Brain activity related to the perception of illusory contours.

We have addressed the question of whether the brain's capacity to resolve an ambiguous retinal image depends upon the activity of early visual areas or whether it involves the investment of the received image with higher order cognitive hypotheses. To resolve the issue, we have used the technique of positron emission tomography to detect increases in regional cerebral blood flow (rCBF) in the brains of humans while they perceive the simple figures described by Schumann (1900) and by Kanizsa (1979). These figures produce striking percepts of surfaces or contours variously described as illusory, subjective, cognitive, or anomalous because they depend upon the brain's ability to complete the figures. If such completion is due to higher order cognitive processes or a combination of higher order and early areas, then, one might expect areas of increased rCBF outside the occipital lobe when subjects perceive these figures. However, if completion is mediated entirely by early visual areas, then the increases in rCBF will be restricted to these regions. Our results show that the perception of subjective contours is associated with significant activity in early visual areas only, particularly in area V2, leading us to conclude that the occipital cortex can contribute to the perception of these stimuli without higher order cognitive influence specific to the completion task.