Visual Illusion: Now you see it, Now you don’t

Psychology 101: The 101 Ideas, Concepts and Theories that Have Shaped Our World - Adrian Furnham 2021

Visual Illusion: Now you see it, Now you don’t

We don’t simply see, we look. (E.J. Gibson, Annual Review of Psychology, 1988)

Vision is the art of seeing things invisible. (Jonathan Swift, Thoughts on Various Things, 1710)

Psychologists and artists have always been interested in visual and optical illusions. They are of particular interest to visual scientists and cognitive psychologists because they give an important insight into the process of perception. That is, we better understand the process of perception and vision by seeing what illusions people experience.

Seeing is a rapid, automatic, unconscious process. It is not a deliberate process, and our awareness of the process of visual perception usually comes only after it is complete — we get the finished product not the details of the process.

How do we see our world? We see colour, movement and depth: we recognize objects and people and indeed there is the whole debate on whether subliminal perception occurs. At the most abstract level it is possible to distinguish three processes: the reception of the light waves by the cornea and iris; translation where this physical energy (light) is coded into neurochemical messages sent to the brain; and decoding or translation of these.

What actually happens from the point at which information enters our senses to us perceiving what is there?


What we see is classified as either the object we are looking at (figure) or background. The classification of an item as a figure or ground is not an intrinsic property of the object but depends on the observer. We can always separate them from each other (figure—ground separation) although sometimes we receive ambiguous clues about what the object is and what the background is. Look at the object above: is it a vase or two faces? The figure and the ground can be reversed revealing two different pictures. There are many examples like this. One question is the difference between individuals who see different objects first.


One of the most important aspects of form perception is the existence of a boundary. If the visual field contains a sharp and distinct change in brightness, colour or texture, we perceive an edge, and we can ’see’ illusory contours (lines that do not exist). In the middle of this picture (on p.427), a triangle can be seen, brighter than the rest of the picture. This follows the gestalt principle of closure, as we tend to complete incomplete forms and fill in the gaps.


One central feature of study is how we ’put together’ or form complete pictures of objects from the separate pieces of information that we have. Between the wars the Gestalt psychologists studied the issue of what is called perceptual organization. They specified the law of proximity and good continuation tried to explain how we see patterns in abstract shapes. Collectively they are known as the laws of grouping and remain accurate descriptions of how we see.

The Gestaltists also became particularly interested in the accuracy of what we see. They devised Gestalt psychology — a theory of form perception with laws of pragnanz that explain how we perceive.

Similarity is where similar parts of a form are more likely to be perceived as belonging together; this might depend on relationships of form, colour, size or brightness.

The Proximity principle holds that surfaces or edges close together are more likely to be part of the same object than those far apart. Others include continuity, common-fate and symmetry.


It has been argued that these illusions can be explained by assuming that previous knowledge of three-dimensional objects is misapplied to these two dimensional patterns.

In the Ponzo illusion (A) the two horizontal lines are exactly the same length even though the lower line appears to be much shorter. This is because the linear perspective created by the converging lines of the rail track suggests the top line is further away. If it has the same retinal size but is further away it must be bigger — our perceptual system is mistakenly taking distance into account.

The Muller-Lyer illusion (B) has a similar explanation. The left line looks like the outside corners of a building, while the right line looks like the ’inside’ corners. These inside corners are, in a sense, further away than the outside ones, so the right line is perceived as further away and, using the same logic as the Ponzo illusion, as it has the same retinal size it is perceived as longer. These show that perception is influenced by factors other than stimulus — in this case, perceived distance and previous experience.


When objects move close or far away, under different lights or turn around, we tend not to see them as different or changing but remaining the same object. There are different types of constancy processes — shape, size, colour, brightness — which can help explain visual illusions.

Pick up this book. Hold it upright and facing you. It is (just) a rectangle. Now flip it over first through a vertical plane, then horizontal. It is no longer the same shape but you see the book as remaining the same. This is shape constancy.

Similarly, when we see an elephant or person walking away, or car driving away from us, it does not appear to be getting smaller though the image on a retina quite clearly is.


Imagine you grew up in an environment where there were no straight lines: no square houses, straight roads, long poles or traditional oblong tables. Your houses were round as were your fields. Your paths were twisting and turning. Would you still be ’fooled’ by visual illusions? If you have never seen a straight road or railway track would you experience the Ponzo illusion? And if yes, if you had never seen the corner of a room or house would you experience the Muller-Lyer illusion?

Various studies have been done with rural African and Aboriginal Groups to test ideas about how learning and experience influences our experience of illusions. One study compared urban and rural Africans who looked with one eye at a trapezoid shape called ’Ames window revolving’ (see previous page). As predicted the rural group saw it oscillating around 180 degrees. Another study found that Zulus from South Africa saw the Ponzo illusion to a greater extent than White South Africans, possibly due to their greater experience of wide open spaces.

Our personal and cultural experiences may make us more or less likely to see visual illusions.


Gregory, R.L. (1997). ’Visual illusions classified’. Trends in Cognitive Sciences. 1 (5): 190—4.

Purves, D., Lotto, R.B., & Nundy, S. (2002). ’Why We See What We Do’. American Scientist. 90 (3): 236—42.