In the present study, a computational model of the automatic centroid extraction based on the processes of local integration of excitatory profiles in the visual pathways has been developed. The model predictions have been compared with the results of our psychophysical examination of the perceptual distortions of the spatial extent. In experiments, the subjects matched two spatial intervals flanked by one of the three types of the contextual objects: either the Müller-Lyer wings, or vertical stripes, or spot pairs. A good resemblance between the theoretical functions and dependencies of the illusions' magnitudes on various spatial parameters of the stimuli has provided evidence that the effects caused by indirect positional coding via centroids are powerful enough to explain the phenomena of the geometric illusions investigated.