1 Biol Cybern (2010) 102:475–487 DOI 10.1007/s00422-010-0379-5 Original Paper CENTER-OF-MASS ALTERATIONS AND VISUAL ILLUSION OF EXTENT Aleksandr Bulatov, Algis Bertulis, Algirdas Gutauskas, Lina Mickiene, Gitana Kadziene Institute of Biology, Kaunas University of Medicine, Mickevičiaus 9, LT-44307 Kaunas, Lithuania Corresponding author: A. Bulatov e-mail: bulatov@vision.kmu.lt phone:+37037326334 Abstract In the present communication, we have developed a computational model related to the conception of positional coding via centers-of-masses (centroids) of the objects’ luminance distributions. The model predictions have been tested by the results of our psychophysical study of geometrical illusion of extent evoked by a modified Brentano figure consisting of three separate spots clusters. In experiments, the centroids of the clusters were manipulated by varying the positions of additional non-target spots flanking the stimulus terminators. A good correspondence between the model predictions and the illusion magnitude changes provided convincing evidences in favor of “centroid” explanation of origin of the illusion investigated. Key words: centroid; visual geometrical illusions; lateral interaction; attentional pooling The original publication is available at www.springerlink.com