Two small contours defined by wiggly lines sit within a similar, larger contour. The contour lines are dual-coloured - yellow on one side, purple on the other.
Media Licence:
Attribution-NonCommercial-NoDerivs 
Media Source:
Pinna, B. and Tanca, M. (2008) “Perceptual organization reconsidered in the light of the watercolor illusion: The problem of perception of holes and the object-hole effect”, Journal of Vision, Vol.8, 8. doi:10.1167/8.7.8

Illusion Credit

Baingio Pinna, experimental psychologist and lecturer, University of Sassari.

Instructions

Look at the space inside the large rectangle, and the space between the smaller rectangles withint it. Are these spaces the same colour?

Effect

You should have the illusory visual experience of a faint yellow colouration in the area within the large rectangle but not within the smaller rectangles within it, when in fact the space inside all contours is the same shade of white. There is also a 'figure/ground' effect meaning that the smaller contours appear as 'gaps' inside a large, well-defined shape.

Illusion Credit

Baingio Pinna, experimental psychologist and lecturer, University of Sassari.
  • Watercolour illusion
    Two small contours defined by wiggly lines sit within a similar, larger contour. The contour lines are dual-coloured - yellow on one side, purple on the other.
    Media Licence:
    Attribution-NonCommercial-NoDerivs 
  • Watercolour illusion
    Two peninsulas seem connected to the the mainland at the top
    Media Licence :
    © 2008 ARVO
    Media Source :
    Pinna, B. and Tanca, M. (2008) “Perceptual organization reconsidered in the light of the watercolor illusion: The problem of perception of holes and the object-hole effect”, Journal of Vision, Vol.8, 8. doi:10.1167/8.7.8
  • Watercolour illusion
    Green watercolour effect
  • Watercolour illusion
    Different coloured watercolour effects
    Media Source :
    Lower panel of Figure 3.11.35 from Stockman, A., & Brainard, D. H. (2009). Color vision mechanisms. In M. Bass, C. DeCusatis, J. Enoch, V. Lakshminarayanan, G. Li, C. Macdonald, V. Mahajan & E. van Stryland (Eds.), The Optical Society of America Handbook of Optics, 3rd edition, Volume III: Vision and Vision Optics. New York: McGraw Hill.

The watercolour illusion exhibits two remarkable phenomena: firstly, there is a long-range ‘spreading’ of a pale colour from one or more brightly coloured contours into the surrounding space; secondly, there is a pronounced figure-ground effect, in which certain elements will be perceived as having a definite shape (the figure) while others are perceived as a formless ground which is being occluded by the figure. In the main image on this page, the space inside the two smaller contours appears as the ground, which we glimpse through gaps in the figure bounded by the larger contour. Note that reversing the juxtaposed colours will cause both the colour-spreading and the figure-ground relation to also reverse: see figure 1.

Figure 1 – Reversal of colours causes reversal of figure and ground

The figure-ground effect in the watercolour illusion is powerful; it defies many of the Gestalt principles which were formulated by the Gestalt school of psychology which emerged in early 20th century Berlin. ‘Gestalt’ roughly translates as ‘form’ or ‘organized whole’ – the Gestaltists sought to uncover the principles of perceptual organization which determine how we perceive objects, groups, and relational properties (see Todorovic 2008 for further discussion). Gestaltism embodies a kind of ‘mid-level’ approach to perception, as opposed to low-level retinal/neural explanations and high-level cognitive/reasoning-based explanations. The figure-ground perceptual experience produced by the watercolour illusion defies Gestalt principles like continuity, closure, symmetry, past experience, and Prägnanz (Pinna 2008; Prägnanz is characteristic of Gestaltism, and means a kind of parsimonious interpretation of stimulus). Gestalt psychology’s popularity has waxed and waned over the years (see Wagemans et al. 2012), not least of all due to its methods, which drew greatly on phenomenology or introspection on the subjective character of experience. However, perceptual organization remains an area of active research; philosopher Tyler Burge discusses Gestaltist concepts as an attempt to explain our objectified representations of the world based on sensory stimulus (Burge 2010).

The coloration effect seen in the watercolour illusion is similar to that of the neon colour spreading illusion, which is often explained with reference to boundary grouping and perceptual filling-in. Figure-ground effects may also be explained by these same processes - see Grossberg (2015).

Boundary grouping is the process or processes in perception that determine what is taken to be all the one boundary or all the one contour of an object ( as opposed to different contours of different objects). This is often not a straightforward task as parts of the countour of an object may be occluded, some parts may be nearer or further away from the perceiver, and so on.

The concept or concepts of ‘filling-in’ are explored in the entry for the Troxler Effect.

References

Burge, T., 2010, Origins of Objectivity, OUP: New York.

Dennett, D., 1991. Consciousness Explained, Penguin.

Dennett, D., 1992. ‘“Filling in” versus finding out: A ubiquitous confusion in cognitive Science’, in P. van den Broek, H. L. Prick and D. C. Knill (Eds), Cognition: Conceptual and methodological issues, American Psychological Association Press.

Friedman, H. S., R. von der Heydt and H. Zhou, 2003. ‘Searching for the Neural mechanism of Color Filling-In’, in L. Pessoa and P. De Weerd (Eds), Filling-in: From Perceptual Completion to Cortical Reorganization, OUP: Oxford.

Grossberg, S., 2015. ‘Cortical Dynamics of Figure-Ground Separation in Response to 2D Pictures and 3D Scenes: How V2 Combines Border Ownership, Stereoscopic Cues, and Gestalt Grouping Rules’, in Front Psycho (6)

Myin, E., and L. De Nul, 2009. ‘Filling-in’, in T. Bayne, A. Cleeremans and P. Wilken (Eds), The Oxford Companion to Consciousness, OUP: Oxford.

Pinna, B., 2008. ‘Watercolor illusion’, Scholarpedia. http://www.scholarpedia.org/article/Watercolor_illusion

Todorovic, D., 2008. ‘Gestalt principles’, Scholarpedia. http://www.scholarpedia.org/article/Gestalt_principles

Wagemans et al., 2012. ‘A Century of Gestalt Psychology in Visual Perception I. Perceptual Grouping and Figure-Ground Organizaiton’, Psychol Bull. 138 (6) pp. 1172-1217.

How To Cite This Article

Author and Citation Info

Please cite this article as follows:

Thomson, G. and Macpherson, F. (July 2017), "Watercolour Illusion" in F. Macpherson (ed.), The Illusions Index. Retrieved from https://www.illusionsindex.org/i/watercolour-illusion.

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This article is licensed under Creative Commons (CC BY-NC_SA 4.0)

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