LIGHT-PROPAGATION IN THE BRAIN DEPENDS ON NERVE-FIBER ORIENTATION

IN THIS STUDY, the penetration of red laser light (632.8 nm) in fresh bovine brain was measured parallel, oblique, and perpendicular to the axis of white matter tracts. The measurements were performed in eight samples with an isotropic light source and detector and were obtained by advancing the detector tip toward the light source in the tissue. A statistically significant difference in the effective attenuation coe fficient of the light (mu(eff)) was found between the parallel and per pendicular directions, 0.47 +/- 0.06 mm(-1) and 0.63 +/- 0.13 mm(-1), respectively(P = 0.005). The measurements taken at an angle of 45 degr ees in the same sample resulted in an intermediate mu(eff) of 0.58 +/- 0.09 mm(-1). These results suggest a preferential guidance of light a long the axis of the white matter tracts of the brain.