Among the response characteristics of single cells of this early mammalian visual cortex were: at least three receptive field geometries, each with simple area summation (i.e., no zones of mutual inhibition); excellent response to diffuse light as well as discrete stimuli; no significant relationship between field area and eccentricity, even between tapetized and non-tapetized retina; binocular responsiveness among 40% of the cells studied with a full spectrum of dominancies represented. Evidence for vertical columnar organization of these dominancies was found.