Category Archives: Visual illusions

Yet more visual illusions

We haven’t had any visual illusions on this blog for a while, and since the poster recently released for Ram Gopal Varma’s Bhoot Returns depends on a visual illusion this seems as good as time as any.

It’s surprising that more films do not choose to use visual illusions in their marketing materials, but some  nice examples based on Disney films by Rowan Stocks Moore can be found here. The Peter Pan and Snow White posters in particular stand out.

Archimedes Lab has many different illusions and oddities from Gianni Sarcone and Marie Waeber, which  you can access here. There is also a great selection of vintage illusions dating back 2500 years.

The finalists for this year’s Illusion of the Year contest can be found here, with attractive celebrities that turn ugly and a great interactive demonstration of the wagon wheel illusion. There is also an illusion inspired by the infamous twisting neck scene from The Exorcist which you can see below if you’re brave enough. The effect is much more eerie than anything you could do with CGI.

io9 has a dedicated illusions channel, which has lots of different examples of visual illusions and articles covering a range of issues including the art of anamorphic illusions and why our pupils contract when looking at illusions that are not bright lights.

This last example comes from the pages of Akiyoshi Kitaoka, and you can find details of his latest work here.

Finally, a good collection of illusions, including some of those listed above and in my other posts on this topic (here and here), can be found in New Scientist’s ‘Friday Illusion’ column.

More Visual Illusions

I like visual illusions – though I must admit that the rotating snakes (Figure 1) from Professor Akiyoshi Kitaoka’a illusion pages makes me feel somewhat queasy.

Figure 1 Rotating snakes from Akiyoshi’s illusion pages (click on the image for a larger version or got to to see the illusion in all its glory).

You can find other versions of this illusion and many others at Akiyoshi’s illusion pages here, along with research papers that discuss the psychological basis of the illusions he features. An fMRI study of the above illusion is Kuriki I, Ashida H, Murakami I, and Kitaoka A 2008 Functional brain imaging of the Rotating Snakes illusion by fMRI, Journal of Vision 8 (10): 16, 1-10, and can be accessed here.

The Daily Cognition has twenty visual illusions here. presents a study of illusions from 1922 – Matthew Luckiesh’s Visual Illusions: Their Causes, Characteristics and Applications – in its entirety.

An interesting introduction to the role of visual illusions in psychological research is David Eagleman’s article on how the study of visual illusions has guided neuroscience research: Eagleman DM 2001 Visual illusions and neurobiology, Nature Reviews Neuroscience 2: 920-926.

Richard Gregory, who died in May of last year, conducetd a large amount of research on visual perception and illusions, and the website dedicated to his memory – and featuring some of his papers on illusion and perception – can be accessed here.

Another researcher in visual illusions is Cornelia Fermüller, and her website can be found here, and includes examples of illusions and her research on a computational theory of optical illusions in video sequences and stereo images. One paper worth reading is Ogale AS, Fermüller C, and Aloimonos Y 2005  Motion segmentation using occlusions, IEEE Transactions on Pattern Analysis and Machine Intelligence 27 (6): 988-992.

We examine the key role of occlusions in finding independently moving objects instantaneously in a video obtained by a moving camera with a restricted field of view. In this problem, the image motion is caused by the combined effect of camera motion (egomotion), structure (depth), and the independent motion of scene entities. For a camera with a restricted field of view undergoing a small motion between frames, there exists in general a set of 3D camera motions compatible with the observed flow field even if only a small amount of noise is present, leading to ambiguous 3D motion estimates. If separable sets of solutions exist, motion-based clustering can detect one category of moving objects. Even if a single inseparable set of solutions is found, we show that occlusion information can be used to find ordinal depth, which is critical in identifying a new class of moving objects. In order to find ordinal depth, occlusions must not only be known, but they must also be filled (grouped) with optical flow from neighboring regions. We present a novel algorithm for filling occlusions and deducing ordinal depth under general circumstances. Finally, we describe another category of moving objects which is detected using cardinal comparisons between structure from motion and structure estimates from another source (e.g., stereo).

This paper from Alex Holcombe looks at the illusion of motion perception in the cinema:

Holcombe AO 2009 Seeing slow and seeing fast: two limits on perception, Trends in Cognitive Neuroscience 13 (5): 216-221. [This paper contains links to movies as part of the paper’s supplementary materials].

Video cameras have a single temporal limit set by the frame rate. The human visual system has multiple temporal limits set by its various constituent mechanisms. These limits seem to form two groups. A fast group comprises specialized mechanisms for extracting perceptual qualities such as motion direction, depth and edges. The second group, with coarse temporal resolution, includes judgments of the pairing of color and motion, the joint identification of arbitrary spatially separated features, the recognition of words and high-level motion. These temporally coarse percepts might all be mediated by high-level processes. Working at very different timescales, the two groups of mechanisms collaborate to create our unified visual experience.

Mel Slater’s paper looks at why we experience a sense of immersion in artificially created environments.

Slater M 2009 Place illusion and plausibility can lead to realistic behaviour in immersive virtual environments, Philosophical Transactions of the Royal Society B 364 (1535): 3549-3557.

In this paper, I address the question as to why participants tend to respond realistically to situations and events portrayed within an immersive virtual reality system. The idea is put forward, based on the experience of a large number of experimental studies, that there are two orthogonal components that contribute to this realistic response. The first is ‘being there’, often called ‘presence’, the qualia of having a sensation of being in a real place. We call this place illusion (PI). Second, plausibility illusion (Psi) refers to the illusion that the scenario being depicted is actually occurring. In the case of both PI and Psi the participant knows for sure that they are not ‘there’ and that the events are not occurring. PI is constrained by the sensorimotor contingencies afforded by the virtual reality system. Psi is determined by the extent to which the system can produce events that directly relate to the participant, the overall credibility of the scenario being depicted in comparison with expectations. We argue that when both PI and Psi occur, participants will respond realistically to the virtual reality.

A different approach to the nature of immersion in visual perception of animated images can be found in this paper from Kenny Chow and Fox Harrell:

Chow KKN and Harrell DF 2009 Material-based imagination: embodied cognition in animated images, Cognition and Creativity, Digital Arts and Culture 2009, Arts Computation Engineering, UC Irvine, California,;jsessionid=A526BE7A0733A59BB004CD0AC05A9EB6.

Drawing upon cognitive science theories of conceptual blending and material anchors, as well as recent neuroscience results regarding mirror neurons, we argue that animated visual graphics, as embodied images whose understanding relies on our perceptual and motor apparati, connect both material and mental notions of images. Animated visual images mobilize a reflective process in which material-based imaginative construction and elaboration can take place. We call this process as “material-based imagination,” in contrast to the general notion of imagination as purely a mental activity. This kind of imagination is pervasive in today’s digitally mediated environments. By analyzing a range of digital artifacts from computer interfaces to digital artworks, we show the important role of imaginative blends of concepts in making multiple levels of meaning, including visceral sensation and metaphorical narrative imagining, to exemplify expressiveness and functionality. The implications of these analyses collectively form a step toward an embodied cognition approach to animation phenomena and toward recentralizing understanding of artistic and humanistic production in cognitive research.

Finally, the 2011 finalists for the Illusion of the Year contest can be found here.

Visual illusions for film studies

This post contains a number of link to various web sites devoted to visual illusions and some papers on visual illusions that are of interest film researchers. (NB: the papers linked to may not be the final published versions).

Possibly the best site devoted to illusions is Michael Bach’s 87 Optical Illusions and Visual Phenomena, which has an impressive array of very nicely presented illusions. As a researcher on visual perception, Bach has published many papers on how we experience the world including Bach M, Poloschek CM (2006) Optical Illusions, Advances in Clinical Neuroscience and Rehabilitation 6 (2): 20–21, which  provides a short general overview.

George Mather is a researcher at the University of Sussex, who has developed the two-stroke apparent motion illusion, and published on this topic: Mather, G (2006) Two-stroke: a new illusion of visual motion based on the time course of neural responses in the human visual system, Vision Research 46: 2015-2018; and Mather G, Challinor KL (2009) Psychophysical properties of two-stroke apparent motion, Journal of Vision 9 (1): 28.

Other papers on similar illusions are Conway BR, Kitaoka A, Yazdanbakhsh A, Pack CC, Livingstone MS (2005) Neural basis for a powerful static motion illusion, The Journal of Neuroscience 25 (23): 5651-5656; and

Mather’s two-stroke illusion won second place at the ‘Illusion of the Year’ contest in 2005. For information on the other visual illusions, the contest’s web site is here.

Another good site is at the University Of Massachusetts Psychology Department and is maintained by David T. Landrigan.

The wagon-wheel illusion in particular is of interest to film researchers as it raises the question of why we experience motion. (The wagon wheel illusion demonstration at Bach’s website is particularly good). Short-range apparent motion is the traditional explanation for the experience of motion in the cinema:

The illusion of continuous motion is called apparent motion to distinguish it from ‘real’ motion, which is perceived when an object moves continuously across a viewer’s visual field. When Sir Laurence Olivier appears to be fencing in a film, he is in apparent motion, whereas a person walking across the theatre in front the screen is in real motion (Ramachandran & Anstis 1986: 102).

Ramachandran and Anstis’s argument is based on an ontological distinction between ‘real’ motion and ‘apparent’ motion, and that there is a one-to-one correspondence between stimulus and experience. Thus a continuous stimulus results in the experience of ‘real’ motion, while a discrete stimulus results in ‘apparent motion’ (Ramachandran ,V.S., and Anstis, S.M. (1986) The perception of apparent motion, Scientific American 254 (6): 102-109.) But what if our perception is discrete, so that we experience the world as a sequence of snapshots? The one-to-one correspondence between stimulus and experience implied by the ‘real’/’apparent’ distinction is not relevant if all our percepts are discrete, and the ontology of the stimulus (continuous/discrete) is irrelevant to our experience. In simple terms, all our perceptions would be discrete irrespective of the nature of the stimulus, and there would be no difference between watching Olivier on film or someone walking in front of the screen. The viewer would experience motion in the cinema because he experiences motion. This problem is raised in the following papers, most of which cite the cinema as a direct example of discrete perception:

Andrews T, Purves D (2005) The wagon-wheel illusion in continuous light, Trends in Cognitive Sciences 9 (6): 261-263.

Purves D, Paydarfar JA, Andrews TJ (1996) The wagon wheel illusion in movies and reality, Proceedings of the National Academy of Sciences 93 (8): 3693-3697.

van Rullen R, Koch C (2003) Is perception discrete or continuous?, Trends in Cognitive Sciences 7 (5): 207-213.

van Rullen R, Reddy L, Koch C (2005) Attention-driven discrete sampling of motion perception, Proceedings of the National Academy of Sciences 102 (14): 5291-5296.

van Rullen R, Reddy L, Koch C (2006) The continuous wagon wheel illusion is associated with changes in electroencephalogram power at ~13 Hz, The Journal of Neuroscience 26 (2): 502-507.

Van Rullen R, Pascual-Leone A, Batelli L (2008) The continuous wagon wheel illusion and the ‘when’ pathway of the right parietal lobe: a repetitive transcranial magnetic stimulation study, Public Library of Science One 3 (8): e2911.

A challenge to the argument for discrete perception argument in the wagon wheel illusion can be found in Klein K, Holcombe AO, Eagleman DM (2004) Illusory motion reversal is caused by rivalry, not by perceptual snapshots of the visual field, Vision Research 44: 2653-2658; and Klein K, Eagleman DM (2008) Evidence against the temporal subsampling account of illusory motion reversal, Journal of Vision 8 (4): 13.

Yves Gallifret deals with the history of retinal persistence and cinema in an English language essay from Comptes Rendues Biologies: Gallifret Y (2006) Visual persistence and cinema?, Compte Rendues Biologies 329 (5-6): 369-385.

An article on a similar subject is Paul St. George’s piece on chronophotography: St. George, P (2009) Using chronophotography to replace Persistence of Vision as a theory for explaining how animation and cinema produce the illusion of continuous motion, Animation Studies 4: 17-26.