​The Poggendorff Illusion was discovered by Johann Christian Poggendorff (1796 - 1877), a German physicist. He discoverd it the drawings of Johann Karl Friedrich Zöllner (1834 - 1882), a German astrophysicist with a keen interest in optical illusions. Poggendorff was editor of the journal to which Zöllner submitted drawings in support of his paper reporting what is now known as the Zöllner Illusion. The Poggendorff Illusion was first published in the journal Annalen der Physik in 1860.

The Poggendorff Illusion is one among a number of illusions where a central aspect of a simple line image – e.g. the length, straightness, or parallelism of lines – appears distorted by other aspects of the image – e.g. other background/foreground lines, or other intersecting shapes. These are sometimes called ‘geometrical-optical illusions’. You can search for other geometric illusions in the Illusions Index.

There are a number of general hypotheses for how the broader type of illusion of which the Poggendorff illusion is an example works. One is that our perceptual systems have a tendency to ‘expand’ acute angles—that is to represent them as larger angles than they really are. In the case of the Poggendorff Illusion, the grey foreground rectangle which intersects the narrow straight lines causes the visual system to enhance the orientation contrast between the lines and rectangle—i.e. to ‘expand’ the acute angles at the relevant points of intersection. This, in turn, causes the appearance of a misalignment in the lines. Exactly why our perceptual systems have a tendency to expand acute angles is still being investigated. For a detailed discussion of the phenomenon of acute angle expansion, see Westheimer (2008).

One piece of evidence which, at the very least, complicates the acute-angle expansion hypothesis in the case of the Poggendorff Illusion is that the illusion does not seem to persist through a rotational change. Consider, for example, the version of the illusion below where the entire image has been rotated by 31 degrees:​

It is perhaps not immediately obvious why, if the acute angle expansion hypothesis were correct, the illusion should no longer be present simply because the image has been rotated 31 degrees so that the narrow lines are vertical and the grey rectangle is not vertical. Those who favour some version of the acute angle expansion hypothesis have endeavoured to explain this phenomenon whilst retaining the hypothesis (for discussion of this and related points, see: Green & Hoyle 1963; Gregory 1968; Day 1973; Weintraub et al. 1980; Greist-Bousquet & Schiffman 1981; Spehar & Gillam 1998; Morgan 1999; Westheimer & Wehrhahn 1997; Westheimer 2008).

The Poggendorff Illusion is 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 Poggendorff 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. For a general discussion of cognitive penetration, see Macpherson (2012).

Philosophers have also been interested in what illusions like the Poggendorff Illusion can tell us about the nature of experience. For example, in the case of experiencing the Poggendorff Illusion, it would seem to be that the one can know that the vertical red lines are straight whilst at the same time experience them as curved. 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 Poggendorff Illusion one would simultaneously believe that the lines were straight and not straight. 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).