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A Photon for Your Thoughts

Photonics Spectra
Jun 1999
Daniel C. McCarthy

Photonics is still a long way away from reading your thoughts, but a new optical scanning technique can at least detect where and when in your brain those thoughts are activating neurons. Unlike spatially resolved techniques for viewing brain functions, the scanner developed at the University of Illinois uses a time-resolved approach to provide images in real time.

Physicists led by Enrico Gratton at Illinois developed the underlying technology to penetrate skeletal muscle, breast and brain tissue. That last application proved useful to a husband-and-wife team of psychologists -- Gabriele Gratton and Monica Fabiani -- at the University of Missouri in Columbia who are applying it to human volunteers. Gabriele and Enrico are brothers.

Called Eros -- for event-related optical signal -- the scanner uses near-infrared laser light to produce real-time imagery of where in the brain neurons respond to visual and auditory stimuli. Test subjects wear motorcycle helmets drilled with holes, through which 16 fiber optic cables are threaded. Each cable delivers a 1-mW beam of 750-nm laser light, which penetrates the scalp and skull and diffuses quickly into the outer 5 cm or so of brain tissue. The helmet also integrates two detectors to collect the optical signal.

"The basic idea is to modulate the intensity of the light source," said Sergio Fantini, a physicist at Illinois. The laser's intensity is modulated at a frequency in the order of 100 MHz. Researchers measure the amplitude and the phase of the modulated light at the scanner's two detectors. Loosely speaking, Fantini said, amplitude relates to the attenuation of emitted light, while phase indicates the time delay of photons traveling through tissue.

From the attenuation and delay of optical signal, the Missouri psychologists can derive where and when neurons have activated. This is because photons travel more easily through active neurons, explained Gabriele Gratton. "Consequently, photons take longer to reach the surface of an active brain, not because they are slowed down, but rather because the path of their travel is altered. In a sense, they're measuring the transmission of light." The analyzed data appear on a monitor as red and yellow images mapped over various parts of the subject's brain.

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