Orbison Illusion

Instructions

Look at the red square and at the outer blue rectangle within which the blue lines are contained. Do the square and the rectangle appear distorted? Hover the cursor over the image and see whether they really are.

Effect

The red square and containing rectangle appear distorted, but they are not, as you can see when you hover the cursor over the image which removes the blue lines.

Illusion credit

William Orbison (1912 - 1952), American psychologist.

The Orbison Illusion was created by William Orbison (1912 - 1952), an American psychologist. Orbison first published the illusion in the American Journal of Psychology, in 1939.

The Orbison 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 geometrical illusions in the Illusions Index.

There are a number of general hypotheses about human vision which would explain the Orbison 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 Orbison Illusion, the blue background lines which intersect the red lines of the square 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 distortion in the square. 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).

The Orbison 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 Orbison 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 Orbison Illusion can tell us about the nature of experience. For example, in the case of experiencing the Orbison Illusion, it would seem to be that the one can know that the vertical red lines form a perfect square whilst at the same time experiencing the figure as if it were a distorted square. 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 undergoing this illusion. For discussion of this general point about whether perceptions are like beliefs, see Crane & French (2016).

References

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.

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.

Orbison, W.D., 1939. Shape as a function of the vector-field. The American Journal of Psychology, 52(1), pp.31-45.

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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