A system for non-invasively measuring blood volume oscillations inside a cranium of a human subject, the system comprising: a first light source adapted to be placed on the skin above the cranium of the human subject configured to emit light which penetrates a superficial space outside the cranium; a detector adapted to be placed on the skin above the cranium of the human subject spaced from the first light source by a first predetermined separation distance (d-24) that causes the detector to detect light which reflects from the superficial space outside the cranium and output superficial output signals; a second light source adapted be placed on the skin above the cranium configured to emit light which penetrates through the superficial space outside the cranium to inside the cranium and the detector is spaced from the second light source by a second predetermined separation distance (d-34) that causes the detector to detect light which reflects from inside the cranium and output cranial output signals; a processing subsystem coupled to the first light source, the second light source, and the detector, the processing subsystem configured to alternately enable the first light source and the second light source and alternately enable the detector to detect the light which reflects from the superficial space outside the cranium and the light which reflects from inside the cranium to generate the superficial output signals and cranial output signals, the processing subsystem responsive to the superficial output signals and the cranial output signals and further configured to reduce contributions from the superficial space existing in the cranial output signals and generate corrected cranial output signals indicative of blood volume oscillations inside the cranium; and a reference sensor adapted to be placed on the human subject and coupled to the processing subsystem, the reference sensor configured to provide reference signals for calculating a differential pulse transit time, a phase shift, or transfer function from one location on the human subject to the corrected cranial output signals. a first light source adapted to be placed on the skin above the cranium of the human subject configured to emit light which penetrates a superficial space outside the cranium; a detector adapted to be placed on the skin above the cranium of the human subject spaced from the first light source by a first predetermined separation distance (d-24) that causes the detector to detect light which reflects from the superficial space outside the cranium and output superficial output signals; a second light source adapted be placed on the skin above the cranium configured to emit light which penetrates through the superficial space outside the cranium to inside the cranium and the detector is spaced from the second light source by a second predetermined separation distance (d-34) that causes the detector to detect light which reflects from inside the cranium and output cranial output signals; a processing subsystem coupled to the first light source, the second light source, and the detector, the processing subsystem configured to alternately enable the first light source and the second light source and alternately enable the detector to detect the light which reflects from the superficial space outside the cranium and the light which reflects from inside the cranium to generate the superficial output signals and cranial output signals, the processing subsystem responsive to the superficial output signals and the cranial output signals and further configured to reduce contributions from the superficial space existing in the cranial output signals and generate corrected cranial output signals indicative of blood volume oscillations inside the cranium; and