This method makes use of the fact that oxygenated and deoxygenated blood have different absorption spectra for light. By illuminating cortical tissue with monochromatic light of a wavelength where the difference in absorption is maximal oxygen consuming areas, that is active areas, can be distinguished from non- oxygen consuming, inactive, areas. The method therefore uses a metabolic marker to image neuronal activity.

Figure:
Tonotopic map, i.e. an orderly neuronal representation of sound frequency, in gerbil primary auditory cortex as revealed by ORIS. Scale bar: 0.5mm.