Abstract: In accordance with the present invention a method and apparatus are disclosed for real time imaging of functional activity in cortical areas of a mammalian brain using intrinsic signals. The present invention enables dynamic, on-line imaging of, for example, a human brain during neurosurgery to permit more accurate identification of dysfunctional cortical areas. Cortical areas are identified on the basis of stimulus evoked changes which occur in the optical properties of the brain so that real time localization of seizure (e.g., epileptic seizure) foci and functional mapping can be observed during neurosurgery. Areas of pathological tissue such as epileptic foci and tumors can thus be identified and removed during neurosurgery without damaging important functional areas in the cortex near the tissue.