Visual filling-in relates to a perceptual phenomenon in which a stimulus pattern apparently undergoes dynamic changes assuming an attribute such as colour, texture, or brightness from the surround. This perceptual completion effect has up to now been shown only for real images. Here, we present filling-in in negative afterimages, a phenomenon not yet reported. Using coloured disk-ring patterns for stimuli, we demonstrate that afterimage filling-in arises independently, and is not simply a replica of filling-in observed in real images. Such filling-in does not occur when the afterimage is elicited dichoptically, suggesting its emergence within the monocular visual pathway. In this way, our findings indicate that filling-in under certain conditions may derive from an active neural mechanism located at low levels of the visual pathway

Adam Geremek

Kai Hamburger

Lothar Spillmann

English

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 An illusion induced by an illusion – 
perceptual filling-in of coloured negative afterimages 
Adam Geremek, Kai Hamburger & Lothar Spillmann 
 
Visual filling-in relates to a perceptual phenomenon in which a stimulus pattern 
apparently undergoes dynamic changes assuming an attribute such as colour, texture, or 
brightness from the surround. This perceptual completion effect has up to now been 
shown only for real images. Here, we present filling-in in negative afterimages, a 
phenomenon not yet reported. Using coloured disk-ring patterns for stimuli, we 
demonstrate that afterimage filling-in arises independently, and is not simply a replica 
of filling-in observed in real images. Such filling-in does not occur when the afterimage 
is elicited dichoptically, suggesting its emergence within the monocular visual pathway. 
In this way, our findings indicate that filling-in under certain conditions may derive 
from an active neural mechanism located at low levels of the visual pathway. 
The study of visual illusions delivers insights into the operating mode of the visual 
system. For instance, with prolonged fixation of a coloured disk-ring pattern, the disk assumes 
the colour of the ring (filling-in) or, less frequently, the ring assumes the colour of the disk 
(filling-out). If one turns the gaze subsequently to these spreading effects and views a white 
background, a negative afterimage of the original stimulus is perceived initially. For example, 
the formerly red ring looks greenish, and the formerly green disk appears reddish.  Yet, soon 
thereafter one of the two colours in the afterimage will give way to the other, producing 
filling-in (i.e., all greenish) or filling-out (i.e., all reddish) in the afterimage as happened in the 
real image during adaptation. The question arises whether these filling-in effects in the 
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afterimage follow the same rules as those observed in the real image or whether they arise 
independently. 
To answer this question, we used binocularly viewed stimulus patterns consisting of a 
coloured disk surrounded by two coloured rings, as shown in Fig. 1 (top). Using such stimulus 
patterns, Hamburger and colleagues1 obtained 4 types of spreading effects, enabling us to 
compare filling-in and filling-out in real images with the effects observed in negative 
afterimages. Figure 1 (bottom) illustrates the 4 modes of spreading: In Effect 1 (E1), the 
colour of the outer ring fills in the inner ring and the enclosed disk, yielding a uniformly 
coloured percept. In Effect 2 (E2), the colour of the inner ring fills in the disk, while the 
colour of the outer ring remains unchanged. In Effect 3 (E3), the colour of the outer ring fills 
in the inner ring, while the colour of the disk remains unchanged. In Effect 4 (E4), colour 
spreading occurs in the opposite direction: from the disk onto the inner ring or from the inner 
ring onto the outer ring, i.e., filling-out (due to the infrequent occurrence of filling-out 
compared to filling-in, both filling-out modes were merged in Effect 4). Finally, Effect 0 (E0), 
not shown here, denotes that the initial percept of the stimulus remains unchanged (neither 
filling-in nor filling-out). 
Our hypothesis was that there should be no difference as to the type and frequency of 
induced colour changes during adaptation to the physical stimulus (i.e., in the real image) and 
in the subsequent afterimage. However, this was not what we found. In fact, when we 
compared the spreading effects observed under the two conditions, they differed in 80.66 % 
of all observations (Fig. 2, second column).  For example, during adaptation to the stimulus, 
both the inner ring and disk may have become entirely filled in by the colour of the outer ring 
(E1), while in the subsequent afterimage the colour of the disk may have spread outward from 
the disk onto the inner ring or from the inner ring onto the outer ring, i.e., filling-out (E4). 
Furthermore, filling-in or filling-out occasionally observed in the negative afterimage, when 
no completion effects occurred in the real image during the preceding adaptation period (E0). 
 3
A statistical analysis (Pearson correlation coefficient) between the two kinds of results 
revealed that there were no systematic correlations between the modes of colour filling 
perceived in the real image and in the subsequent afterimage (for the statistical analysis see 
caption Figure 2). 
Since opponent colours are known to be differently strong inducers1, we tested in a 
control experiment colour spreading in a stimulus whose colours were complementary to 
those in the original stimulus. Specifically, we used a green/red/yellow stimulus pattern (outer 
ring/inner ring/disk) instead of a red/green/blue stimulus, and, vice versa, a yellow/red/green 
pattern instead of a blue/green/red stimulus. This was done so that the colours of the 
afterimage would conform to the colours of the original stimulus. Yet, also for this condition 
we found a difference of 67.34 % between the type of colour spreading in the real image and 
in the subsequent afterimage (Fig. 2, fourth column). Furthermore, there were no systematic 
correlations between the filling-in of real stimuli and filling-in of afterimages having the same 
colours (cross-comparison). 
These data suggest that filling-in (or filling-out) of the complementary colours in the 
negative afterimage do not follow the same rules as filling-in (or filling-out) in the real image. 
We therefore assume that afterimages are treated by the visual system as independent 
“stimuli” capable of eliciting perceptual filling-in effects of their own, suggesting that colour 
spreading in the afterimage arises independently of the real image. 
A phenomenological effect apparently similar to ours was reported by Shimojo and 
colleagues2. After prolonged adaptation to a Varin figure3, consisting of four white ‘pacmen’ 
in the corners, with four red wedges abutting the sides, they observed an illusory square in the 
complementary colour (green) to that of the four wedges filling the space between the four 
black pacmen. Shimojo et al. concluded that the complementary colour of the filled-in square 
was an afterimage of the red square induced by the four red wedges during adaptation, 
implying that filling-in occurred first, with the negative afterimage of the filled-in image 
 4
occurring in second place. In contrast, we argue that afterimage filling-in in our experiment is 
prompted by the afterimage of the primary stimulus pattern, acting as an inducing stimulus. 
One fundamental question about filling-in is the presumptive site of origin within the 
visual system. In general it is commonly assumed that filling-in is of cortical origin since the 
properties of an active perceptual completion mechanism are assumed to be too complex for 
early visual areas4,5. However, up to now no common neural mechanism for filling-in has so 
far been found and it is conceivable that different types of perceptual completion such as 
filling-in of the blind-spot, of artificial scotomas and of stabilized images involve different 
mechanisms at different sites within the visual pathway. 
Shimojo et al.2 suggested that their observation was the result of cortical processing, 
since the filled-in square occupied an area outside the adapted regions of the red wedges on 
the retina. To find out whether our effect was cortical or precortical, we tested for colour 
spreading with a dichoptic stimulus. Using a mirror stereoscope, a green disk was presented to 
one eye and a red ring to the other, such that when perceptually combined they were 
concentric and in register. Under these conditions observers perceived colour spreading 
during stimulus adaptation, but never in the afterimage. These findings suggest that colour 
spreading in the negative afterimage originates at a precortical stage, i.e., prior to binocular 
fusion. On the other hand, filling-in due to Troxler’s effect, has already been proposed earlier 
to originate at a lower level of the visual pathway6,7. 
Our findings support the assumption that filling-in in afterimages may originate at a 
precortical stage. Yet, while the retina can generate coloured afterimages, it is unclear 
whether retinal horizontal cells can mediate filling-in. On the other hand, color-selective 
center-surround neurons in the lateral geniculate nucleus (LGN) in conjunction with feedback 
connections from the visual cortex may give rise to such activity8. Top-down influences from 
cortical neurons, which themselves are capable of filling-in, directly to the rich interneuron 
circuits within the LGN have been demonstrated9,10,11. However, most studies on filling-in 
 5
concentrate on cortical structures as origin of this illusory effect12, 13. Following our study, 
further neurophysiological investigations in primates or fMRI studies in humans might change 
this view and focus on the role of the LGN in coding for filling-in in coloured stimuli, 
especially since recent fMRI reports provide contradictory results for (the early visual cortex 
as) the  origin of colour filling-in5,13,14. 
 
References: 
1. Hamburger, K., et al., Vision Res., 46, 1129-1138 (2006). 
2. Shimojo, S. et al., Science, 293, 1677-1680 (2001). 
3. Varin, D. Rivista di Psicologia, 65, 101-128 (1971). 
4. von der Heydt, Friedmann, H.S., Zhou, H., , 106-127 In: L. Pessoa, P. De 
Weerd, eds., Oxford University Press, Oxford (2003). 
5. Sasaki, Y., Watanabe, T., PNAS, 101,  18251-18256 (2004). 
6. Clarke, F.J.J., Belcher, S.J., Vision Res., 2, 51-68 (1962). 
7. Kotulak, J.C., Schor, C.M., Perception, 15, 7-15 (1986). 
8. Gove, A., Grossberg, S., Mingolla, E. Vis. Neurosci., 12, 1027-1052 (1995). 
9. Fitzpatrick, D., et al., Vis. Neurosci., 11, 307-315 (1994). 
10. Komatsu, H., Nature Rev. Neurosci., 7, 220-231 (2006). 
11. Cudeiro, J., Silito, A.M., Trends  Neurosci., 29, 298-306 (2006). 
12. Meng, M., Remus, D.A., Tong, F., Nature Neurosci., 8, 1248-1256 (2005).  
13. Cornelissen, F.W., et al., J. Neurosci., 26, 3634-3641 (2006). 
14. Cornelissen, F.W., Vladusich, T., Nature Rev. Neurosci., 7, (2006). 
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Figures and figure captions: 
 
Fig. 1: Top: Adaptation (real/physical) stimulus consisting of a disk surrounded by two rings. 
Stimuli were cut from cardboard and pasted on a white background. The disk and ring 
subtended 3.5 deg each while the outer ring subtended 7 deg. Observation distance was 65 
cm. Four equiluminant colours were used, representing Munsell 5B7/8, 5G7/8, 5R7/8 and 
5Y7/8. In the comparing condition the colours were combined as follows (disk | inner ring | 
outer ring): red/green/blue; control condition: green/red/yellow. In the dichoptic condition 
subjects were presented at the same time a red ring and a green disk separately to each eye, 
such that both were in register. Five observers with normal or corrected-to-normal vision 
participated. Middle: Corresponding negative afterimage as perceived following 30 seconds 
of binocular adaptation and viewing a white background. Bottom: Four modes of colour 
spreading as observed in the experiment in the afterimages and real images. 
 
Fig.2: Frequencies (in percent) of same or different colour spreading effects for the 
real/physical image and for the afterimage. Five subjects took part in the experiment 
performing n = 30 trials for each condition. The bars indicate standard errors. The result of the 
T-test for the difference between the types of colour spreading were: for the physical stimulus 
and the subsequent afterimage: t = 7.899, df = 4, p = .001, and for the inverted stimulus and 
the subsequent afterimage: t = 1.640, df = 4; p = .176 (Due to the small sample in our 
experiment and a relatively large standard error the difference in the control condition was not 
statistically significant. However, there is a obvious descriptive difference between the types 
of filling-in in the inverted stimulus (column 3 and 4). We also performed a correlation 
analysis to test whether the single completion effects perceived in the afterimages correlated 
with the filling-in effects perceived in real images. The results of the correlation analysis 
(Pearson correlation coefficient) were: E0 r=.236, p=.703; E1 r=.547, p=.340; E2 r=-.784, 
 7
p=.116; E3 r=.379, p=.529; E4 r=-.241, p=.696. The results for the inverted stimulus and the 
subsequent afterimage were: E0 r=-.248, p=.687; E1 r=.809, p=.097; E2 r=.222, p=.720; E3 
r=.117, p=.851; E4 r=1.00, p<.001. The last correlation refers to the reports of only one 
subject for both the real image and the afterimage.  


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An illusion induced by an illusion -perceptual filling-in of coloured negative afterimages

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An illusion induced by an illusion -perceptual filling-in of coloured negative afterimages

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