It is important for near-infrared imaging to estimate the sensitivity of detected signal to the change in absorption of tissue resulting from brain activation and the volume of tissue interrogated for a specific source-detector spacing. In this study, light propagation in adult head models is predicted by Monte Carlo simulation in order to investigate the effect of the thickness of the superficial tissues on the partial optical path length in the brain and on the spatial sensitivity profile. The effect of thickness of skull on the partial optical path length and spatial sensitivity profile is almost the same as that of the scalp. The effect of thickness of cerebrospinal fluid layer on the partial optical path length and spatial sensitivity profile is different from that of the scalp and skull. The partial optical path length mainly depends upon the depth of inner skull surface whilst the spatial sensitivity profile is considerably affected by the thickness of the cerebrospinal fluid layer.