In 20 healthy volunteers light evoked changes in blood flow of the posterior cerebral artery were studied with color coded Doppler sonography. Doppler flow profiles of the PCA were recorded before, during and after exposition with light. Under the treatment with light the RI decreased in the mean form 0.61 down to 0.50, indicating an increase of flow. After redarking the RI increased again up to a mean of 0.62. The applied test could be a helpful tool to clarify the pathophysiological mechanism in defect seeing.