Introducing the Illusions Index

The following is a guest post by Fiona Macpherson and Umut BaysanCentre for the Study of Perceptual Experience (, University of Glasgow.

The Illusions Index ( is a fully searchable interactive curated collection of illusions. It consists of images (many of which are interactive), sounds, and videos, together with instructions for experiencing the illusions, descriptions of their effects, historical information about the discovery of the illusions, explanations of the physiological mechanisms that underlie such illusions, and philosophical commentaries about them. Each commentary is appended with bibliographical information for further reading. The content of the articles is open-access via a Creative Commons license (CC BY-NC_SA 4.0 (

The Index has highly functional search options. For example, you can search illusions according to their type, such as which sensory modality they are in, whether they are cross-modal, whether they are size, shape or constancy illusions, whether they are ambiguous figures or involve after-effects, to name but a few. And one can search for illusions that satisfy more than one of these criteria. In creating The Illusions Index we selected a wide variety of illusions including not only visual illusions, but also illusions from other sensory modalities (touch and hearing) and cross-modal illusions. The Illusions Index is distinctive because of the detailed psychological and philosophical commentaries about each illusion with cross-references to entries within the index. The Illusions Index is being regularly updated with new illusions, so it is an ever-growing comprehensive open-access source available for both academic and non-academic purposes.

One of us (Macpherson) had hosted a basic page of illusions for over ten years and had been surprised by the number of hits that it received: on average around 450 per month, with no advertisement of the page. She decided that making a better version of this would satisfy the intense curiosity that the public have about illusions, which manifested itself not only in the popularity of this page, but also in viral internet phenomena, such as ‘The Dress’ ( Together with her postdoc, Keith Wilson (, and local Glaswegian web design company, Mucky Puddle (, she designed the features of the Illusions Index that would make it a unique and novel resource. She also worked with postgraduates at the Centre for the Study of Perceptual Experience, Gavin Thompson, John Donaldson and Raul Morales to produce the content that would go on the site. This work was funded by research incentivisation money from the University of Glasgow. Then, with grants from the School of Humanities, the College of Arts and the Knowledge Exchange fund at the University of Glasgow she worked together with her postdoc, Umut Baysan, uploading the content onto the site, editing the content, and writing up accounts of more illusions. The result was launched on 19 September 2017.

We have been delighted with the response from people thus far. In one week we have had over 2,500 visitors to the website. The response on Twitter has been great. People have told us that the Index is “a cool resource”, “[a] very useful database of perceptual illusions + commentary!”, that they will use it in their teaching of perception, and that it is “[a]mazing, well worth a gander” and ”[m]ind blowing”. People have written to us via the web form to suggest illusions to include on the index. And we’ve been able to respond to one of those requests, adding the sound-induced flash illusion to the website. We plan to add lots more illusions in the future.

We explain the nature of illusions to people in the Illusions Index. Our senses evolved to enable us to perceive the world around us. To a great extent, they are successful in doing so; they help us experience our surroundings accurately. In such cases, we have veridical experiences of the world. But occasionally, our senses let us down, and as a result, we fail to perceive the world around us accurately. We misjudge the sizes, colours and the distances of objects around us, we see or hear things that are not really there, and so on and so forth. In such cases, we have non-veridical experiences.

Philosophers of perception typically make a distinction between two types of non-veridical experiences: illusions and hallucinations. In cases of illusion, we perceive real, physical, objects around us, but we fail to perceive one or more of their properties accurately. In cases of hallucination however, it is typically thought that we have experiences of objects that are not there in physical reality. (However for an alternative perspective see Macpherson and Batty (2016) ( Although illusions and hallucinations are cases where things go wrong, they serve a great purpose in helping us understand how our senses work. By studying the conditions under which our senses fail us, we get great insight into how they work when they do, and how they usually do this so astonishingly well. For this reason, illusions and hallucinations intrigue philosophers and scientists of perception. They also challenge us to explain the nature of our apparent awareness of objects and properties when there aren’t corresponding objects and properties in the world in front of us.

Some illusions both within and across sensory modalities are similar, and they exploit similar physiological mechanisms. For this reason, they also raise the same philosophical questions. The Index makes this very clear, and makes the comparison of similar illusions easily manageable. For example, if you want to want to find out about various sorts of visual aftereffects, you can choose the tags “visual” and “aftereffect”. If you do so, then you get a number of results, including negative afterimages, positive afterimages, the Waterfall Illusion (which is a motion aftereffect) and so on and so forth. When you remove the tag “visual”, the list will include aftereffects from other sensory modalities, for example the tactile aftereffect.

This function of comparing similar illusions highlights the interesting similarities between illusions from different sensory modalities. Consider the tactile aftereffect, which is an illusion that can be experienced by pressing the palm of your hand and fingers against the curved surface of a salad bowl for about half a minute, and then pressing them on a flat surface. When you press your hands to the flat surface, you experience the flat surface as if it were curved in the opposite direction to the surface of the salad bowl for a few seconds. As Vogels and colleagues (1996) ( observed in experiments, we are more likely to judge a flat surface to be convex if our hands were previously exposed to a concave surface (and vice versa). This illusion bears a very striking structural similarity to motion aftereffect illusions experienced in vision, for example the well-known Waterfall Illusion. Once your eyes are exposed to continuous motion in one direction, when you move your eyes to a stationary scene, you experience illusory motion in the direction that is opposite of the motion you experience in the conditioning period. Robert Addams (1834) popularised this illusion in after a trip to the Falls of Foyers in Scotland, but illusions of this sort were known much before 19th century, and was even discussed by Aristotle. The physiological explanation of this illusion involves neurons being less sensitive at various sites throughout the brain. See, for just one example of work on this topic, Kohn & Movshon (2003). According to this explanation, when you are watching the stimulus with motion (for example, the moving water in a waterfall), the neurons that detect continuous movement in one direction (e.g., downward) become less sensitive. As a result, when you look away, neurons that detect movement in the opposite direction (e.g., upwards) are more active in comparison. This results in the appearance of the stationary object moving in the latter direction (upwards). This is similar to the tactile aftereffect illusion in which the illusory shape you experience has the opposite feature to the shape that you were experiencing in the conditioning: convex and concave.

We are also carrying out research on the nature of the experience had in the Waterfall illusion because there is disagreement among experts about what it is like to experience it. We hope that people will tell us about their experience to further our work on this topic. To do so please fill out our questionnaire after looking at the waterfall illusion available at: This is one way in which, the Illusions Index will be able to further research on perception, and genuinely create knowledge exchange between the public and academics.

Another case in point illustrating the similarities between illusions from different sensory modalities is the similarity between the Shepard Scale illusions, on the one hand, and some “impossible” figures on the other. Shepard Scale illusions are auditory illusions in which one experiences an illusory ever-increasing or an ever-decreasing pitch (Shepard 1964). What is actually happening is that the very same sequence of eight complex tones is being played over and over again. The repetition of the same octave creates the illusory experience of a continuous ascent or descent. The illusion occurs because each tone is composed of many pitch frequencies that are carefully crafted to create ambiguity. The illusory experience of an ever descending (or ascending) pitch is reminiscent of the illusory appearance of an ever-descending (or ever-ascending) barber pole or the staircase depicted in Escher’s lithograph ‘Ascending and Descending’. Common to all these illusions is that they create an experience of something that is impossible. In the case of Escher’s staircase what is impossible is that the stairs constantly descend or ascend and yet traveling along them one ends up at the same point as one started. In the case of the Shepard Scale what is impossible is that there is an ever descending or rising series of tones, that doesn’t become too low or too high for us to hear, and that ends up back at the same tone that we first heard. These illusions raise interesting questions about the nature of the representation or presentation of the illusory qualities: what is that nature such that it allows for apparently impossible features to be experienced?

A particularly interesting issue that we were sensitive to when creating the index is whether a particular experience is illusory or hallucinatory. For example, consider the Hermann Grid (Herman 1870). This is a visual stimulus that consists of a black and white grid. Because of a lightness contrast effect, one experiences grey dots at the intersection points of white lines (and these dots disappear when one tries to focus on them). Since the grey dots are not really there, we might categorise this as a case of hallucination (see Macpherson 2013); however, one might argue that one is experiencing the white intersections as grey, and hence categorise this as a case of illusion. Since there is room for debate here, we also used “hallucination” as a tag for the Herman Grid stimulus. Other cases in point are negative and positive afterimages, and a tactile effect which is known as Aristotle’s Illusion. The latter can be experienced when you cross your index and middle fingers and touch a small spherical object (e.g., a frozen pea or a marble) with the “inside” parts of your crossed fingers. As a result, you should feel as if your fingers are touching two objects, not one. Since there is only one object present however, it would be tempting to conclude that at least one of the spherical objects that you seem to touch is not there. Thus, it is tempting to characterise this experience as hallucination. Nonetheless, there is some reason to resist this explanation and to think of the case as one of illusion, for what is going on bears an interesting similarity to cases of double vision, in which one has a visual experience as of two objects when only one is present. Lowe (2000, p.109) argues that this phenomenon is best thought of as perceiving the finger twice—once with each eye—that results in two experiences of one finger. Again, since there is room for philosophical controversy here, we thought it would be best to use “hallucination” as one of the tags for the relevant entry.

We hope that the public and professionals will enjoy exploring the Illusions Index and their own perception. We would love to know people’s reactions to the site and have a feedback form ( on the site for that purpose. We also hope that people will suggest more illusions for the site. Indeed, if any professional philosophers or psychologists would like to write new entries for The Illusions Index then we urge them to get in touch ( Finally, we would like to create more interactive elements on the site that will enable us to gather data about perception. We are raising money in order to do that so if people would like to donate ( to the site, then we would be very grateful.


Addams, R. (1834). “An account of a peculiar optical phenomenon seen after having looked at a moving body”, London and Edinburgh Philosophical Magazine and Journal of Science, 5: 373-374

Hermann, L., (1870). “Eine Erscheinung simultanen Contrastes”, Pflügers Archiv für die gesamte Physiologie, 3: 13-15.

Kohn, A. & Movshon, J.A. (2003). “Neuronal Adaptation to Visual Motion in Area MT of the Macaque”. Neuron, 39: 681–69

Lowe, E. J. (2000) An Introduction to the Philosophy of Mind, Cambridge: Cambridge University Press.

Macpherson, F., (2013). “The Philosophy and Psychology of Hallucination: An Introduction,” in Hallucination: Philosophy and Psychology, MIT Press: Cambridge, MA.

Macpherson, F. and Batty, C. (2016) “Redefining Illusion and Hallucination in Light of New Cases”, Philosophical Issues, 26: 263 – 296.

Shepard, R. N. (1964) “Circularity in Judgments of Relative Pitch”, The Journal of the Acoustical Society of America, 36: 2346–2353.

Vogels I. M., Kappers A. M., and Koenderink J. J. (1996). “Haptic aftereffect of curved surfaces.” Perception 25: 109-19.

Header image: ‘McGurk Effect’, via the Illusions Index (

One comment

  1. I can’t resist a quick comment on the grey strawberries illusion. Because the image is perceived to depict objects seen through a translucent blue-green film, the colours of the image are necessarily quite different from the colours of the virtual objects we perceive in the scene. We can’t say that the *strawberries* are “in fact” grey, only that the areas of the image that depict them “should” appear grey. Colour constancy is not working against us when we see the *strawberries* as red. The “illusion” is just that the colours we perceive as belonging to these virtual objects are so salient that it is very difficult to attend to colours pertaining to the image itself (though not so much for the image colour for the white plate, interestingly). However if you look attentively at a small area of one strawberry and judge the *image* colour in relation to a nearby bit of white outside the image, you may suddenly perceive the area as grey, or at least very much greyer than the red object colour you perceive when viewing the image globally.

Comments are closed.

Back to Top