Visual completion of three-dimensional, chromatic, moving stimuli in humans Marja Liinasuo a, * , Ilpo Kojo b , Jukka Ha ¨kkinen c,d , Jyrki Rovamo e,f a Department of Biosciences, Division of Animal Physiology, P.O. Box 65, University of Helsinki, FIN-00014 Helsinki, Finland b Center for Knowledge and Innovation Research, Helsinki School of Economics, P.O. Box 1210, FIN-00101 Helsinki, Finland c Department of Psychology, General Psychology Division, P.O. Box 9, University of Helsinki, FIN-00014 Helsinki, Finland d Visual Communications Laboratory, Nokia Research Center, P.O. Box 407, FIN-00045 Nokia Group, Finland e Department of Biosciences, University of Helsinki, FIN-00014 Helsinki, Finland f Department of Optometry and Vision Sciences, Cardiff University, Cardiff, UK Received 20 May 2003; accepted 15 September 2003 Abstract We studied whether visual completion can be produced within three-dimensional (3-D), moving, chromatic objects. Rotating thin and thick discs with red sectors, forming the corners of Kanizsa triangle, were presented on a display. Observers also rotated a hand-held real thin Kanizsa triangle. An illusory, moving contour encompassing an illusory colourful triangle was generally observed. This indicates that the visual system is capable of filling in complex (3-D, moving, chromatic) percepts. q 2003 Elsevier Ireland Ltd. All rights reserved. Keywords: Vision; Perception; Illusion; Three dimensional; Color spreading; Visual completion Visual completion means that something is perceived in a specific visual field location when physical presence occurs only in the surrounding area. These processes operate, for example, when a blind spot or scotoma is completed with surrounding visual attributes. Visual completion also takes place in illusory figure perception when illusory brightness or colour (i.e. neon colour [18,19]) appears to spread within illusory contours (for an overview, see [16]). A typical example is Kanizsa triangle [8], consisting of three discs with cut-out sectors forming the corners of a triangle. It results in the perception of illusory contour and surface, i.e. an illusory triangle with brightness enhancement inside the area defined by the cut-out discs. When the sectors have colour, it appears to spread within the illusory contours, producing a tinted illusory surface. Nakayama and Shimojo [15] presented a cross stereo- gram consisting of two limbs, all of uniform luminosity, and added disparity only to the outer edges of the horizontal limb. Then, the whole limb captures the depth information and the limb appears to lie in front or behind the vertical limb, depending on the disparity information given. Bakin et al. [1] studied the cortical area V2 of the alert macaque monkey using single cell recording. Instead of the above horizontal limb, they presented a larger square with only the ends of the vertical limb extending beyond the square containing disparity information. No disparity information or stimulus contrast were presented within the cell’s receptive field, located in the rectangle in the middle of the square. Neurons, normally responding to stimulus disparity, responded as if receiving depth information from the illusory contour modally completed across the homo- geneous stimulus area. This suggests that V2 has a role in the three-dimensional (3-D) visual completion. Motion and pictorial cues including colour can also mediate 3-D perception in normal viewing. Illusions extending through several depth planes have been studied using rotating black-and-white [3,9], or black-and-white [4] or chromatic [12] bended stimuli. To study further the chromatic 3-D visual completion, we used a moving, three- dimensional Kanizsa configuration, i.e. black discs with red sectors forming the apices of a triangle in front of a white Neuroscience Letters 354 (2004) 18–21 www.elsevier.com/locate/neulet 0304-3940/03/$ - see front matter q 2003 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.neulet.2003.09.069 * Corresponding author. Tel.: þ358-9-191-59832; fax: þ 358-9-191- 59810. E-mail address: marja.liinasuo@helsinki.fi (M. Liinasuo).