We hypothesized that it would be possible to detect the distribution of cortical activation by using a sensitive, rapid, high-resolution infrared imaging technique to monitor changes in local cerebral blood flow induced by changes in focal cortical metabolism. In a prospective study, we recorded in 21 patients the emission of infrared radiation from the exposed human cerebral cortex at baseline, during language and motor tasks, and during stimulation of the contralateral median nerve using an infrared camera (sensitivity 0.02°C). The language and sensorimotor cortex was identified by standard mapping methods (cortical stimulation, median nerve somatosensory-evoked potential, functional magnetic resonance imaging), which were compared with infrared functional localization. The temperature gradients measured during surgery are dominated by changes in local cerebral blood flow associated with evoked functional activation. The distribution of the evoked temperature changes overlaps with, but extends beyond, functional regions identified by standard mapping techniques. The distribution observed via infrared mapping is consistent with distributed and complex functional representation of the cerebral cortex, rather than the traditional concept of discrete functional loci demonstrated by brief cortical stimulation during surgery and by noninvasive functional imaging techniques. By providing information on the spatial and temporal patterns of sensory-motor and language representation, infrared imaging may prove to be a useful approach to study brain function. Ann Neurol 2003