood selectively absorbs it, increases in blood levels reduce reflection. Precise spacing of light emitters and detectors on a headband allows researchers to gauge the depth at which most of the light is reflected; the target is the prefrontal cortex. "That's where your decision-making goes on, and where most of your societal inhibitions reside-if you have any," Chance says. "That was what we wanted to study. Knowledge and inhibition. Fear and deceit." Chance is developing the technology not only for studying the cortex in action-for lie detection and cognition studies-but for other applications like breast cancer screening. In his University of Pennsylvania lab, Chance, 89, showed Technology Review senior editor David Talbot how to use light to find truth.