Pii: S0304-3940(00)00818-1

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. Surprisingly, 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 by the neural processes of visual area V2 [15] between some physically existing objects, known as inducers. A related phenomenon is neon colour spreading [14], the apparent spreading of colour or luminance (cf. ®lling-in within V1 [7]) to its neighbouring area without this physical quality. Classically, neon colour within illusory ®gures is produced by using a colour or luminance different from the background and inducers. This makes the illusory ®gure to appear as colored, transparent and often glowing [1]. An example of a one-dimensional illusory ®gure is an illusory contour created between two abutting gratings with a phase shift [11]. Two-dimensional illusory ®gures have contours and surfaces in one depth plane. For example, three pacmen (disks with a sector removed) forming the apices of a triangle usually elicit the perception of an illusory Kanizsa [6] triangle. Perceptions of thin Kanizsa squares bending in 3-D space have been produced with stereo pairs [2]. Neon spreading in stationary stimuli seems to be twodimensional [1]. In a line drawing consisting of black horizontal and vertical crossing lines so that some line elements are replaced by different colour or luminance, the surroundings of the line with deviant colour or luminance appears to be ®lled with this colour or brightness [13]. In illusory ®gures, deviant colour or brightness seems to produce a uniform spreading within the illusory ®gure [16]. Then, the illusory area looks transparent and colour-tinted or different in brightness. In early vision, illusory contour [15], surface, and neon spreading formation is prevented by a nearer surface [3]. This has been assumed to re ̄ect early neural processes [4]. Based on this, we wanted to study whether neon spreading in stationary images extends across several depth planes when inducers and the induced contours/surfaces have different depth signs. Hence, we used three-dimensional Kanizsa-type triangles, somewhat similar to the curved Kanizsa-type squares [2], but produced by stereopairs of black disks with red sectors forming the apices of a triangle on a white surface and bending in depth, thus, making illusory triangles to appear convex or concave towards the observer. We recruited 18 ®rst-year students of psychology without experience in perception studies. All had normal vision, with or without correction, and received course credit for Neuroscience Letters 281 (2000) 119±122 0304-3940/00/$ see front matter q 2000 Elsevier Science Ireland Ltd. All rights reserved. PII: S0304-3940(00)00818-1 www.elsevier.com/locate/neulet * Corresponding author. Tel.: 1358-50-307-5955; fax: 1358-9191-8681. E-mail address: marja.liinasuo@helsinki.® (M. Liinasuo) their participation. It was checked that participating students were able to perceive (i) stereo ®gures by presenting red and green stereo pairs, (ii) illusory ®gures by presenting a Kanizsa triangle with three black pacmen drawn on a white paper, and (iii) neon colour spreading by presenting a photo of black pacmen with red sectors usually resulting in the perception of neon spreading within a Kanizsa triangle. Five students were eliminated due to failure in perceiving neon colour spreading. Thus, 13 students (aged 19±35, mean 23 years) served as subjects in the experiments. The stimulus ®gures were drawn with Illustrator, printed on ®lm with Digital Palette equipment, and presented as colour prints using a lenticular stereoscope, Stereopticse (EARTHings Corp., USA) allowing stereo pair observation by means of parallel fusion. The device is made of black cardboard and has holes with lenses allowing eyes to focus on the plane of the stimulus. Viewing distance was 13.5 cm. The stimuli were black inducers with white surrounds and red sectors forming the apices of Kanizsa triangles (Fig. 1). For all stimuli, the inter-centre distance between the pacmen was 2 cm (8.58) and their diameter was 1.5 cm (6.48). The tip of the upper sector and the horizontal edges of the lower sectors were always on the same depth level relative to each other. Only the three-dimensional curving and location of the Kanizsa triangle, relative to the surround, were varied. In the ®rst and second stimulus, the tip and the horizontal edges of the lower sectors had null disparity with respect to the contours of the inducers, and should be perceived to be in the same plane. The sectors of the ®rst stimulus should appear convex and above the level of the black inducers (Fig. 1a, the two on the right/left ®gures for crossed/ uncrossed fusion). In the second stimulus, the sectors should appear concave and further in depth than the black inducers (Fig. 1a, the two ®gures on the right/left for uncrossed/ crossed fusion). In the third stimulus, the tip and horizontal edges of the lower sectors should appear to be further from the observer than the inducers. The rest of the convex sectors should appear closer than the inducers and even penetrate the white ®eld bearing the inducers (Fig. 1b, the two ®gures on the right/left for crossed/uncrossed fusion). The fourth stimulus was the reverse of the third. The tip and horizontal edges of the sectors should appear closer to the observer than the inducers. The rest of the concave M. Liinasuo et al. / Neuroscience Letters 281 (2000) 119±122 120 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. sectors should reach into the distance, penetrating the white ®eld so that they are further from the observer than the inducers (Fig. 1b, the two ®gures on the right/left for uncrossed/crossed fusion). The subjects observed each stereo pair using Stereopticse and reported their impressions aloud. After the subject gave a spontaneous response, the experimenter enquired, if necessary: (1) whether the subject saw the illusory triangle: (2) if yes, whether the subject saw contours around the triangle; (3) about the quality of the illusory surface, and (4) about its three-dimensional properties, that is, were some parts nearer or further away from the observer than others. Twelve subjects out of 13 perceived neon colour spreading in the ®rst stimulus. Eleven of these perceived a reddish convex illusory triangle with its apices on the pacmen but otherwise in front of them and `above' the white surround. One subject (S.S.) perceived the illusion in the same way but behind the white surface that was transparent to some degree. Subject (S.R.), who did not perceive neon colour spreading, described a glossy uncoloured transparent triangle curving above the white surface. In the second stimulus eleven subjects did not perceive neon colour spreading but saw the pacmen as holes through which the three apices of a red concave triangle were visible; the other parts of the triangle were behind the white surface. Thus, the whole triangle appeared to be deeper in depth than the white surface. Two subjects (T.T. and M.K.) perceived an illusory triangle with neon colour spreading behind the white surface bearing the contours of the holes. The white surface appeared suf®ciently transparent to reveal the red illusory surface, which was not transparent enough to reveal the black distant surface visible through the holes. Eleven subjects perceived neon colour spreading in a convex illusory triangle in the third stimulus. The pacmen were described as holes or funnels into which the apices penetrated, while remaining visible. Otherwise the convex surface was located in front of the white ®eld bearing the contours of the holes. Two subjects (S.R. and S.S.) reported a curved triangle, partly occluded by a white plane so that only apices were visible through the holes in the white opaque plane, and no neon colour spreading. In the fourth stimulus twelve subjects reported a concave triangle, partly occluded by a white surface, with apices visible since they reached through holes in the white surface, and no neon colour spreading. Hence, the apices of the triangle appeared above the white surface whereas the rest of the triangle was occluded by it. One subject (S.S.) reported that a concave triangle with neon colour spreading was visible through the white surface which was partly transparent. When the tip of the upper sector and the horizontal edges of the lower sectors were further away from the observer than the rest of the con®guration (the ®rst and third stimulus), a convex illusory triangle with neon colour spreading was perceived by 12 (®rst stimulus) and 11 (third stimulus) subjects out of 13. When the above mentioned parts of the sectors were closer to the observer than the rest of the con®guration, a concave, partly-occluded triangle was usually reported, and the neon effect was reported less frequently, by only two (second stimulus, x…1† ˆ 121, P , 0:001) and one (fourth stimulus, x…1† ˆ 144, P , 0:001) subjects. Although illusory contours existed without neon colour in one case (subject S.R. with the ®rst stimulus), no one saw neon effects without illusory contours. The other individual differences in the perceptions seemed to depend on the perceived location of the white surface with respect to the red sectors and whether or not the surface appeared transparent to any degree. Subjects S.S. (the ®rst stimulus) and S.R. and S.S. (the third stimulus) perceived the relative depth of the illusory or partly occluded triangle differently from the other subjects. This could be due to a subtle de®cit in stereo vision. We found that three-dimensional neon spreading can also be produced within steady illusory ®gures curving in depth, extending previous ®ndings [1] where neon effects were obtained in three-dimensional settings but with ̄at shapes. Junctions seem to be important in creating neon colour spreading in ̄at frontoparallel illusory ®gures [17]. Our experiments show that these junctions do not have to consist of planar curves but neon effect can also spread from elements curved in depth. Illusory contours, surfaces and neon spreading have been shown to be prevented by a nearer sharp-edged object [3]. This has been assumed to re ̄ect early neural processes [4]. However, our experiments with stimulus three showed that spreading can be induced above the intervening surface even when the inducers are below the surface, which is probably re ̄ecting later neural processes. The reason for earlier misinterpretation and limited neural modelling is the experimental restriction of each set of inducers to a single depth plane. Additionally, our experiments showed no neon spreading when inducers were perceived behind the intervening surface appearing opaque. These results indicate that neural computation of transparency/opaqueness modulates the construct of early processes. Black inducers generate a glowing white surface, that is, brightness enhancement in plain illusions. The glowing in neon ®gures could also be due to this brightness enhancement generated by luminance contrast between inducers and surround. The use of the same basic neural mechanisms [1] is also supported by the similar three-dimensional appearance in curved Kanizsa ®gures with (present experiments) and without [2] colour spreading. Anatomical, neurophysiological and psychophysical evidence [5,8,10,12,18] suggests that in early vision illusory contours, illusory surfaces and neon spreading are processed in parallel. However, there is evidence that ®lling-in takes place in V1 [7], illusory contours are processed in V2 [15], and V4 is the main colour area in primates [19]. Serial M. Liinasuo et al. / Neuroscience Letters 281 (2000) 119±122 121