The Ebbinghaus illusion was discovered by the German psychologist Hermann Ebbinghaus (1850–1909).The illusion was popularized by Edward B. Titchener in a 1901 textbook, in English, which is why the illusion is sometimes called "Titchener circles".

The Ebbinghaus illusion is one of several size contrast illusions to have been studied by both philosophers and cognitive scientists. Bela Julesz (1928-2003), a Hungarian neuroscientist and experimental psychologist hypothesised that the illusory effect of the Ebbinghaus discs cannot be explained entirely in terms of retinal processes (Julesz 1971).

In normal vision, the monocular information from each eye is sent along the optic nerve and undergoes binocular fusion in the cerebral cortex. This produces a `cyclopean’ perception, wherein the subject possesses a single point of view but has greatly improved powers of depth perception owing to binocular cues such as parallax and convergence (the tendency to go very slightly ‘cross-eyed’ as we look at an object close-up). The Ebbinghaus illusion occurs even when the pattern of discs exists only as a disparity between the stereographic images, i.e. the discs are discernible only after the cortical fusion of two random-dot images. Julesz 1971 concluded that the illusion must therefore be generated beyond the eye and the optic nerve, existing on a metaphorical ‘cyclopean retina’ which highly suggests the image of cortical processes. For an in-depth discussion of the relative roles of monocular depth cues (e.g. size, perspective) and binocular cues in the Ebbinghaus illusion, (see Papathomas et al. 1996).

Philosophers of perception are often interested in the nature of perceptual experience and its phenomenal character (namely, ‘what it is like’ for a subject to undergo that experience). Representationalism is the view that perceptual experiences possess content which represents objects, properties and relations as being a certain way, and that phenomenal character is either identical to or supervenes on this content. A representationalist may claim that, in additon to a conscious representation of an object (which some argue physically exists in the ventral stream in the brain) there is also a pragmatic representation of an object which guides action and which is unconsicous (which is in the dorsal stream in the brain). It has been shown that although people report consciously seeing one disk as larger than the other, when people reach out to pick up the disks, they open their fingers the same distance apart in each case to accurately grab each disk. Thus it has been argued that the pragmatic, dorsal representation accurately represents the Ebbinghaus discs, but we have introspective access only to the non-veridical, ventral representation (Nanay 2014).

A common anti-representationalist view is relationalism, whose proponents propound the ‘naïve realist’ thesis that we have direct access to the perceptible properties of physical objects (i.e. when we look at a tomato, the redness we perceive is just the redness of the tomato). Nanay (2015) argues that Ebbinghaus-type illusions pose a problem for relationalists: if perception is a direct relation between the perceiver and the physical object, is what is perceived the physical property tracked by our dorsal (action) stream or that which informs our ventral (perceptual) stream? The objection is that a single mind-independent property of disc size cannot be responsible for two pairwise inconsistent perceptual verdicts: (1) our reports that the disks seem unequal and (2) our grasping action that indicates that (in some sense) the disks seem equal to us. Assuming that the relationalist does not want to say that the disc possesses more than one kind of size, they must give some account of this phenomenon.

The Ebbinghaus Illusion is also interesting because it is relevant to debates about modularity, cognitive penetration, and the nature of experience. To explain: on the hypothesis that the mind is modular, a mental module is a kind of semi-independent department of the mind which deals with particular types of inputs, and gives particular types of outputs, and whose inner workings are not accessible to the conscious awareness of the person – all one can get access to are the relevant outputs. So, in the case of the Ebbinghaus Illusion, a standard way of explaining why experience of the illusion persists even though one knows that one is experiencing an illusion is that the module, or modules, which constitute the visual system are ‘cognitively impenetrable’ to some degree – i.e. their inner workings and outputs cannot be influenced by conscious awareness. However, Vishton et al. (2007) suggests that the strength of the Ebbinghaus illusion is significantly diminished for subjects who were informed that they were soon to perform a grasping task. This is some evidence towards thinking that perception can be cognitively penetrated, hence challenging the modularity hypothesis considered above. For a general discussion of cognitive penetration, see Macpherson (2012).

Philosophers have also been interested in what illusions like the Ebbinghaus Illusion can tell us about the nature of experience. For example, in the case of experiencing the Ebbinghaus Illusion, it would seem to be that the one can know that the two orange discs have the same size whilst at the same time one experiences the one on the right bigger. If so, then this might count against the claim the perceptual states are belief-like, because if perceptual states were belief like then, when experiencing the Ebbinghaus discs one would simultaneously believe that the discs were, and were not, the same size. This would seem to entail that one was being irrational, because one would simultaneously be holding contradictory beliefs. But it seems highly implausible that one is being irrational when under going this illusion. For discussion of this general point about whether perceptions are like beliefs, see Crane & French (2016).

The Dynamic Ebbinghaus is an animated version of the Ebbinghaus illusion using only one central disc and one set of inducing discs. It shows that continuously scaling the size of the inducers around a central disc produces a very weak illusory effect when the central disc is stationary, but a very powerful effect if the entire figure is set in motion. This very clearly demonstrates a failing of size constancy, our capacity to perceive the size of an object as invariant under changes in retinal stimulus. Perceptual constancy is defined as a subject’s capacity to perceive some property of an object (e.g. lightness, colour or shape) as being independent of external conditions (e.g. lighting, distance or viewing angle). Size constancy is usually considered to be an invariance under perceived distance, but the discoverers of the Dynamic Ebbinghaus argue that it shows the existence of a kind of dynamic size constancy wherein expansion and contraction of circles produces depth cues (Mruczek et al. 2015).

https://www.youtube.com/embed/hRlWqfd5pn8?rel=0&showinfo=0
The Dynamic Ebbinghaus Illusion (Mruczek et al.) was voted Illusion of the Year 2014.

Philosopher Tyler Burge (2010) argues for a form of representationalism in which perceptual constancy is necessary for any form of perception. Burge claims that constancies, understood as capacities for objectification of sensory stimuli—roughly, the capacities to track objects and properties—provide veridicality conditions for our contentful representations of the world. Burge discusses constancy mechanisms throughout the animal kingdom, including visual colour constancy in bees and auditory location constancy in owls. It is interesting to note, in this context, that size constancies are rather variable in different perceivers. For example, baboons seem to fail to perceive any illusory effect in the Ebbinghaus circles, whereas in pigeons the effect appears to be reversed (Salva et al. 2014). Doherty et al. (2010) found that young children under the age of seven are not deceived by the Ebbinghaus illusion, and even ten-year olds are less affected than adults. Finally, Doherty et al. (2008) found that size perception is more context-sensitive in Japan than in the UK, and so Japanese people will experience the Ebbinghaus illusion more strongly than people in the UK.