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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.

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