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Neon Colour Spreading in Three-Dimensional Illusory Objects in Humans

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Neon Colour Spreading in Three-Dimensional Illusory Objects in Humans Neuroscience Letters 281 (2000) 119±122 www.elsevier.com/locate/neulet Neon colour spreading in three-dimensional illusory objects in humans Marja Liinasuoa,*, Ilpo Kojob, Jukka HaÈ kkinenc, Jyrki Rovamod aInstitute of Biomedicine, Department of Physiology, P.O. Box 9, 00014 University of Helsinki, Helsinki, Finland bThe Finnish Institute of Occupational Health, Brain Work Laboratory, Topeliuksenkatu 41 a A, 00250 Helsinki, Finland cDepartment of Psychology, General Psychology Division, P.O. Box 13, 00014 University of Helsinki, Helsinki, Finland dDepartment of Optometry and Vision Sciences, University of Wales, College of Cardiff, P.O. Box 905, Cardiff CF1 3XF, UK Received 11 October 1999; received in revised form 7 January 2000; accepted 10 January 2000 Abstract We studied whether neon spreading can be induced within three-dimensional illusory triangles. Kanizsa triangles were induced by black pacman disks consisting of red sectors with curved sides. Viewing our stimuli monocularly produced two-dimensional illusory contours and surfaces as well as neon spreading in each ®gure. Triangles appeared concave or convex under stereoscopical viewing. Neon colour spreading was induced within illusory ®gures bending in three-dimensional space, suggesting that neural contour completion and surface ®lling-in interact across depth. Surpris- ingly, neon spreading was induced above the intervening surface even when the inducers were below the surface. Neon colour and illusory con®guration were preserved behind the intervening surface only when it appeared transparent. q 2000 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Vision; Perception; Illusion; Three-dimensionality; Colour; Neon spreading Illusory ®gures with one to three dimensions are created zontal and vertical crossing lines so that some line elements by the neural processes of visual area V2 [15] between some are replaced by different colour or luminance, the surround- physically existing objects, known as inducers. A related ings of the line with deviant colour or luminance appears to phenomenon is neon colour spreading [14], the apparent be ®lled with this colour or brightness [13]. In illusory spreading of colour or luminance (cf. ®lling-in within V1 ®gures, deviant colour or brightness seems to produce a [7]) to its neighbouring area without this physical quality. uniform spreading within the illusory ®gure [16]. Then, Classically, neon colour within illusory ®gures is produced the illusory area looks transparent and colour-tinted or by using a colour or luminance different from the back- different in brightness. ground and inducers. This makes the illusory ®gure to In early vision, illusory contour [15], surface, and neon appear as colored, transparent and often glowing [1]. spreading formation is prevented by a nearer surface [3]. An example of a one-dimensional illusory ®gure is an This has been assumed to re¯ect early neural processes illusory contour created between two abutting gratings [4]. Based on this, we wanted to study whether neon spread- with a phase shift [11]. Two-dimensional illusory ®gures ing in stationary images extends across several depth planes have contours and surfaces in one depth plane. For example, when inducers and the induced contours/surfaces have three pacmen (disks with a sector removed) forming the different depth signs. Hence, we used three-dimensional apices of a triangle usually elicit the perception of an illu- Kanizsa-type triangles, somewhat similar to the curved sory Kanizsa [6] triangle. Perceptions of thin Kanizsa Kanizsa-type squares [2], but produced by stereopairs of squares bending in 3-D space have been produced with black disks with red sectors forming the apices of a triangle stereo pairs [2]. on a white surface and bending in depth, thus, making illu- Neon spreading in stationary stimuli seems to be two- sory triangles to appear convex or concave towards the dimensional [1]. In a line drawing consisting of black hori- observer. We recruited 18 ®rst-year students of psychology without * Corresponding author. Tel.: 1358-50-307-5955; fax: 1358-9- experience in perception studies. All had normal vision, 191-8681. with or without correction, and received course credit for E-mail address: marja.liinasuo@helsinki.® (M. Liinasuo) 0304-3940/00/$ - see front matter q 2000 Elsevier Science Ireland Ltd. All rights reserved. PII: S0304-3940(00)00818-1 120 M. Liinasuo et al. / Neuroscience Letters 281 (2000) 119±122 their participation. It was checked that participating students sectors were always on the same depth level relative to each were able to perceive (i) stereo ®gures by presenting red and other. Only the three-dimensional curving and location of green stereo pairs, (ii) illusory ®gures by presenting a the Kanizsa triangle, relative to the surround, were varied. Kanizsa triangle with three black pacmen drawn on a In the ®rst and second stimulus, the tip and the horizontal white paper, and (iii) neon colour spreading by presenting edges of the lower sectors had null disparity with respect to a photo of black pacmen with red sectors usually resulting in the contours of the inducers, and should be perceived to be the perception of neon spreading within a Kanizsa triangle. in the same plane. The sectors of the ®rst stimulus should Five students were eliminated due to failure in perceiving appear convex and above the level of the black inducers neon colour spreading. Thus, 13 students (aged 19±35, mean (Fig. 1a, the two on the right/left ®gures for crossed/ 23 years) served as subjects in the experiments. uncrossed fusion). In the second stimulus, the sectors should The stimulus ®gures were drawn with Illustrator, printed appear concave and further in depth than the black inducers on ®lm with Digital Palette equipment, and presented as (Fig. 1a, the two ®gures on the right/left for uncrossed/ colour prints using a lenticular stereoscope, Stereopticse crossed fusion). (EARTHings Corp., USA) allowing stereo pair observation In the third stimulus, the tip and horizontal edges of the by means of parallel fusion. The device is made of black lower sectors should appear to be further from the observer cardboard and has holes with lenses allowing eyes to focus than the inducers. The rest of the convex sectors should on the plane of the stimulus. Viewing distance was 13.5 cm. appear closer than the inducers and even penetrate the The stimuli were black inducers with white surrounds and white ®eld bearing the inducers (Fig. 1b, the two ®gures red sectors forming the apices of Kanizsa triangles (Fig. 1). on the right/left for crossed/uncrossed fusion). For all stimuli, the inter-centre distance between the pacmen The fourth stimulus was the reverse of the third. The tip was 2 cm (8.58) and their diameter was 1.5 cm (6.48). The tip and horizontal edges of the sectors should appear closer to of the upper sector and the horizontal edges of the lower the observer than the inducers. The rest of the concave Fig. 1. Stimuli used in the experiments. Although only parallel fusion was used, stimuli for converging fusion are also provided. The instructions are given for parallel fusion; for converging fusion, the opposite applies. Thus, when instructed to fuse the two leftmost ®gures, the convergers should fuse the two rightmost ®gures and vice versa. (a) Fusing the two leftmost ®gures (®rst stimulus) usually produced the perception of a convex illusory triangle with a reddish surface above the contours of the black inducers. Fusing the two rightmost ®gures (second stimulus) resulted in the perception of a concave red triangle. Typically, only its apices were visible through three holes showing a black background, without illusory contours or neon colour spreading. (b) For most observers, fusing the two leftmost ®gures (third stimulus) produced a perception of a convex illusory triangle with neon colour spreading. The apex of the upper sector and the horizontal edges of the lower sectors were further away from the observer than the contours of the black inducers; the rest of the illusory ®gure curved in front of them. The black areas were perceived as holes or funnels. When the two rightmost ®gures were fused (fourth stimulus), most observers perceived a red concave triangle, only its apices appearing to be closer than the contours of the black inducers. The rest of the sectors appeared to be further away from the observer than the white surround that appeared to occlude the central part of a real triangle. M. Liinasuo et al. / Neuroscience Letters 281 (2000) 119±122 121 sectors should reach into the distance, penetrating the white subjects out of 13. When the above mentioned parts of the ®eld so that they are further from the observer than the sectors were closer to the observer than the rest of the inducers (Fig. 1b, the two ®gures on the right/left for con®guration, a concave, partly-occluded triangle was uncrossed/crossed fusion). usually reported, and the neon effect was reported less The subjects observed each stereo pair using Stereo- frequently, by only two (second stimulus, x2 1121, pticse and reported their impressions aloud. After the P , 0:001) and one (fourth stimulus, x2 1144, subject gave a spontaneous response, the experimenter P , 0:001) subjects. enquired, if necessary: (1) whether the subject saw the illu- Although illusory contours existed without neon colour in sory triangle: (2) if yes, whether the subject saw contours one case (subject S.R. with the ®rst stimulus), no one saw around the triangle; (3) about the quality of the illusory neon effects without illusory contours. The other individual surface, and (4) about its three-dimensional properties, differences in the perceptions seemed to depend on the that is, were some parts nearer or further away from the perceived location of the white surface with respect to the observer than others. red sectors and whether or not the surface appeared trans- Twelve subjects out of 13 perceived neon colour spread- parent to any degree.
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