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PSYCHOLOGY 



ORIGINAL RESEARCH ARTICLE 

published: 08 October 2013 
doi: 10.3389/fpsyg. 201 3.00707 




Afterimage watercolors: an exploration of contour-based 
afterimage filling-in 

Simon J. Hazenberg * and Rob van Lier 

Donders Institute for Brain, Cognition and Behaviour, Radboud University Nijmegen, Nijmegen, Netherlands 



Edited by: 

Galina Paramei, Liverpool Hope 
University, UK 

Reviewed by: 

Daw-An Wu, Caltech, USA 
Baingio Pinna, University of Sassari, 
Italy 

Correspondence: 

Simon J. Hazenberg, Centre for 
Cognition, Donders Institute for 
Brain, Cognition and Behaviour, 
Radboud University Nijmegen, 
Montessorilaan 3, PO Box 9104, 
6500 HE Nijmegen, Netherlands 
e-mail: s.j. hazenberg@donders. ru. nl 



We investigated filling-in of colored afterimages and compared them with filling-in of 
"real" colors in the watercolor illusion. We used shapes comprising two thin adjacent 
undulating outlines of which the inner or the outer outline was chromatic, while the 
other was achromatic. The outlines could be presented simultaneously, inducing the 
original watercolor effect, or in an alternating fashion, inducing colored afterimages of 
the chromatic outlines. In Experiment 1 , using only alternating outlines, these afterimages 
triggered filling-in, revealing an "afterimage watercolor" effect. Depending on whether the 
inner or the outer outline was chromatic, filling-in of a complementary or a similarly colored 
afterimage was perceived. In Experiment 2, simultaneous and alternating presentations 
were compared. Additionally, gray and black achromatic contours were tested, having an 
increased luminance contrast with the background for the black contours. Compared to 
"real" color filling-in, afterimage filling-in was more easily affected by different luminance 
settings. More in particular, afterimage filling-in was diminished when high-contrast 
contours were used. In the discussion we use additional demonstrations in which we 
further explore the "watercolor afterimage." All in all, comparisons between both types of 
illusions show similarities and differences with regard to color filling-in. Caution, however, 
is warranted in attributing these effects to different underlying processing differences. 



Keywords: color, contours, afterimages, filling-in, illusion 



INTRODUCTION 

Perception of color does not always reflect what is physically 
present. This is clearly demonstrated in the case of colored 
afterimages where, after adapting to a colored stimulus, a vivid 
afterimage of a complementary hue is seen when the stimulus is 
removed or if one changes their gaze to a blank wall. Although 
the neural locus of the afterimages is still debated, data from a 
recent study indicates that the appearance of colored afterimages 
arise from signals originating in the retina (Zaidi et al., 2012). 
It has been argued further that the signals coding for colored 
afterimages may be, like retinal signals coding for "real" colors, 
subject to various kinds of contextual modifications. In this study 
we focused on the influence of luminance contours on the per- 
ception of colors. Specifically, we investigated filling-in of colored 
afterimages between luminance contours and compared this with 
color filling-in that is induced by "real" colors in the well-known 
watercolor illusion. 

There have been various studies showing that the perception 
of colored afterimages can be modulated by luminance defined 
contours. For instance, an afterimage appears much more salient 
and saturated when it is surrounded by a luminance contour 
(Daw, 1962). More recently, the dependence of afterimages on 
contours have been emphasized by showing that a colored after- 
image spreads within a test outline and fills in regions that were 
not adapted to color (van Lier et al., 2009). They used adapt- 
ing stimuli consisting of multiple colors and an achromatic inner 
region and showed that the location that was not adapted to color 
revealed an afterimage color that depended on the subsequent 



presented outline (see Figure 1). They additionally demonstrated 
that when afterimages of multiple colors are contained within a 
contour, the colors within that contour tend to mix (van Lier 
et al, 2009; Anstis et al, 2012a). The color of an afterimage not 
only depended on the color that was positioned inside a subse- 
quent contour, but also on the color that was positioned outside 
the subsequent contour. The color inside the contour produced a 
complementary color, whereas the color outside the contour pro- 
duced a color that was similar to the inducing color, although the 
latter effect was found to be less strong (van Lier et al, 2009). 
Two possible routes have been suggested previously to explain the 
latter colored afterimage. Along the first route, the outside color 
induces a contrasting color across the boundary into the interior 
of the figure during the adapting period which then produces a 
complementary colored afterimage during the test phase. Along 
the second route, the outside color produces a complementary 
colored afterimage during the test phase, which then induces a 
contrasting color into the interior of the outline (Anstis et al, 
1978). 

Interactions between contours and "real" colors are known as 
well. Chromatic sensitivity seems to be enhanced when a colored 
region is bound by luminance edges (Montag, 1997). Similarly, 
"real" color filling-in occurs in the Boynton illusion in which 
color spreads out from a colored region to (nearly) isoluminant 
achromatic areas until it reaches luminance -defined contours or 
illusory contours (Feitosa-Santana et al., 2011). Furthermore, in 
the neon color illusion color diffuses from colored parts of, for 
example, multiple concentric lines and is blocked by illusory 



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Hazenberg and van Lier 



Afterimage watercolors 




Appearance Reddish Greenish 



FIGURE 1 | Illustration of the stimuli used in van Lier et al. (2009). The 

first row shows the stimuli. The second row shows the resulting percepts. 



contours (van Tuijl, 1975; Bressan et al, 1997; van Lier, 2002). 
The typical percept is of an illusory colored transparent disc float- 
ing in front of the inducing elements. Finally, Pinna et al. (2001) 
developed a clear demonstration that even a pair of juxtaposed 
thin colored undulating outlines induces color filling-in, i.e., the 
so-called watercolor illusion. Figure 2A shows a typical example 
of the watercolor illusion. By positioning the light orange outline 
on the inside and the dark purple outline on the outside of the 
star-like shape, the interior of the shape appears to be filled- in 
with a color that is similar, but less saturated than the color of the 
inner contour. When the colors of the outlines are reversed, color 
appears to spread outwards (Figure 2B). Color then, spreads in 
the direction where the luminance contrast between the con- 
tour and the background is lowest. In addition, the strength of 
the effect depends on luminance contrast between the two out- 
lines. When the outlines are isoluminant, color spreading is rather 
weak and appears to spread both inwards and outwards, but 
becomes more vivid when the luminance contrast between the 
outlines is enhanced (Devinck et al, 2005). Additionally, Cao 
et al. (2011) showed that there is an optimal contrast after which 
color spreading diminishes again. 

In any case, it seems that the appearance of surface color 
strongly depends on edge information. This is in line with a 
study in which the activity of single neurons in VI and V2 were 
recorded (Friedman et al., 2003). They showed that some cells 
that code for color are also orientation selective, indicating that 
the representation for form and color are tightly linked (Von der 
Heydt and Pierson, 2006). Activity in these cells might be respon- 
sible for filling-in of both afterimage colors and "real" colors. 
Similarly, activity of cells with receptive fields along the edges 
might also underlie perceived contrast induction of colors across 
edges. Indeed, in a version of the watercolor illusion in which the 
inner contour of a shape was achromatic (black) and the outside 
contour was colored, a complementary color was perceived in the 
interior of the shape (Pinna, 2006). 




FIGURE 2 | Examples of the watercolor illusion. (A) By juxtaposing the 
lighter colored contour (orange) to a darker colored contour (purple) inside 
an enclosed area, color appears to spread inwards and uniformly fills the 
interior of the star-like shape. (B) Reversing the colors of the contour 
results in color spreading outwards. 



Given the similarity of the observed interactions between col- 
ors and luminance contours in both "real" colors and afterimage 
colors it seems plausible that the observed effects tap from a 
common mechanism. In this study, we further explored this by 
comparing performance on two color judging experiments in 
which filling-in could be induced by afterimage colors or by "real" 
colors. Similar to the stimuli used to investigate the watercolor 
illusion, we only used thin outlines. In Experiment 1, we first 
investigated whether the afterimage filling-in effects described by 
van Lier et al. (2009) also occur when using thin colored outlines 
similar to the watercolor illusion. In Experiment 2, we used such 
stimulus configurations to compare filling-in of afterimage colors 
with filling-in in the watercolor illusion. 

EXPERIMENT 1 

In this experiment, a chromatic contour alternated over time with 
an achromatic contour. Thus, when the achromatic contour is 
presented, an afterimage of the previously presented chromatic 
contour should be perceived. The chromatic contour could be 
positioned inside or outside a subsequently presented achromatic 
contour. Participants had to judge the color of the interior of the 
achromatic outline. If afterimage colors of thin outlines are suffi- 
cient to induce color filling-in, the interior of the figures should 
reveal complementary color filling-in when the chromatic out- 
line is placed inside the achromatic outline, whereas filling-in 
of a similar color as the chromatic outline should be induced 
when this outline is positioned outside the achromatic outline. 
Following van Lier et al. (2009) we further expected to find weaker 
color filling-in for outer chromatic contours as compared to inner 
chromatic contours. 

METHODS 

Participants 

Twenty-one observers participated in Experiment 1 (aged 17-24, 
one participant had an age of 64; six males). All had normal or 
corrected to normal visual acuity. In addition, all participants 
had normal color vision as screened with the AO Hardy Rand 
and Rittler Pseudoisochromatic Plates (2nd edition). Participants 
received payment or course credits. All participants were naive to 
the purpose of the experiment. 



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Afterimage watercolors 



Stimuli 

Stimuli were presented on a gray background [ClE^ y ) = 
0.3128,0.3303] with a luminance of 73.87 cd/m 2 . The stimuli 
consisted of closed, arbitrarily undulating, contours. Three out- 
lines or contours were created that differed in shape, size, and 
thickness (see Figure 3 for examples of the contours). A second 
set of contours was created such the shapes of these contours 
fit neatly within the shapes of the former set of contours. Thus, 
for each shape in Figure 3, we distinguished between an "outer" 
contour and an "inner" contour. 

Both inner and outer contours could be used as adapting 
stimuli or as test stimuli. The contours of the chromatic adapting 
stimuli could be colored either green [CIE( Xj y) = 0.3514, 0.4417; 
L = 65.69 cd/m 2 ], orange [CIE (xj) = 0.3774, 0.3694; L = 
58.65 cd/m 2 ], blue [CIE (x , y) = 0.2600, 0.3058; L = 52.90 cd/m 2 ], 
or pink [CIE (xj) = 0.3814, 0.2733; L = 29.87 cd/m 2 ]. The 
colors were chosen such that two colors were approximately 
complementary to each other (e.g., orange-blue; pink-green). 
The interior of the adapting contour was gray having a luminance 
that was equal to the luminance of that contour. This was done as 
luminance borders between the inducing color and the interior 
area may block afterimage filling-in (van Lier et al., 2009). The 
contours of the achromatic test stimuli were gray and had a 
luminance of 55 cd/m 2 . Note that all stimuli were darker than 
the background. Examples of stimuli in which the inner contour 
is the adapting chromatic contour are shown in Movie 1 and 
examples of stimuli in which the outer contour is the adapting 
chromatic contour are shown in Movie 2. 

Procedure 

The experiment was run on a PC and an 18-inch CRT monitor 
with a 120 Hz refresh rate. The monitor was calibrated using an 
X-Rite Color Monitor Optimizer. The experiment was designed 
and presented using Presentation (Version 14.8, Neurobehavioral 
Systems, Inc.). 

Once written informed consent was given and the instruc- 
tions were read, the experiment started. The sequence of events 
is shown in Figure 4. Each trial started with the presentation 
of a small fixation dot that was presented on the center of the 
screen for 1000 ms. Afterwards a chromatic adapting stimulus 
was presented for 1000 ms which was followed by an achromatic 
test stimulus that was presented for another 1000 ms. The adapt- 
ing stimulus and test stimulus kept alternating until a response 
was made. Once a response was made, the next trial started 
automatically after 1000 ms. 




FIGURE 3 | Examples of adapting stimuli. 



We distinguished between two types of trials (See Figure 4). 
In the first type, the outer contour was the chromatic adapting 
stimulus and the inner contour was the achromatic test stim- 
ulus. In the second type, the inner contour was the chromatic 
adapting stimulus, while the outer contour was the achromatic 
test stimulus. Participants were instructed to judge whether the 
inside of the achromatic test stimulus appeared to be filled in with 
a color. In order to respond, five disks, four of which were col- 
ored, were shown at the bottom of the screen. The colors of the 
response disks were similar to the inducing colors. When partic- 
ipants perceived color filling-in, they had to choose which of the 
colored response disks best matched the perceived color. The fifth 
response disk comprised the same gray as the background and 
could be chosen whenever participants did not perceive any color 
filling-in. Participants were asked to observe at least three pre- 
sentation cycles (chromatic contour, achromatic contour) before 
responding. Responses were given by pushing one of five buttons 
corresponding to the five disks. 

There were 24 unique trials; stimulus shape (3 levels) x color 
(4 levels) x trial type (2 levels). Prior to the experiment, partic- 
ipants completed a small practice block consisting of three trials 
to get familiar with the task. The main experiment was admin- 
istered in four blocks. In each block, each of the 24 trials was 
presented once in a randomized order. Participants controlled the 
time between the blocks and started the next block by pressing 
one of the five response buttons. 

RESULTS 

One participant was unable to perceive any afterimages and was 
not included in the analysis. In Figure 5 the responses are plotted. 
The responses for the colors orange, blue, pink and green were 
assigned to coordinates (1, 0); (-1, 0); (0, 1); (0, -1), respectively. 
For example, a 100% response for the color orange is represented 
by plot coordinate (1, 0). Additionally, a no color response was 
assigned to coordinates (0, 0). Figure 5 shows the coordinate plots 



color 
outside 



color 
inside 




1000 ms 1000 ms 
► 



FIGURE 4 | Sequence of events in Experiment 1. Upper row: an example 
of a trial in which the outer contour is the chromatic stimulus and the inner 
contour is the achromatic test contour. Lower row: an example of a trial in 
which the inner contour is the chromatic stimulus and the outer contour is 
the achromatic test contour. 



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Hazenberg and van Lier 



Afterimage watercolors 



pink 
1.0 



blue — O 



1.0 ~ -0.5 n ao 



-0.5 



O Color inside 
□ Color outside 



□ 



-1.0 1 
green 



FIGURE 5 | Mean response coordinates when the inner or the outer 
contour was the adapting chromatic stimulus (indicated by circles and 
squares, respectively). The color of the symbols within the graph depict 
the colors of the adapting stimuli (e.g., the rightmost blue circle indicates 
the mean response for the adapting stimuli in which the blue contour was 
positioned inside the gray contour). 



for trials in which the inner contour was the chromatic con- 
tour and for trials in which the outer contour was the adapting 
contour. 

To analyze the data, we calculated the proportions each partic- 
ipant responded with "same color" "complementary color" and 
"no color." The resulting proportions were transformed using the 
arcsine transformation in order to obtain a normal distribution 
of the data. All statistical tests were performed on these trans- 
formed proportions. Paired f-tests showed that when the inner 
contour was chromatic, the proportion complementary color 
responses was larger as compared to the proportion same color 
responses [t(i9) = 11.98, p < 0.001]. In contrast, when the outer 
contour was chromatic, there were more same color responses as 
compared to complementary color responses [t(\9) = 5.18, p < 
0.001]. We also compared the proportions no color responses to 
check whether the probability of perceiving any color filling- in 
differed between conditions. A paired f-test revealed significant 
results [t(i9) = 2.41, p < 0.05], showing that participants were 
more likely to respond with no color when the outer contour was 
chromatic as compared to when the inner contour was chromatic. 

DISCUSSION 

The results showed that afterimages of chromatic outlines filled in 
regions that were not adapted to color. The color of the filled-in 
region depended on the position of the chromatic contour in the 
adapting stimulus: when the chromatic contour was positioned at 
the inside, filling- in of the complementary color in the inner area 
was perceived; when the chromatic outline was positioned out- 
side, filling-in of the same color in the inner area was perceived. 
Afterimage filling-in induced by the outer contour appeared less 
strong as compared to afterimage filling-in induced by the inner 
chromatic contour, although there appears to be some variability 
within the different colors (e.g., compare green inside and outside 
in Figure 5 with the other colors). 

The fact that thin colored outlines were sufficient to induce 
spreading within closed boundaries strengthened our initial idea 



that the filling-in mechanisms triggered by afterimage colors pro- 
ceeds in a similar fashion as the filling-in processes that are at 
work in the original watercolor illusion. The next experiment was 
set up to further test similarities between both types of filling-in. 

EXPERIMENT 2 

We used a color judging experiment similar to Experiment 1. In 
addition to presenting the chromatic and achromatic contours 
alternately, watercolor-like stimuli were created by presenting the 
contours simultaneously. We expected filling-in of different col- 
ors to depend on whether the contours alternated or whether they 
were presented simultaneously. For example, an inner chromatic 
contour should induce color spreading of a similar hue when the 
contours are presented simultaneously, but the same chromatic 
contour should induce complementary color filling-in when the 
contours alternated. Previous studies found that the strength of 
the watercolor illusion depends on the asymmetric luminance 
profiles of both contours (Pinna et al., 2001; Devinck et al., 2005). 
To test the effect of luminance contrast between chromatic and 
achromatic outlines, we used two kinds of achromatic contours. 
One was of a same gray as in Experiment 1, while the other was 
much darker (i.e., black). We expected that when the contours 
were presented simultaneously, stronger color filling-in should 
be perceived when black achromatic contours were used as com- 
pared to gray achromatic contours. Note that in Experiment 1, the 
inner area was isoluminant to the chromatic contour to enhance 
filling in. However, to allow a better comparison with the usual 
static watercolor illusion, in the current experiment the inner area 
has the same luminance as the surrounding background, which is 
different from the luminance of the chromatic contour. 

METHODS 

Participants 

Twenty observers participated in Experiment 2 (aged 17-24; four 
male). All had normal or corrected to normal visual acuity. In 
addition, all participants had normal color vision as screened 
with the AO Hardy Rand and Rittler Pseudoisochromatic Plates 
(2nd edition). Participants received payment or course credits. All 
participants were naive to the purpose of the experiment. 

Stimuli 

The stimuli we used in this experiment were similar to those we 
used in Experiment 1, but with the following changes. Firstly, in 
addition to presenting the contours in an alternating fashion, the 
chromatic and achromatic contours where also presented simul- 
taneously to induce the classic watercolor illusion. Examples of 
these stationary stimuli are shown in Figure 6. Secondly, the inte- 
rior of the adapting contours was always of the same white as 
the background. In order to enhance the watercolor effect, stim- 
uli were presented on a white background (100 cd/m 2 ). Lastly, in 
addition to the gray contour that was used in Experiment 1, we 
also used black contours (0.39 cd/m 2 ). 

Procedure 

For the alternating presentation condition, the same procedure 
as in Experiment 1 was followed. Similarly, in the simultaneous 
presentation condition, participants had to judge whether the 
interior of the stimulus appeared colored or not, using the same 



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Hazenberg and van Lier 



Afterimage watercolors 



response categories as in Experiment 1 . The stimuli remained on 
the screen until participants responded. 

There were 96 unique trials: stimulus shape (3 levels) x color 
(4 levels) x trial type (2 levels; inner or outer contour was chro- 
matic) x presentation type (2 levels; alternating or simultaneous) 
x achromatic contour (2 levels; black or gray). Each unique trial 
was presented twice. The experiment was administered in four 
blocks in which the variables presentation type and achromatic 
contour were blocked while the other trials were presented ran- 
domly. The presentation order of the blocks was counterbalanced 
across participants. In each block, each of the 24 trials was pre- 
sented once in a randomized order. Participants controlled the 
time between the blocks and started the next block by pressing 
one of the five response buttons. 



color inside color outside 




FIGURE 6 | Example stimuli that were used to induce the watercolor 
effect. Both the inner contour (left panels) and the outer contour (right 
panels) could be chromatic. The achromatic contour could either be gray 
(upper panels) or black (lower panels). 



RESULTS 

We have plotted the results in a similar fashion as the results of 
Experiment 1. First we consider the results of trials when the 
achromatic contours were gray (see Figure 7). 

The results were analyzed according to the response cate- 
gories specified in Experiment 1. As in Experiment 1, all analyzes 
were performed on the arcsine transformation of the propor- 
tions. As can be seen in Figure 7, for the alternating presentation 
condition, when the outer contour was chromatic, the propor- 
tion participants responded with the same color was greater as 
compared to the proportion participants responded with the 
complementary color [t(i9) = 4.13, p < 0.01]. However, when 
the inner contour was chromatic, the same color responses and 
the complementary color responses did not appear to differ 
(p > 0.1). For the simultaneous presentation condition, the same 
color response prevailed as compared to the complementary color 
response for both the inner and the outer chromatic contour 
[t (19) = 7.80, p < 0.001; f(i 9 ) = 3.53, p < 0.01; respectively]. To 
analyze whether the probability of perceiving any color filling- 
in differed between conditions, we also compared the no color 
responses. Paired f-tests showed no difference for the alternating 
presentation condition (p > 0.1). For the simultaneous presenta- 
tion condition, participants were more likely to respond with no 
color when the outer contour was chromatic as compared to when 
the inner contour was chromatic [t(i9) = 3.89, p < 0.01]. 

Next we consider responses on the same conditions, but now 
for the stimuli with black achromatic contours (See Figure 8). 

As can be seen in Figure 8, the same pattern as before was 
found. For the alternating presentation condition, when the outer 
contour was chromatic, the proportion participants responded 
with the same color was greater as compared to the proportion 
participants responded with the complementary color [t(i9) = 
3.40, p < 0.01]. However, when the inner contour was chro- 
matic, the same color responses and the complementary color 
responses did not differ (p > 0.1). For the simultaneous pre- 
sentation condition, participants were more likely to respond 
with the same color response as compared to the complementary 
color response for both the inner and the outer chromatic 



Alternating 



pink 
1.0 



Simultaneous P' nk 

1.0 



blue 



1X) Dl5 ^00 



O U o.5 



-o-n 



-1.0 
green 



orange blue 



0.5 

( 

c 

~o %i TO 

-o9 



O Color inside 
□ Color outside 



-1.0 
green 



FIGURE 7 | Mean response coordinates when the inner or the outer contour 
was the adapting chromatic stimulus (indicated by circles and squares, 
respectively) and the achromatic contour was gray. Left panel: mean 



responses for the alternating presentation condition. Right panel: mean 
responses in the simultaneous presentation condition. The color of the 
symbols within the graph depicts the colors of the adapting stimuli. 



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October 2013 | Volume 4 | Article 707 | 5 



Hazenberg and van Lier 



Afterimage watercolors 



Alternating ^P ink 



0.5 



□ 




-0.5 



-1.0 
green 

FIGURE 8 | Mean response coordinates when the inner or the outer contour 
was the adapting chromatic stimulus (indicated by circles and squares, 
respectively) and the achromatic contour was black. Left panel: mean 



contour [f (19) = 8.80, p < 0.001; f ( i 9) = 2.48, p < 0.05; respec- 
tively] . Paired t-tests on the proportions participants responded 
with no color revealed no difference for the alternating pre- 
sentation condition (p > 0.1). However, for the simultaneous 
presentation condition, participants were more likely to respond 
with no color when the outer contour was chromatic as com- 
pared to when the inner contour was chromatic [f(i9) = 3.43, 
p < 0.01]. 

For the alternating presentation condition, we noticed that 
when black achromatic contours were used, color filling- in 
appeared to be attenuated as compared to when the lighter 
gray achromatic contours were used. Paired f-tests confirmed 
this, showing that for both inner [t(i$) = 2.85, p < 0.05] and 
outer chromatic contour [t(i9) = 5.18, p < 0.001], the propor- 
tion participants responded with no color was greater when black 
contours were used as compared to when gray contours were 
used. No such effect was found for the simultaneous presentation 
condition. 

DISCUSSION 

In this experiment we compared filling- in of afterimage col- 
ors with filling-in of "real" colors. We used chromatic and 
achromatic contours that could be presented simultaneously 
or alternately. When gray achromatic contours were used, for 
the alternating condition, an outer chromatic contour induced 
filling-in of a similar color whereas an inner chromatic con- 
tour hardly induced filling-in of the expected complementary 
color. In contrast, for the simultaneous presentation condition, 
the probability of perceiving color filling-in was greater for 
an inner chromatic contour as compared to an outer contour. 
Furthermore, filling-in induced by an inner or an outer chro- 
matic contour was most likely to be similar to the color of the 
contour. Although we expected to find stronger color spreading 
for the simultaneous presentation condition when black achro- 
matic contours were used, no such effect was found. Moreover, 
the use of black contours in the alternating presentation condition 
greatly diminished filling-in induced both by an inner and outer 
chromatic contour. 



Simultaneous P' nk 

1.0 



0.5 

• ft 



O0.5 




orange 



-1.0 
green 

responses for the alternating presentation condition. Right panel: mean 
responses in the simultaneous presentation condition. The color of the 
symbols within the graph depicts the colors of the adapting stimuli. 



At first glance, the results from the alternating condition in 
the present experiment appear to be at odds with the results 
from Experiment 1. In the first experiment, an inner chromatic 
contour generally induced stronger afterimage filling-in as com- 
pared to an outer chromatic contour, while this did not reveal a 
significantly different effect in Experiment 2. A plausible cause 
for these apparent differential results lies in the different back- 
ground luminance. More in particular, in Experiment 1, the 
interior area of the adapting stimulus was always isoluminant 
with the chromatic contour, while in this experiment, the inte- 
rior area was of a different luminance. As mentioned, this was 
done to allow a better comparison with the typical watercolor 
displays, but it also caused a luminance border between the 
inner chromatic contour and the interior area. This luminance 
border, which likely remained in the afterimage (with a con- 
trast polarity in the opposite direction), apparently prevented 
the colored afterimage of the chromatic contour from spread- 
ing. In contrast, color filling-in by an outer chromatic contour 
should be influenced less by such afterimage luminance bor- 
der as this type of filling-in depends on color induction across 
luminance borders (Anstis et al, 1978). Indeed, comparing the 
plot in Figure 5 with the left plot in Figure 7, it appears that color 
spreading induced by the inner chromatic contour was atten- 
uated in Experiment 2 (running additional independent f-tests 
confirmed this observation [t@$) = 2.65, p < 0.05], while the 
strength of color spreading induced by the outer chromatic con- 
tour appears more or less the same across experiments [p > 0.1]). 
When black achromatic contours were used, the greater lumi- 
nance contrast in the afterimage appeared to interfere with both 
effects. 

Note that although afterimages of a luminance border between 
a contour and the adjacent area may weaken or even prevent 
the spreading of afterimage colors, similar luminance borders 
do not prevent color spreading of "real" colors in the cur- 
rent watercolor displays. In fact, when contours were presented 
simultaneously, we replicated previous findings on the water- 
color illusion (Pinna et al., 2001; Devinck et al., 2005; Cao et al., 
2011) and showed that an inner chromatic contour triggered 



— orange blue — 
1.0 -1.0 

O Color inside 
□ Color outside 



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Hazenberg and van Lier 



Afterimage watercolors 



same-color filling-in. Apparently, afterimage filling-in is more 
sensitive to luminance borders between the inducing contours 
and the adjacent area than filling-in triggered by "real" col- 
ors. Note that the current difference in strengths in fact deal 
with the induction of the filling-in, i.e., the flow from con- 
tour colors to the adjacent area. This is different from the 
earlier observation that, once afterimages are generated, their per- 
ceived strength strongly depends on the position of luminance 
contours (e.g., Daw, 1962; van Lier et al., 2009; Powell et al., 
2012). 

An additional difference between the current filling-in of after- 
image colors and "real" colors appeared when filling-in was 
triggered by the outer chromatic contours. For afterimage filling- 
in, our results were in line with Experiment 1 and previous 
research (Anstis et al., 1978; van Lier et al., 2009). As it is likely 
that the effect depends on contrast induction (either of after- 
image colors or of "real" colors), we also expected to find, like 
Pinna (2006), contrast induction when "real" colors were used. 
However, this is not what we found; not only was color filling- 
in triggered by the outer contours stronger for afterimage colors 
as compared to "real" colors, color filling-in triggered by "real" 
colors was, unexpectedly, more likely to be of the same color as 
the inducing contour (see Figures 7, 8, right plots). We speculate 
that given the very weak color appearance for these configura- 
tions, color judgments were biased toward the color of contour 
that was actually present in the display. The use of different stim- 
ulus configurations across studies may further offer a solution 
for the diverging results. To illustrate this, consider the shapes in 
Figure 9. In the upper left and upper right, one of the shapes used 
in our experiment (Figure 9 A) and an angular version of that 
shape (Figure 9B) are shown. Both shapes produce rather weak, 
but an approximately similar color appearance based on contrast 
induction (i.e., orangish filling-in) in the interior area, indicating 
that the type of contour (smoothly undulating or angular) does 
not seem to matter much. However, the effect can be enhanced 
by reducing the to be filled-in area by adding a gray contour 
inside the inner area (Figure 9C). The enclosed area between 
the gray contours now reveals a stronger orangish impression. 
In Figure 9D we have added an additional blue contour outside 
that area. As a consequence the inner part of the figure has a 
bluish tint and the area between the gray contours appears even 
more orangish. In fact, the latter figure is similar to the examples 
provided by Pinna (2006). These informal observations illustrate 
that our stationary stimuli were perhaps not optimized for strong 
filling-in based on contrast induction. Note further that, contrary 
to shapes such as Figure 9D in which contrast induction can be 
compared with watercolor filling-in within the inner region, our 
stimuli were presented in isolation, which may have made the task 
of judging color filling-in of an already weak effect even more 
challenging. 

GENERAL DISCUSSION 

We showed that, similarly to the watercolor illusion, afterim- 
ages of thin colored outlines spread within a region bounded by 
luminance contours. Spreading induced by an afterimage of an 
inner chromatic contour appears stronger as compared to spread- 
ing induced by an afterimage of an outer contour. The probability 




FIGURE 9 | Examples of shapes that may induce contrast induction of 
the blue contour. (A) A slightly bigger version of a shape that is taken from 
Experiment 2. (B) An angular version of panel (A). (C) Similar to panel (B), 
but with an additional gray contour. (D) A jagged annulus that is flanked on 
both sides by a blue contour. 



of perceiving filling-in depended on whether it is induced by 
"real" colors or afterimage colors. For instance, it appears that 
spreading of afterimage colors can be more easily affected by 
changes in luminance settings as compared to spreading of "real" 
colors. In addition, afterimage colors were, in contrast to "real" 
colors, more likely to induce color contrast across boundaries. 

Filling-in demonstrated in this study may be related to other 
filling-in phenomena that appear to depend on mechanisms that 
are related to boundary processing. For example in Troxler fad- 
ing, prolonged fixation causes stimuli in the periphery (Troxler, 
1804), or, as has been shown more recently, even entire scenes 
(Simons et al, 2006) to disappear from view. As adaptation 
causes luminance boundaries to break down, color may spread 
beyond that boundary. Filling-in due to Troxler fading has been 
studied using "real" colors (Hamburger et al, 2005) and after- 
image colors (Hamburger et al, 2012). They also found that 
different types of filling-in triggered by "real" colors and after- 
image colors did not perfectly match. As has been mentioned 
in the introduction, another possible related instance of filling- 
in occurs in the neon color illusion. Afterimages of neon color 
spreading have also been reported (Shimojo et al., 2001), which 
have been shown to be the result of adaptating to the illusory 
filled-in surface. However, in our demonstrations the situation 
is different. As the chromatic contours in our stimuli are not 
likely to induce filling-in or contrast induction by themselves 
(a second contour is necessary), it is likely that for our stim- 
uli, filling-in mechanisms act on the afterimage of the chromatic 
contours instead. 

Several theories of form and surface perception account for 
filling-in phenomena (Komatsu, 2006). For example (Grossberg 
and Mingolla, 1985) proposed that visual input is processed into 
two parallel systems; a boundary contour system and a feature 
contour system. Boundary and edge information are processed 
in the boundary contour system, while feature information such 



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October 2013 | Volume 4 | Article 707 | 7 



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Afterimage watercolors 



as color and brightness are processed in the feature contour sys- 
tem. A perception of a surface is formed when information of 
both systems are combined. Color and brightness information in 
the feature contour system spread across surfaces and are bound 
by edge information in the boundary contour system. The the- 
ory has been used to explain filling-in phenomena such as the 
neon color effect (Grossberg and Mingolla, 1985) and also the 
watercolor illusion (Pinna and Grossberg, 2005). In fact, the lat- 
ter authors constructed a stimulus, the so-called two -dot limiting 
case, to explain both similarities and differences between the neon 
color illusion and the watercolor illusion. 

Recently Francis (2010) used a model based on the boundary 
contour system and the feature contour system to simulate the 
perceived afterimages in van Lier et al. (2009). Initially, the results 
from the model were consistent with the idea that boundaries 
block color spreading. In a second study however, some predic- 
tions of the model did not entirely match their experimental data 
(Kim and Francis, 2011). Particularly, in one instance they used 
similar star-like stimuli (see Figure 1) as used in van Lier et al. 
(2009), but varied the size of the test outline. Contrary to pre- 
dictions of the model, their experimental results revealed that 
afterimage filling-in was less likely to be perceived when the test 
contour did not exactly match the edges of the inducing color. 
The results were explained by the fact that for a smaller test con- 
tour, the region that receives afterimage color signals is relatively 
smaller, while for larger test contour, a larger region had to be 
filled in. Both of these factors may have diluted color spreading. 
An alternative interpretation is that the alignment of a test con- 
tour with the edges of the inducing colored region is important 
for filling-in to occur, because of repeated activation of orienta- 
tion selective neurons that are also coding for color (Friedman 
et al., 2003). Kim and Francis (201 1 ) additionally found that when 
the test contour was larger as compared to matching contours, 
the probability of perceiving filling-in of an unexpected color 
became higher, possibly due to the fact that larger test contours 
included afterimages from two complementary colors. However, 
when the test contour was smaller than the inducing colored 
region, the probability of perceiving no color filling-in became 
higher. In addition to their explanation, this result may have been 
caused by the fact that while one portion of the inducing colored 
region fell inside the test contour, another portion fell outside the 
test contour. It is possible that the similarly colored afterimages 
induced by these outer colors negated the complementary colored 
afterimages induced by the inner colors. 

To illustrate this, consider the examples in Movie 3. In addition 
to adapting figures with only one contour (on the right), we cre- 
ated adapting figures comprising two chromatic contours, one of 
which is positioned inside and the other outside the test contour 
(on the left). When both contours are of the same color, com- 
plementary colored filling-in induced by the inner contour and 
similarly colored filling-in induced by the outer contour appear 
to cancel each other out, so no color filling-in is perceived (com- 
pare top left with top right animation). When the color of the 
outer contour is changed to purple (bottom left), both inner and 
outer contour induce approximately the same purplish colored 
afterimage, leaving a stronger purplish impression as compared 



to when only an inner green contour is used (compare bot- 
tom left with bottom right). Another example is provided in 
Movie 4. Here the chromatic contour is sequentially followed by 
two achromatic contours, juxtaposed to the chromatic contour; 
one positioned inside the interior area, and one positioned out- 
side the interior area. After viewing a few cycles (remain fixated 
on the central dot) one may see a color change with regard to the 
afterimage filling-in, corresponding with the position of the con- 
tour. These examples illustrate that both effects play an important 
role in afterimage filling-in and should be integrated in any model 
accounting for afterimage filling-in. 

All in all, the current color filling-in triggered by alternat- 
ing juxtaposed chromatic and achromatic contours largely reveals 
similar phenomenological impressions as the original watercolor 
illusion. Nevertheless, there are also different sensitivities for both 
types of filling-in. It should be noted, however that phenomeno- 
logical differential effects for "real" colors and afterimage colors 
do not necessarily point to fundamentally different processing 
mechanisms between these types of color. For example, it has 
been shown that when having appropriate color settings (e.g., 
having "real" colors that are more comparable to the relatively 
weak and desaturated afterimage colors), both types of colors 
can be effectively blocked and gated by luminance contours (e.g., 
Anstis et al., 2012a,b). Further investigations (e.g., Powell et al., 
2012) may clarify whether different sensitivities are merely a 
result of different stimulus parameters and color properties (like 
saturation) or whether they are caused by different underlying 
mechanisms. The current afterimage watercolors may provide a 
suitable entrance to further examine the conditions under which 
colors straddle the boundaries. 

SUPPLEMENTARY MATERIAL 

The Supplementary Material for this article can be found 
online at: http://www.frontiersin.org/journal/10.3389/fpsyg. 
2013.00707/abstract 

Movie 1 | Examples of stimuli in which the inner contour is the chromatic 
adapting contour. Observers tend to perceive color filling-in 
complementary to the color of the adapting contour. Note, the effect 
appears somewhat stronger after a couple of alternations (please set your 
video player to loop the movie and fixate on the small dot in the middle of 
the four stimuli). 

Movie 2 | Examples of stimuli in which the outer contour is the chromatic 
adapting contour. Observers tend to perceive color filling-in similar to the 
color of the adapting contour. 

Movie 3 | Examples of stimuli with two chromatic adapting contours. In 

the top left a stimulus is shown comprising both an inner and an outer 
green adapting contour. Compared with the figure in the top right, color 
spreading appears less strong, because filling-in induced by both contours 
appear to cancel each other out. In contrast, when the outer contour is 
changed to purple (lower left), both contours induced a similar colored 
afterimage. The color should be similar to color filling-in induced by the 
stimulus in the lower right. 

Movie 4 | Examples of chromatic contours that are followed subsequently 
by two different achromatic contours. After a few cycles the color of the 
afterimages switches according to the achromatic contour. 



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Conflict of Interest Statement: The 

authors declare that the research 
was conducted in the absence of any 
commercial or financial relationships 
that could be construed as a potential 
conflict of interest. 



Received: 31 May 2013; paper pend- 
ing published: 24 June 2013; accepted: 
16 September 2013; published online: 08 
October 2013. 

Citation: Hazenberg S J and van Lier 
R (2013) Afterimage watercolors: an 
exploration of contour-based afterim- 
age filling-in. Front. Psychol. 4:707. doi: 
1 0. 3389/fpsyg.201 3. 00707 
This article was submitted to Perception 
Science, a section of the journal Frontiers 
in Psychology. 

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