Determination of local optical properties of the rat barrel cortex during neural activation: Monte-Carlo approach to light propagation

Spatially-spectrally-resolved reflectance measurements allow in vivo measuring the optical coefficients of absorption and scattering within the cortical tissue. This method, if applied to neural tissue during enhanced activity, could allow a straightforward monitoring of the blood oxygen saturation changes occurring in the brain cortex during hemodynamic responses. Furthermore, it may provide valuable information on possible absorption and scattering changes occurring during stimulation. The feasibility of such measurements was investigated by carrying a preliminary numerical study using a Monte-Carlo light propagation routine. Experimental parameters such as the geometry of the optical probe, baseline cortex optical coefficients retrieved from the literature and anatomical characteristics of the rat barrel cortex were used as an input for the simulations. The sensitivity of the probe to the local variations of optical coefficients was investigated with this numerical approach. Additionally, the influence of the barrel cortex dimensions and the probe positioning relatively to the activated region were studied for instrumental optimization purpose. It was found that typical variations of optical coefficients can be detected if the activated region of barrel cortex has a volume of typically 1 mm³ or larger. The decay of the probe sensitivity to changes was studied as a function of the depth of the activated region. The results showed that the best sensitivity is achieved by placing the light injection fiber of the optical probe aligned onto the center of the cylindrical barrel.