The measurement of the optical signal evoked or optical signal related to the event (, EROS) is a technique used in the field of the Neuroscience. It uses the fact that the activity of the Neuron S modifies their properties Optique S. By measuring the quantity of light diffused, one can thus deduce the activity from the neurons which it crosses.

It is about a relatively recent method (developed at the end of the years 1990) but it puts back on a phenomenon highlighted since 1949 by D.K. Hill, an English neurologist: the nervous fabrics when they are in activity diffuse less the light which crosses them than when they are at rest. The explanation of this phenomenon remains badly included/understood but it could be related to the ic transfers Ion through the cellular Membrane of the Neuron S (which would modify the chemical composition of the cellular environment and thus its Index of refraction) like to the simultaneous exchanges of molecules of Eau (which, by modifying the volume of the cellular body, distend or retract the membrane and thus modifies its transparency).

Just as for the spectroscopic Imagery near infra-red, one uses the close range Infrarouge spectrum (between 700 and 1000nm) which crosses organic fabrics relatively well. That makes it possible to measure starting from the surface of the Scalp, the activity of nervous fabrics until a depth from approximately 3 to 5 cm.

The interest of this methodology rests on its temporal resolution: one records modifications of the optical properties which occur at the same speed as the ionic exchanges, i.e. at the same speed as the neuronal activity itself, contrary to the spectroscopic Imagerie near infra-red which is based on the hemodynamic Réponse which spends several seconds to be appeared after the change of activity of the neurons.