The Ehrenstein figure was first introduced in 1941 by German psychologist Walter Ehrenstein (1899-1961) of the early 20th century Berlin school of Gestalt psychology. For most people, the Ehrenstein figure will produce the visual experience of apparent contours defining white discs. Of course, these discs do not actually exist as part of the figure – we are really perceiving only fragments of lines. Most individuals will experience an illusory ‘filling-in’ of colour such that the apparent discs are filled with a solid white that is brighter than the rest of the figure. A third effect is one of illusory depth – the illusory discs are perceived as figures which occlude the undefined ground which continues behind them in a different plane.

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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.

In contrast to this, the triangle that one seems to see pointing downwards in the Kanizsa traingle image, that appears to be partly behind the upwards pointing triangle that we previously mentioned, provides an example of amodal completion. The experience that one has of the downward pointing triangle does not consist in experienced boundaries consisting of colour, lightness or texture corresponding the occluded portion of the triangle. Yet, nonetheless, it does seem as if a triangle is present. This is a case of amodal completion, and it contrasts with modal completion in that it occurs when part of an object is experienced as occluded and is reported as having a particular shape, yet the occluded portion of the object is not experienced as being defined by colour, lightness or texture boundaries. The horizontal and vertical lines in the Ehrenstein figure are usually perceived as amodally completed – they appear to continue behind the disc - but they are not experienced in virtue of an experience of an apparent luminance or colour boundary. A good discussion of these phenomena from a psychological perspective is given in Gerbino, W., and R. van Lier, 2015. Philosophical accounts of modal and amodal perception can be found in Nanay (2010), Briscoe (2011), and Macpherson (2015).

The concepts of modal/amodal completion and the figure/ground distinction were introduced by Gestalt theorists. They are typical of Gestalt psychology’s emphasis on perceptual organization, i.e. how it comes to pass that we perceive objects, groups and relations. (‘Gestalt’ roughly translates as ‘form’.) Cognitivist theories like that of Rock and Anson (1979) view perceptual organization as resulting from a kind of unconscious inferential process, in which the ‘poverty of the stimulus’ means that sensory information can only provide cues for a number of perceptual hypotheses. A symmetrical arrangement of line elements precisely outlining the edges of a disc is a relatively unlikely stimulus, and so the perceptual system resolves the ambiguous scene as one in which continuous elements (the lines) are being occluded by particular objects (the discs), while the discs themselves are camouflaged in their surroundings – in this sense the perceptual system rejects hypotheses that require a high degree of coincidence. Objects being camouflaged or partially hidden is common in our natural biological environment, and so perceptual systems producing these hypotheses would be selected for. However, similar results are predicted by the Gestalt principles of perceptual organization, such as prägnanz (parsimonious interpretation of stimuli) or the role of past experience (Todorovic 2008). As far as neurophysiology goes, Peterhans et al. (1986) suggest that the illusory completed contour can be explained by the action of end-stopped neurons in the visual cortex. These cells correspond to elongated receptive fields on the retina and can fire selectively for both length and orientation of stimulus. Activity in spatially separated, end-stopped cells may trigger a gating mechanism, allowing for communication between neurons at previously inactive synapses.

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.

A special version of the Ehrenstein figure (Figure 2 below) provides a nice example of neon colour spreading, discussed in another entry in the Illusions Index.

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