The use of time- and frequency-domain spectroscopy for the detection of brain oxygenation and the localization of tissue regions exhibiting brain-bleeding has been suggested recently by Chance and coworkers. In this study, we explore the effect of the skull (with minimal absorbance and greater scattering properties) upon measurements of photon migration within the underlying brain tissue (of comparatively greater absorbance and smaller scattering properties). Using the diffusion approximation to understand photon migration within the layered medium, we show that the effect of a transmitting layer can be significant, yet may considered negligible at significant source and detector separations in time- and frequency-domain measurements. Using Monte Carlo simulation of photons migrating within a model consisting of (i) a central core ((mu) sb equals 8 cm-1, (mu) ab equals 0) and (ii) an annulus ((mu) ss equals 24 cm-1, (mu) a equals 0), we demonstrate the contribution of an additional 'time-of-flight' to time-domain spectra (TDS) and an additional phase-shift to frequency-domain spectra (FDS) due to the presence of the highly scattering layer. Under conditions of changing absorbance ((mu) ab equals 0 - 0.075 cm-1) in the central core, our results show that the diffusion approximation also provides description of the changes seen in TDS and FDS derived from Monte Carlo simulation. Finally, upon mapping of volumes sampled by the migrating photons, we characterize the capacity for 'light-channelling' within the highly scattering layer as a function of source-detector separation and core absorbance.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.