In study of the brain, oxygenation changes in the cerebral cortex are increasingly monitored using optical methods based on near-infrared spectroscopy (NIRS). When monitoring blood oxygenation in the cerebral cortex, at depth of approximately 15 mm - 20 mm from the skin surface, separation distance between source and detector becomes significant. Many studies show that by increasing the source-detector distance, illuminating light penetrates deeper into tissue. In this work, we use optical phantoms to determine experimentally the minimum source-detector distance between that allows sensing of the cerebral cortex, particularly the grey matter of the brain. A multilayered forehead phantom was fabricated and a silicon tube was added inside the phantom at depths of 15 mm and 19 mm, measured from the surface of the skin mimicking layer. This depth corresponds to the grey matter layer of the brain. The phantom's optical properties were specifically designed to mimic the optical properties of tissue layers of the forehead and to facilitate near-infrared sensing. Optical sensing of liquid movement within the tube was measured by varying the distance between the near-infrared light source and the detector. Based on our measurements, we can conclude that it is possible to sense pulsations from a grey matter mimicking layer of the brain using near-infrared spectroscopy at a source-detector distance of 3 - 4 cm.