Completion Illusions

[Gisberto Letter]

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A striking example of the constructive nature of visual perception is how the human visual system completes contours of occluded objects. To date, it is unclear whether perceptual completion emerges during early stages of visual processing or whether higher-level mechanisms are necessary. To answer this question, we used transcranial magnetic stimulation to disrupt signaling in V1/V2 and in the lateral occipital (LO) area at different moments in time while participants performed a discrimination task involving a Kanizsa-type illusory figure. Results show that both V1/V2 and higher-level visual area LO are critically involved in perceptual completion. However, these areas seem to be involved in an inverse hierarchical fashion, in which the critical time window for V1/V2 follows that for LO. These results are in line with the growing evidence that feedback to V1/V2 contributes to perceptual completion.

The visual phantom illusion was first discovered by Rosenbach in 1902 and named 'moving phantoms' by Tynan and Sekuler in 1975 because of its strong dependence on motion. It was later revealed that phantoms can be generated by flickering the grating (flickering phantoms) or by low-luminance stationary gratings under dark adaptation (stationary phantoms). Although phantoms are much more visible at scotopic or mesopic adaptation levels (scotopic phantoms) than at photopic levels, we proposed a new phantom illusion which is fully visible in photopic vision (photopic phantoms). In 2001, we revealed that the visual phantom illusion is a higher-order perceptual construct or a Gestalt, which depends on the mechanism of perceptual transparency. Perceptual transparency is known as a perceptual product based upon brightness and contrast. We furthermore manifested the shared mechanisms between visual phantoms and neon color spreading or between visual phantoms and the Petter effect. In our recent study, the visual phantom illusion can also be seen with a stimulus of contrast-modulated gratings. We assume that this effect also depends on perceptual transparency induced by contrast modulation. Moreover, we found that the Craik-O'Brien-Cornsweet effect and other brightness illusions can generate the visual phantom illusion. In any case, we explain the visual phantom illusion in terms of surface completion, which is given by perceptual transparency.

You will likely experience illusory discs where the lines would intersect. These will appear 'filled-in' with a brighter white than the surrounding area. You may also experience these discs as 'figures' which sit in a higher depth plane than the lines and so occlude them.

Experiences of the Ehrenstein figure exhibit both modal and amodal completion of objects. In modal completion one has a visual experience as of an object in virtue of experiencing edges that appear to be created by a luminance, colour or texture boundary. On reflection, one can tell that there is no such boundary and there is not a difference in luminance, colour or texture where there appears to be one; but, nonetheless, that is what we experience. For example, look at the Kanizsa triangle on the right. The triangle that one seems to see pointing upwards, in virtue of a difference in luminance between it and the background, is a classic example of modal completion. The apparent discs in the Ehrenstein figure are also an example of modal completion, as they are experienced in virtue of experiencing an apparent lightness boundary where none is present.

Perceptual filling-in is a well-attested phenomenon. For example, if a subject is presented with the stimulus of a green disc surrounded by a red annulus, and the green region is retinally fixed (moves with the eye), after a short time the subject will report that they see only a red disc (Krauskopf 1963). This is because retinally fixed stimuli quickly fade from the visual field due to neural adaptation (see the entries for the Troxler Effect and Negative Afterimages). In the Krauskopf experiment, the green disc was filled in by the surrounding red in a process similar to the filling-in of the blind spot which is inherent in the anatomy of the human eye. The physiological mechanisms of filling-in are unclear; Friedman et al. (2003) suggest that visual perception is based on a neural image operating as a 2-dimensional array, in which colour signals diffuse in all directions until meeting a contour signal. It can be assumed that the filling-in of the apparent discs in the Ehrenstein figures is mediated by the same mechanism, although in this case the contour is illusory rather than actual. For more detailed discussion, see Grossberg (2015) for an accessible introduction to the FACADE (Form and Colour and DEpth) neural model, which predicts perceptual filling-in as well as the apparent brightness increase and figure-ground effects seen in Ehrenstein figures.

The figure is often experienced as a solid triangle pointing upwards that is lighter than the background, which occludes an inverted triangle pointing downwards, and a set of black discs which are also occluded by the solid bright white triangle that points upwards. Surprisingly, none of these shapes are actually present in the figure.

Kanizsa (1955) makes a distinction between modal and amodal completion of contours. In modal completion one has a visual experience as of an object in virtue of experiencing edges that appear to be created by a luminance, colour or texture boundary. On reflection, one can tell that there is no such boundary and there is not a difference in luminance, colour or texture where there appears to be one; but, nonetheless, that is what we experience. In the Kaniza triangle the triangle that one seems to see pointing upwards, in virtue of a difference in luminance between it and the background, is a classic example of modal completion. The apparent discs in the Ehrenstein figure are also an example of modal completion, as they are experienced in virtue of experiencing an apparent lightness boundary where none is present.

It is a given of life that nothing is permanently and finally satisfying or fulfilling. It is a given of the mind that somewhere there is a person or thing that will be permanently satisfying or fulfilling. Such a chimerical belief and the restless desperate seeking that may follow it can be deeply disheartening and self-defeating.

The search for completion in life is a red-herring. In the context of feeling and purpose, completion is the idea that life will be fundamentally different later, after something is completed. For instance, one may feel that social relations will be different after one has 'completed' getting married, or that life will be different after one 'completes' making a lot of money, or that one can start expecting to be treated well after one 'completes' proving oneself. Circumstances change of course, but the feeling of completion never comes. One occaisionally meets people who can report 'content' but never one who can report 'completion.'

There is nothing to make in life, and nothing to complete. Mission and goals can give life meaning, but what detracts from feeling and purpose is the lack of full participation in relationships based on the idea or feeling of 'something pending.' Also, a great deal of insensitivity to the present situation results from pursuing 'completion' for something else. Still more, the illusion that something is complete, leads to insensitivity. For instance, it is common to view a relationship as 'complete' at marriage, and so the marriage becomes (in the experiencing anyway) a static thing, not a relationship. In life nothing has been completed, and nothing will be completed.

Working on of signal and scene reconstruction, I have found there are certain limitations to the underlying computations. Perfect signal reconstruction is not always possible. This gave rise to a few theories of illusions.

(xiii) Similarly to neon color spreading, the watercolor illusion induces a complementary color when one of the two juxtaposed contours is achromatic and the other chromatic (Pinna & Grossberg, 2005).

(v) The watercolor illusion determines grouping and figure-ground segregation more strongly than the classical Gestalt principles. The principles of grouping by Wertheimer (1923) determine how elements group in parts that in their turn group in overall objects. The question they answer is how individual elements create larger wholes separated from others. The principles of figure-ground segregation by Rubin (1921) represent the rules of belongingness of the boundaries to the figure and not to the ground. The question they answer is what appears as a figure and what as a background. The figure-ground effect of the watercolor illusion wins against the following gestalt principles: proximity, good continuation, Prgnanz, relative orientation, closure, symmetry, convexity, past experience, similarity, surroundedness and parallelism.

The study of the relationship between the coloration and the figure-ground effects is a basic problem of Vision Science, with at least two main questions: Are color and figure-ground processes independent? How and where does the integration between the two processes occur?

Pinna & Reeves (2006) showed that coloration without a figure-ground effect is induced through a dichoptic composite stimulus, where the orange contour is presented to one eye while the purple one to the other eye. Under these conditions, the coloration is clearly perceived within the inner edges, but the figure-ground effect is totally absent.The binding between the coloration and the figure-ground effects is weakened by reversing the figure-ground organization where the coloration occurs.

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