The temporal/spatial evolution of optical signals in human cortex.

Intraoperative measures of functioning brain are an important aspect to understanding normal and diseased cortical response. Previous studies, in animal models, have used optical reflectance maps to illustrate the location and timing of functional activity. We used optical reflectance mapping in patients undergoing parietal tumor resection to reveal the temporal/spatial evolution of perfusion and other related metabolic responses of sensorimotor cortex to peripheral somesthetic stimulation. The somatosensory cortex of seven anesthetized patients was mapped in response to transcutaneous electrical median and ulnar nerve stimulation using optical reflectance imaging. The time course and spatial extent of this response were measured and correlated with evoked potential maps collected during the same conditions. Observable signals first appeared within 1-2 sec, peaked at 3 sec, and disappeared by 9 sec. These signals colocalized with the largest evoked potentials in both the sensory and motor regions and demonstrated topological specificity with median and ulnar nerve stimulation. Maps of this temporal/spatial resolution illustrate the integrative and dynamic nature of the neuronal, vascular, and metabolic responses of human cortex. These data also provide insight to the mechanisms responsible for signals obtained using other brain imaging techniques such as PET and fMRI.