BRAIN ACTIVITY RELATED TO THE PERCEPTION OF ILLUSORY CONTOURS

We have addressed the question of whether the brain's capacity to reso lve an ambiguous retinal image depends upon the activity of early visu al areas or whether it involves the investment of the received image w ith higher order cognitive hypotheses. To resolve the issue, we have u sed the technique of positron emission tomography to detect increases in regional cerebral blood flow (rCBF) in the brains of humans while t hey perceive the simple figures described by Schumann (1900) and by Ka nizsa (1979). These figures produce striking percepts of surfaces or c ontours variously described as illusory, subjective, cognitive, or ano malous because they depend upon the brain's ability to complete the fi gures. If such completion is due to higher order cognitive processes o r a combination of higher order and early areas, then, one might expec t areas of increased rCBF outside the occipital lobe when subjects per ceive these figures. However, if completion is mediated entirely by ea rly visual areas, then the increases in rCBF will be restricted to the se regions. Our results show that the perception of subjective contour s is associated with significant activity in early visual areas only, particularly in area V2, leading us to conclude that the occipital cor tex can contribute to the perception of these stimuli without higher o rder cognitive influence specific to the completion task. (C) 1996 Aca demic Press, Inc.