Scientists do not have a window into your thoughts yet, but they can now shine beams of light into your head and watch information flow around your brain. Gabriele Gratton and Monica Fabiani, neuroscientists at the University of Illinois, are pioneering the new technique based on the way brain tissue transmits light.

A test subject wears a helmet that allows Gratton and Fabiani to apply intense near-infrared illumination to the skull using fiber-optic cables. Sensitive detectors pick up faint reflections as the light bounces off countless microscopic surfaces on the brain’s outer layer, the cortex. (This “scattering” is the reason a laser pointer, pressed into a fingertip, makes the finger glow red.) Heightened activity among neurons increases the scattering, producing a so-called event-related optical signal. One explanation is that when neurons fire they absorb water, which deforms cell membranes, forcing the light rays to travel along longer paths with measurable delay.

Gratton and Fabiani can produce a three-dimensional map of neuronal events in the cortex, including the regions that process input from the eyes, ears and other sensory organs. Light penetrates the cortex to a depth of three centimeters, and activity can be pinpointed to regions 0.5 centimeter across. “There may be areas we can’t observe,” Gratton says. “Nevertheless, we see most.” The technique is blazingly fast —it can resolve events separated by milliseconds, whereas each slice of a functional MRI scan spans about one second and PET scans average data over 40-second intervals or longer. This fine temporal resolution means that Gratton and Fabiani can watch the order in which rapid neural events originate and then migrate to other regions of the cortex. For instance, they hope to understand how skilled operators such as air traffic controllers and military commanders interpret complex visual displays and prepare to take action. With their novel technique, thought processes may one day be made truly transparent.