This paper presents an overview of a new method for the non-invasive measurement of brain function, the event-related optical signal (EROS). This technique is based on measures of the optical properties of cortical brain tissue, which change while the tissue is active. These changes are likely to be due to changes in light scattering, and are very rapid and localized, being related to phenomena occurring within or around the neuronal membrane. EROS, therefore yields images of cortical activity that combine spatial specificity (i.e. they can be related to patches of tissue less than a cubic centimeter in size) with temporal resolution (i.e. they depict the time course of the neural activity in the cortical areas under measurement). A limitation of this technique is its reduced penetration into the head (less than 3-5 cm). EROS appears to be a suitable technique for studying the time course of activity in selected cortical areas, and for providing a bridge between hemodynamic and electrophysiological imaging methods.