Watercolor illusion

History

Baingio Pinna discovered the watercolor illusion in 1987, reporting it in Italian. Jack Broerse and Robert P. O'Shea independently discovered it in 1995, reporting it in English, although they called it "spread colour", relating it to neon colour spreading. Broerse, Tony Vladusich, and O’Shea, demonstrated the phenomenon in 1999 (Figure 1). Pinna, Gavin Brelstaff, and Lothar Spillmann published the first account of the phenomenon in English in 2001, giving it its current name. Since the discovery many experiments have been performed and analyzed to understand the concept of perception of the illusion compared to various Gestalt factors and the neural processes that create the illusion.

Watercolor illusion compared to Gestalt principles

The watercolor illusion has had much debate over whether it can be described by Gestalt psychology. Watercolor illusion has been considered a case of the Gestalt principles by some because of the similarity principles that describe the figure-ground (perception). According to the similarity principles (principles of grouping), elements are grouped together based on its color, brightness, size and shape. There are seven Gestalt factors that the Watercolor Illusion filling of the figure-ground organization were compared to: proximity, good continuation, closure, symmetry, convexity, amodal completion, and past experience. These seven factors were tested in a series of experiments by Pinna, Werner, and Spillman to determine the strength of each factor compared to the illusion.

The first experiment tested the watercolor effect versus proximity to determine the figure-ground segregation. According to the Gestalt factor of proximity, closer elements are more likely to be grouped together. The stimuli had different spacing between the set of contour/flank lines. Each stimuli used had a vastly different response, but the watercolor illusion held true even in the wide spaces of the illusion. In some cases, the figure-ground areas were reversed as the filling-in of the orange flank was stronger than the filling in of the purple.

The second experiment tested the watercolor effect versus good continuation. In good continuation, the smooth continuation areas tend to be grouped together. With different variations of a square-wave pattern and basic contours with fringes, the good continuation of the stimuli was studied. It was determined that the uniform watercolor illusion is seen in only closed space.

The third experiment studied the watercolor illusion stimulus against the idea of closure and surroundness. According to the closure principle, piece creating a closed figure are grouped together. When one region encompasses another region completely, the surrounding region is perceived as ground, and the feature that is perceived as figure according to the surroundness principle. When the four purple rectangles were surrounded by a larger rectangle, the large rectangle was rarely perceived as figure while the four rectangles were seen as figures. When orange contours bordered the inside of the large rectangle, but outside the four smaller rectangles, the larger rectangle was perceived as figure while the small rectangles were perceived as holes. This showed that the closure and surroundness were weaker than the watercolor illusion.

The fourth experiment was watercolor effect versus symmetry. Parallel contours are grouped together according to the Gestalt principle of symmetry. Parallel wavy lines (rivers) were spaced apart with the purple contours on the inside and orange on the outside. Opposite of the principle, the rivers were not perceived as filed in, but the interspaces between the rivers were perceived to be filled in, or as figure in this case.

The fifth experiment was watercolor illusion compared to convexity. According to the “law of the inside” the concave regions of the stimulus should be perceived as ground and the convex ones perceived as figure. The stimuli used had different sets of concave and convex arcs alternating between two horizontal lines. The concave regions were typically perceived as figure whether the purple was flanked by red or orange fringes. However, as the curvature was increased the effect was decreased when the red fringes were used.

The sixth experiment was amodal completion compared to the watercolor illusion. Amodal completion is not a classical principle of figure-ground segregation that helps explain the perception of an object’s hidden regions. This applies to both figure and ground in the organization. From the experiments, amodal completion does not hold true when the watercolor illusion reversed the perceived segregation of components.

The seventh experiment was to determine if the observer would see the color spread effect if the stimulus was of a common object. From this, it was determined that spaces with prior knowledge (familiar words, shapes, etc. ) are more likely to be grouped together.

Strength of color spread effect experiments

Several experiments were performed to determine the necessary criteria to have the watercolor illusion viewed. The outcome of the experiments affected the properties that were defined by the coloration effect (below).

The first experiment was done to determine the distance over which the color spreading effect occurred. 25 stimuli of different dimension were hand–drawn with magic markers: purple for the outside border and orange for the inner fringe. The stimuli were presented 50 cm away from the observer with no time limit on determining the color spread. It was determined that the color spreading reported from the experiment decreased with increasing length of the shorter axis. The determined threshold was a height of 45 degrees of visual angle for the height of the surface.

The second experiment was to determine the duration of the exposed stimuli to see the illusion. Again, the stimuli were shown to the observer 50 cm away in an ambient lighted room. An electromagnetic shutter was in front of one eye. The smallest interval for the electromagnetic shutter was 100 ms, and the continuous color could be seen at the time duration. It was determined that the watercolor illusion could be perceived instantaneously in the given conditions.

The third experiment was to determine the optimal line thickness. Various thicknesses of the border and fringe were drawn for the stimulus pattern. The observers compared the strength and uniformity of the illusion between the various stimuli. The color spreading was perceived the strongest when the contour and fringe subtended a visual angle of 6 arcminutes; The strength of the illusion was discovered to decrease as the thickness of lines increased.

The fourth experiment was evaluating the waviness of the lines used compared to the strength of the illusion. The stimulus patterns varied in frequency of sinusoidal waves. The results showed increase of strength with increasing spatial frequency of the sinusoidal waves. The effects are the strongest with the wave patterns, however, the watercolor illusion is still strong for a stimulus with straight borders.

The fifth experiment tested inducing color. The colors were drawn with magic marker with pairs of red, green, blue, and yellow lines. Red and blue pairs produced the strongest effects while green and yellow produced the weakest illusion effect. All combinations of two colors produced a clearly visible spreading effect.

The sixth experiment tested the contrast between the different color lines to see which produced the most striking effect. Pinna first discovered the watercolor illusion with high contrast lines (a black outer line and lighter fringe). When the luminance between the two lines is different, the color spreading effect is the strongest. As the luminance between the two lines becomes closer, the spreading effect grows weaker but is still present.

Limitations of the watercolor illusion were studied. The color-spreading effect occurs on colored backgrounds in addition to the white or gray. In the case of a colored background, the watercolor illusion color spread does not mix with the background color but does get superimposed onto the colored background. For lighting conditions, the color spreading effect declines as the illumination in the room increases. It is the strongest at medium illumination. Lastly, the watercolor illusion persists even when dotted lines are used for the purple contour and orange fringe instead of continuous lines.