Color Spreading, Neon Color Spreading, and Watercolor Illusion
The color spreading is a long-range assimilative spread of color emanating from a thin colored contour running in the same direction/continuation or being contiguous/adjacent to a darker chromatic contour and imparting a figure-ground effect across a large area. The two main examples of color spreading are the well-known neon color spreading and the watercolor illusion.
Neon Color Spreading
This phenomenon was independently reported a few years later from Varin’s discovery by van Tuijl , who named it “neon-like color spreading.” He used a lattice of horizontal and vertical black lines, where segments of different colors (e.g., blue) create an inset diamond shape. The main outcome reveals again a tinted transparent diamond-like veil above the lattice (Fig. 1b).
Geometrically, the main geometrical property of all the known cases of neon color spreading is the continuation of one contour, usually black, in a second contour with a different color or, differently stated, a single continuous contour changing from one color to another. Phenomenally, color spreading manifests a coloration and a figural effect described in detail in the following sections.
Coloration Effects in Neon Color Spreading
The phenomenology of the coloration effect within neon color spreading depends on the luminance contrast between the two inducing contours and is summed in the next points [2, 3]. (i) The color is perceived as a diffusion of a certain amount of pigment of the inset chromatic segments. (ii) The appearance of the coloration is diaphanous like a smoggy neon upon the background or (under achromatic conditions) like a shadowy, dirty, or filmy transparent veil. (iii) Under conditions where the inset figure is achromatic and the surrounding inducing elements chromatic, the illusory veil of the inset figure appears tinted not in the achromatic color of the embedded elements, but in the complementary color of the surrounding elements, e.g., the achromatic components appear to spread greenish or bluish illusory colors, respectively, with red or yellow inducers.
Figural Effects in Neon Color Spreading
Coloration Effects in the Watercolor Illusion
As in neon color spreading, the illusory color is approximately uniform. As shown in Fig. 5, the coloration, within the regions where the orange contours are closer, is the same as within the regions where they are distant.
The coloration extends up to about 45°.
It is complete by 100 ms.
Similarly to neon color spreading, all the colors can generate the illusory coloration, as shown in Fig. 6, where an undefined irregular peninsula appears filled with a light blue tint. It should be noted that this peninsula is the Mediterranean Sea when the two adjacent chromatic contours are reversed.
The coloration occurs on colored and black backgrounds. In Fig. 7, an undefined irregular peninsula (the Mediterranean Sea when the contours are reversed) appears filled with a purple tint.
The optimal contour thickness is approx. 6 arcmin.
The contour with a lower luminance contrast relative to the background spreads proportionally more than the contour with a higher luminance contrast.
The color spreads in directions other than the contour orientation.
By reversing the colors of the two adjacent contours, the coloration reverses accordingly.
Phenomenally, the coloration appears solid, impenetrable, and epiphanous as a surface color.
Similarly to neon color spreading [1, 2, 3], the watercolor illusion induces a complementary color when one of the two juxtaposed contours is achromatic and the other chromatic (see Fig. 10) . The inside of the zigzagged annulus appears yellowish.
Figural Effects in the Watercolor Illusion
As in neon color spreading, the figural effect of the watercolor illusion is clearly perceived although it occurs in a different mode of appearance. The figure shows a strong depth segregation and a volumetric rounded and three-dimensional attribute, while the perceived variation of color, going from the boundaries to the center of the object, is seen as a gradient of shading, as if light were reflected onto a volumetric and rounded object. Figure 11 shows undefined, rounded, and volumetric shapes differing from one row to another on a shapeless empty space due to the unilateral belongingness of the boundaries. The stars are totally invisible.
By reversing the colors of the two adjacent contours, the figure-ground segregation reverses accordingly. In Fig. 12, the same elements of Fig. 11, illustrated with reversed purple-orange contours, appear like juxtaposed stars. The undefined shapes differing from one row to another are now invisible.
Under the previous conditions, the figure-ground segregation is not reversible and unequivocal.
The watercolor illusion determines grouping and figure-ground segregation more strongly than the Gestalt principles of proximity, good continuation, Prägnanz, relative orientation, closure, symmetry, convexity, past experience, similarity, surroundedness, and parallelism [6, 7, 10]. In Fig. 13, some examples showing the watercolor illusion respectively against and in favor of surroundedness, relative orientation, good continuation, past experience, and parallelism are illustrated.
By reversing the luminance contrast of the background, e.g., from white to black, while the luminance contrast of the contours is kept constant, the figure-ground segregation reverses (Fig. 14) . Going from the bottom to the top of the figure, the crosses become stars. These results are in contrast to Gestalt claim that the currently figural region is maintained even on black/white reversal.
This suggests that the watercolor illusion includes a new principle of figure-ground segregation, the asymmetric luminance contrast principle, stating that, all else being equal, given an asymmetric luminance contrast on both sides of a boundary, the region whose luminance gradient is less abrupt is perceived as a figure relative to the complementary more abrupt region, which is perceived as a background .
Similarities and Differences In Between the Two Illusions
By summing up the phenomenology of coloration and figural effects in both neon color spreading and watercolor illusion, the former differs from the latter in both the appearance of the coloration (respectively, transparent vs. solid and impenetrable and diaphanous vs. epiphanous) and in the figural effects (respectively, transparent vs. opaque and dense and appearance as a “light,” a “veil,” a “shadow,” or a “fog” vs. rounded thick and opaque surface bulging from the background).
In spite of these differences, the two illusions are phenomenally similar in their clear color spreading and depth segregation. It is suggested  that, while the similarities may depend on the local nearby transition of colors, equivalent in both illusions, the differences may be attributed to the global geometrical boundary conditions, dissimilar in the two illusions. As a matter of fact, while the neon color spreading is elicited by the continuation in the same direction of two contours of different colors, the watercolor illusion occurs through their juxtaposition.
A Limiting Case
Gradual steps toward the final combination of the two illusions in a limiting case are illustrated in Figs. 15b and c. Geometrically, in Fig. 15b, the orange inset arcs are reduced to short dashes, creating a condition in between neon color spreading and the watercolor illusion: from the neon color spreading perspective, the inducing elements are contours continuing in short dashes (or elongated dots), but from the watercolor perspective, the terminations of the inducing arcs contain juxtaposed short dashes. Under these conditions, a coloration effect, not weaker than that of Fig. 15a, is perceived. However, it manifests a poor diaphanous and surface appearance. The illusory figure appears as a fuzzy square annulus, yellowish and brighter than the background. It is worthwhile to note that the further reduction of the dashes to dots does not change significantly the strength of these effects.
The geometrical reduction in between neon color spreading and the watercolor illusion and opposite to the one of Fig. 15b is illustrated in Fig. 15c. Under these further conditions, all else being equal, short dashes become the purple arcs of Fig. 15a. Now the coloration effect is weaker than that of Fig. 15a.
Given these geometrical prerequisites, the final step toward the limiting case becomes immediate and consists in putting together the previous opposite reductions as shown in Fig. 15d. The results show that by reducing both the purple and orange arcs of Fig. 15a to short dashes, the coloration and figural effects do not change significantly . This is corroborated by previous outcomes according to which the watercolor illusion occurs not only by using juxtaposed lines but also by using juxtaposed chains of dots [6, 7, 10]. Under these conditions both coloration and figural effects become weaker and weaker as the density of the dots becomes sparser and sparser.
The two-dot juxtaposition of Fig. 15d can be considered as a true limiting case for neon color spreading and the watercolor illusion. As a matter of fact, (i) the two-dot limiting case can be considered as the geometrical common condition, beneath. (ii) The strength of both coloration and figural effects does not change significantly; therefore, the specific mode of appearance of coloration and figural effects in the two illusions is elicited by different local and global distributions of nearby transitions of colors that, in their turn, induce different boundary organizations. (iii) Phenomenally, the differences between the two illusions, where the inner changes are based on continuation and juxtaposition of contours, can now be reconsidered and unified in terms of transition. This is not only a linguistic alternative but also it can bring advantages by providing support for a simple common neural model. (iv) The limiting case can suggest variations of the two illusions that manifest coloration and figural attributes in between the neon color spreading and the watercolor illusion, as shown in the next section.
Near the Limiting Case
Taken together, these figures suggest that (i) the modes of appearance of coloration are strongly related to boundary conditions that induce specific figural effects; (ii) by changing the boundary conditions, coloration and figural attributes are perceived more similar to one, to the other illusion, or in between; and (iii) given this variety of appearances on the basis of different conditions, a simpler set of boundary cases, like in the limiting case, can unify both effects using local transitions of colors and can help to explain similarities and dissimilarities of the two illusions.
Neural Mechanisms Underlying the Two Illusions
On the basis of the previous results, coloration and figural effects may derive from parallel processes. At a feature processing stage, the short-range interaction area around and in between the two dots produces the color spreading common to both illusions, and at a parallel boundary processing stage, the different geometrical structures in both illusions organize the color spreading to elicit different figural effects. Moreover, the reduction of the neon color spreading and watercolor illusion to a common limiting case can suggest a common and an easier explanation that can be based on the FACADE neural model of biological vision . The model posits that two processes, boundary grouping and surface filling-in [11, 12] substantiated by the cortical interblob and blob streams, respectively, within cortical areas V1 through V4, are responsible of how local properties of color transitions activate spatial competition among nearby perceptual boundaries, with boundaries of lower-contrast edges weakened by competition more than boundaries of higher-contrast edges. This asymmetry induces spreading of more color across these boundaries than conversely. These boundary and surface processes show complementary properties that can also predict how depth and figure-ground effects are generated in these illusions.
Other related findings to both illusions [13, 14, 15] showed that neurons in V2 respond with different strength to the same contrast border, depending on the side of the figure to which the border belongs, implying a neural correlate process related to the unilateral belongingness of the boundaries. Figure-ground segregation may be processed in areas V1 and V2, in inferotemporal cortex and the human lateral occipital complex. Also the color spreading of the two illusions might have its explanation in the cortical representation of borders .
The color spreading is a long-range assimilative spread of color emanating from a thin colored contour running in the same direction/continuation or being contiguous/adjacent to a darker chromatic contour and imparting a figure-ground effect across a large area. Two main examples of color spreading are the well-known neon color spreading and the watercolor illusion. The coloration and the figural properties of the two illusions, studied using phenomenal and psychophysical observations, can be reduced to a common limiting condition, i.e., a nearby color transition called the “two-dot limiting case,” which explains their perceptual similarities and dissimilarities and suggests a common explanation.
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