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The two red vertical lines can appear to be curved outwards, despite actually being straight.
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Illusion Credit

Hering, E. 1861. Beiträge zur Physiologie. I. Zur Lehre vom Ortssinne der Netzhaut. Leipzig: Engelmann


Look at the red lines and see if they seem parallel or slightly bowed outwards. Hover the cursor over the image to make theblue lines disappear. What do you experience now?


When the blue lines are present the red lines appear to be bowed outwards when they are not. 

The two red vertical lines can appear to be curved outwards, despite actually being straight.
Media Licence:

Illusion Credit

Hering, E. 1861. Beiträge zur Physiologie. I. Zur Lehre vom Ortssinne der Netzhaut. Leipzig: Engelmann
  • Hering Illusion

The Hering Illusion was created by Karl Ewald Konstantin Hering (1834 - 1918), a German physiologist, known primarily as Ewald Hering. Hering first published the illusion in 1861.

The Hering 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. The Hering Illusion is especially similar to the Wundt Illusion, as each is an inverted version of the other.

There are a number of general hypotheses about why we experience the Hering illusion. 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 Hering Illusion, the blue radial lines which intersect the vertical red lines cause the visual system to enhance the orientation contrast between the red and blue lines – to ‘expand’ the acute angles at the point of intersection between the blue and red lines. This, in turn, causes the appearance of the red lines bending outwards near the centre. 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. Other explanations include positing a tendency of the visual system to exhibit certain biases when extrapolating 3D information from 2D retinal images (Nundy et al. 2000; Howe and Purves 2005).

One piece of evidence which might support the acute-angle expansion hypothesis in the case of the Hering Illusion is that the illusion seems to persist through certain transformational changes, such as rotation. Consider, for example, the below version of the illusion where the entire image has been rotated by 90 degrees (this image is taken from Westheimer 2008):

The Hering Illusion is interesting because it is relevant to debates about modularity, cognitive penetration, and the nature of experience. 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 Hering 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 Hering Illusion can tell us about the nature of experience. For example, in the case of experiencing the Hering 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 Orbison Illusion one would simultaneously believe that the lines were, and were 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).


Crane, T., and French, C., 2016. The Problem of Perception. In: Zalta, E. N., ed. The Stanford Encyclopedia of Philosophy. Metaphysics Research Lab, CSLI, Stanford University.

Hering, E. 1861. Beiträge zur Physiologie. I. Zur Lehre vom Ortssinne der Netzhaut. Leipzig: Engelmann.

Howe, C.Q. and Purves, D., 2005. Natural-scene geometry predicts the perception of angles and line orientation. Proceedings of the National Academy of Sciences of the United States of America, 102(4), pp.1228-1233.

Macpherson, F., 2012. Cognitive penetration of colour experience: Rethinking the issue in light of an indirect mechanism. Philosophy and Phenomenological Research, 84(1), pp.24-62.

Nundy, S., Lotto, B., Coppola, D., Shimpi, A. and Purves, D., 2000. Why are angles misperceived?. Proceedings of the National Academy of Sciences, 97(10), pp.5592-5597.

Westheimer, G., 2008. Illusions in the spatial sense of the eye: Geometrical–optical illusions and the neural representation of space. Vision research, 48(20), pp.2128-2142.

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Please cite this article as follows:

Donaldson, J. and Macpherson, F. (July 2017), "Hering Illusion" in F. Macpherson (ed.), The Illusions Index. Retrieved from https://illusionsindex.org/KKjkcL/Elaeagnaceae1836124/33TtH90qN.

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