The goal of the following experiments was to empirically demonstrate that involvement and to test the appropriateness of the proposed completion mechanism. Two functional magnetic resonance imaging experiments were carried out to measure and characterize the cortical activity during the processing of illusory contours (Chapters 2 and 3). To show the functional necessity of V1 neurons for illusory contour completion I utilized a variant of the neuropsychological 'lesion' approach. Healthy observers' illusory contour completion ability was tested in the region corresponding to the physiological 'blind spot', i.e. in that region of the retina where the optic nerve exits the eye (Chapter 4). Since this region is strictly monocular in layer 4C of the primary visual cortex, it may serve as a monocular scotoma model. If the completion mechanism relies on these neurons, it should be impaired in this region of the visual field under monocular viewing conditions.