One of the most common principles of cortical organization is that neurons with similar response properties are clustered together in space. Thereby the environment is represented in an orderly fashion on the cortical surface in a so-called "cortical map". In primary visual cortex, for instance, neurons with similar orientation preferences are grouped together, forming the orientation preference map. Optical imaging of intrinsic signals allows to investigate the organization of such maps in vivo. Neuronal activity was measured utilizing the fact that the transition from oxy-hemoglobin to hemoglobin in active brain areas can be detected optically by recording changes in light reflectance with a high resolution CCD-camera. When using this technique to look at the exact patterning of orientation preference maps in cat visual cortex a novel principle for the organization of cortical maps was observed: orientation was not organized in parallel bands as had previously been thought but iso-orientation domains were organized radially; orientations from 0 to 180 degrees were laid out in a pinwheel-like fashion around singularities which we termed "orientation-centers". After observing pinwheel patterns in orientation preference maps in adult cat visual cortex it was also investigated how these meticulously arranged maps develop in the cortex of young kittens. Performing chronical recordings in kittens from the age of postnatal day 17 on we were able to observe how orientation maps form already during the third week of life and--under normal conditions--remain largely unchanged thereafter.(ABSTRACT TRUNCATED AT 250 WORDS)