Non-Invasive Detection of Functional Brain Activity with Near-Infrared Diffusing-Wave Spectroscopy

Abstract

Near-infrared dynamic multiple scattering of light (diffusing-wave spectroscopy, DWS) is introduced as a new method for non-invasive measurement of functional brain activity in humans. DWS was used to detect activation of the somato-motor cortex in 11 right-handed volunteers who performed a finger opposition task separately with their right and with their left hand. Temporal autocorrelation functions g(1)(¿ ) of the scattered light field were measured during 100 s periods of motor task alternating with 100 s resting baseline periods. Analyzing the experimental data with an analytical theory for g(1)(¿ ) from a 3-layer geometry with optical and dynamical heterogeneity representing scalp, skull and cortex allowed to isolate the di®usion coe±cient in cortical regions. During the motor task the cortical di®usion coe±cient was found to be significantly increased, indicating enhanced activation-related dynamics in active cortical tissue. As expected from known brain hemispheric asymmetry for right-handed subjects the contralateral response was more pronounced than the ipsilateral response. These results suggest that near-infrared DWS can be usedto non-invasively investigate subcellular dynamics in the cerebral cortex.