Category: Illusions

Here’s The First Evidence That Even Lizards Succumb To Optical Illusions

Central bearded dragon on the rock

By Emma Young

It’s been known for centuries that we experience all kinds of optical illusions, and in the past few decades, researchers have shown that some animals, including monkeys, pigeons, and dogs, do too. Now the first ever study of this kind in reptiles has found that even the bearded dragon falls for an optical illusion that we humans succumb to.

Perceptual illusions — subjective interpretations of physical information — are interesting to psychologists because they reveal important insights into how we construct our representations of the world. This new work, published in the Journal of Comparative Psychology, provides evidence that at least one reptile can be counted among the animals don’t simply passively process retinal signals, but actively interpret visual data, too.

Continue reading “Here’s The First Evidence That Even Lizards Succumb To Optical Illusions”

A psychologist noticed this cool chair illusion in his office

10.1177_2041669517752372-fig1.gifBy Alex Fradera

A short paper in the journal i-Perception presents a disconcerting visual illusion spotted “in the wild”: how stackable chairs, viewed from a certain angle, mess with your head. This is an unedited image, but your mind resists accepting it could be real. The illusion was first noticed in the office of lead author Nick Scott-Samuel at the University of Bristol, who notes in the paper that “it obtains in real life as well as in images, even when sober”.

The cause of the trick appears to be the two “edges” seen coming up from the near-base of the stack – marked AD and BC in the annotated version of the image below – which “suggests a change in depth along those lines which does not actually exist” and makes it seem as if the bars that run from one “edge” to another compose an outward face.


160px-Penrose-dreieck.svg.pngIt misleads us a little like the Penrose triangle (see right), but whereas the triangle is actually impossible, the chairs only appear to be so – as demonstrated in this video where you can see the effect dip in and out. Scott-Samuel and his colleagues experimented with the chair stack and it appears that you need at least four chairs to create the effect.

You know the angles, you know the number: turn up to the meeting room early next time and use that recipe to astound even the most jaded eyes.

Stacking Chairs: Local Sense and Global Nonsense

Alex Fradera (@alexfradera) is Staff Writer at BPS Research Digest

Believers in conspiracy theories and the paranormal are more likely to see “illusory patterns”

GettyImages-177765632.jpgBy Emma Young

Democratic bankers caused the global financial crisis to get Barack Obama elected. 

Horoscopes are right too often for it to be a coincidence. 

Irrational beliefs – unfounded, unscientific and illogical assumptions about the world – are widespread among “the population of normal, mentally sane adults” note the authors of a new study in European Journal of Social Psychology. It’s been proposed that they arise from a mistaken perception of patterns in the world. But though this idea is popular among psychologists, there’s been surprisingly little direct evidence in favour of it. The new work, led by Jan-Willem van Prooijen at the Free University of Amsterdam, helps to fill the void.

Continue reading “Believers in conspiracy theories and the paranormal are more likely to see “illusory patterns””

Investigating the weird effects treadmills have on our perception

Treadmill workoutBy Christian Jarrett

Anyone who’s been on a treadmill at the gym has probably had that strange perceptual experience afterwards – once you start to walk on stable ground again, it feels for a time as though you’re moving forward more quickly than you really are. The illusion, which is especially striking for treadmill newbies, was first documented scientifically in a Nature paper 20 years ago. Since then psychologists have come to better understand what’s going on and the ways the effects can manifest. Continue reading “Investigating the weird effects treadmills have on our perception”

First brain scan study to feature THAT dress

Figure and caption from Schlaffke et al., 2015.

Earlier this year a dress nearly broke the internet. A photo of the striped frock (which is actually blue and black) was posted on Tumblr and it quickly became apparent that it looked very different to different people, spawning furious arguments and lively scientific commentary.

Specifically, people disagreed vehemently over whether it was white and gold (that’s my perception) or blue and black. Now, writing in the journal Cortex, researchers in Germany have published the first study to scan people’s brains while they look at the dress, and the neural findings appear to support earlier, psychological explanations of the phenomenon.

When the dress story went viral, psychologists were quick to explain that this dress provided a striking example of how our perception of the world arises from a combination of incoming sensory information and our interpretation of that information. In the case of colour perception, when light bounces off an object and hits your retina, its mix of wavelengths is determined by the colour of the object and the nature of the light source illuminating it. Your brain has to disentangle the two. Usually it does this very well allowing for something called “colour constancy” – the way that objects of the same colour are perceived the same even under different illumination conditions. However, the mental processing involved in colour perception does leave room for interpretation and ambiguity, especially when the nature of the background illumination is unclear as is the case with the photo of the dress (another illusion that hacks the limitations of this aspect of our visual system is the checker shadow illusion).

For the new study, Lara Schlaffke and her colleagues scanned the brains of 28 people with normal vision while they looked at the photo of the dress. Fourteen of the participants see the dress as white and gold and 14 see it as blue and black. The key finding is that the people who see the dress as white and gold showed extra activation in a raft of brain areas, including in frontal, parietal (near the crown of the head) and temporal (near the ears) regions. Yet, no group differences emerged in a control condition when the participants simply looked at large coloured squares that matched two of the colours that feature in the dress, but without any contextual information also visible (see figure, above).

These results are broadly consistent with the idea that the white/gold perceivers were engaged in more interpretative mental processing when looking at the dress. To oversimplify, their perceptual experience of the dress is based less purely on the “bottom up”, raw sensory information arriving at their eyes, and is distorted more by their own assumptions and expectations about the background illumination. The extra activity in their brains during the dress viewing is likely, at least in part, a neural correlate of all this interpretative, “top down” processing.

What the new study can’t answer is whether this extra neural processing (or which aspects of it) in the white/gold group is the cause of their perceptual experience of the dress, or the consequence. However, the researchers describe some future approaches that could help address this quasi-philosophical conundrum: for example, by using transcranial magnetic stimulation (TMS) to temporarily disrupt the extra localised neural activity seen in the people who experience the dress as white and gold, we could ask: will they still experience the illusion?

Meanwhile, as someone who’s firmly in the white/gold camp, I take satisfaction from this study: I might see the dress as the “wrong” colours, but at least this isn’t due to simple-mindedness, but rather it’s because my brain’s working overtime, doing clever tricks in the background. I’m pretty sure that must be an advantage in at least some situations.


Schlaffke, L., Golisch, A., Haag, L., Lenz, M., Heba, S., Lissek, S., Schmidt-Wilcke, T., Eysel, U., & Tegenthoff, M. (2015). The brain’s dress code: How The Dress allows to decode the neuronal pathway of an optical illusion Cortex, 73, 271-275 DOI: 10.1016/j.cortex.2015.08.017

further reading

Visual illusions foster open-mindedness

Post written by Christian Jarrett (@psych_writer) for the BPS Research Digest.

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Psychologists use magic to study the illusory feeling of free choice

Most of the time, when a magician asks you to “pick a card” she makes it feel as though you have a free choice, but you don’t really. The authors of a new paper say this is a microcosm for many real-life situations in which we feel free to choose, but in fact our choices are heavily influenced and constrained. Jay Olson, a magician and psychologist, and his colleagues, have put a classic card trick technique under the spotlight as a way to study the psychology behind this experience of illusory free choice.

For each of 118 participants approached on the street or on campuses, Olson “riffled” through a pack of cards before asking the participant to “pick a card”. The 30-second riffling procedure is part of a “forcing” technique in which the magician uses their thumb to pull up and gradually release one end of the deck, ostensibly to give the participant a glimpse of the available cards in rapid succession. It appears a casual gesture, but the technique is carefully performed so that one card – the target card – is displayed substantially longer than the others, and in fact is often the only card shown long enough to be identifiable.

Nearly 100 per cent of participants ended up picking this target card, which the magician duly anticipated and showed to the participants, thus seeming to read their minds. The researchers then quizzed the participants about the experience. Nearly all those who chose the target card felt that they’d had a free choice over which card they’d selected from the pack. Asked why they’d picked the card they had, most said “no reason”, others said it had “stood out”, while the remainder confabulated, such as claiming they’d been thinking of that card earlier, or that the target card had been a bright colour (even when it was black).

Next, the research moved to more controlled laboratory conditions. The basic riffling procedure was repeated but using a computer simulation, in which cards were shown briefly in succession with one “target card” presented for significantly longer than 25 other possible choices (150ms vs. 20 to 70ms).  Participants were again asked to “pick a card”. The simulation was less effective than the real magic trick, with the target card now selected by participants around 30 per cent of the time (of course this still shows a heavy influence on participants’ choices).

The researchers next asked participants whether they’d noticed that one card was displayed for substantially longer than the others: 60 per cent said they had. Particularly interesting differences emerged between those aware of this fact, and those unaware. Among the unaware, personality factors were associated with whether they chose the target card – for example, people with a more external locus of control (they feel their lives are controlled by outside factors) were more likely to have picked the target card. Among those aware that one card had been shown for longer than others, personality factors were irrelevant to whether they picked the target card. Instead, features of the target card became significant, with more visually salient and memorable target cards picked more often by this group.

Olson and his colleagues said their findings have practical significance – they show the potential for using magic to study how people’s decisions can be influenced without them knowing, perhaps ultimately to help them make wiser, healthier decisions. Of course such findings could also be used for malicious ends, although this wasn’t mentioned by the researchers! They did add that their findings also have clinical significance: they say the current study demonstrates feelings of control in the absence of objective control, which is the converse of the experience of some patients with schizophrenia and other conditions, in which they feel their choices are being influenced by outside agents, when in fact they are not.

Olson’s team have made their new data freely available for others to access. “By doing so,” they explained, “we hope to help researchers participate in this growing field [of “forcing” and the factors that influence choice]. In particular, we hope that similar methodologies which combine the realism of the performing environment with the control of the laboratory will foster collaboration between the art of magic and the science of psychology.”


Olson, J., Amlani, A., Raz, A., & Rensink, R. (2015). Influencing choice without awareness Consciousness and Cognition DOI: 10.1016/j.concog.2015.01.004

further reading
The new psychology of everyday playing cards

Post written by Christian Jarrett (@psych_writer) for the BPS Research Digest.

The illusion that gives you sensations in a rubber tongue

Our sense of where our bodies begin and end usually feels consistent and reliable. However psychologists have been having fun for decades, exposing just how malleable the body concept can be.

You may have heard of the “rubber hand illusion” (video). By visibly stroking a rubber hand in time with stroking a participant’s hidden real hand, you can induce for them the feeling of sensation in the rubber hand.

The rubber hand illusion is thought to occur because the brain tends to bind together information arising from different sensory modalities. The stroking sensation arrives at one’s real hand, but the stroking is seen occurring at the same time at the rubber hand. The brain binds these two experiences and the visual modality wins, transferring the felt sensation to the rubber appendage.

Because the rubber hand illusion depends on the dominance of vision, Charles Michel and his colleagues wondered if a similar illusion would still occur for the tongue – one of our own body parts that we feel but rarely see.  The researchers purchased a fake tongue from a magic shop (see pic), and for forty seconds they stroked this tongue with a cotton bud (Q-tip) at the same time as they stroked each participant’s real tongue. The participants could see the stroking of the fake tongue, but the stroking of their own tongue was hidden from view.

Averaging responses from the 32 student participants, there was an overall sense among the students of being stroked on the rubber tongue, “thus demonstrating,” the researchers said, “visual capture over the felt position of the tongue for the very first time.” Further evidence for an illusory effect came from the fact that sensation in the rubber tongue was stronger when the stroking of the fake and real tongues was performed in synchrony as opposed to out of time. This synchronous stroking also led to more agreement from the students that they felt as though they could move the fake tongue, and that the fake tongue was their own.

Next, the researchers shone a laser pointer on the fake tongue as the participants watched. Twenty-two of the participants said that this triggered sensations in their real tongue – some said it felt cold, others warm, tactile and/or tingly. “I felt vibrations on my tongue moving in synchrony with the light movement,” one student said.

The researchers say their results have shown that an illusion, similar to the rubber hand illusion, can be experienced with the tongue. We call this “the Butcher’s Tongue Illusion,” they said.


Michel, C., Velasco, C., Salgado-Montejo, A., & Spence, C. (2014). The Butcher’s Tongue Illusion Perception, 43 (8), 818-824 DOI: 10.1068/p7733

-Further reading-
Psychologist magazine interview with cognitive neuroscientist and ‘master of illusion’ Henrik Ehrsson.

Post written by Christian Jarrett (@psych_writer) for the BPS Research Digest.

Welcome to the weird world of weight illusions

Normally bigger objects weigh more; breaking this rule provokes illusory perceptions

Visual illusions are useful to psychologists because, by tricking the brain, they provide clues about how it works. The same is true for weight illusions, it’s just that they’re far less well known. Now Gavin Buckingham at Heriot-Watt University has published a handy review of weight illusions, and he explores some of the thinking about their likely causes.

Among the most studied is known as the “size-weight illusion (SWI)”. When a person is presented with two objects – one large, one small – that weigh the same, the smaller object feels heavier. The illusion persists even when the perceiver is told what’s going on.

One explanation for the SWI is that, anticipating it will be heavier, we use stronger force to lift the bigger object than the smaller one. The greater force used to lift the bigger object leads to the perception of lightness, so the argument goes. But this explanation can’t account for the fact that the illusion persists regardless of how many times we perform the lifts, and as we adjust our anticipatory force.

Although the SWI illusion persists through repeated liftings, it can actually be reversed through extensive training. Buckingham describes a study in which groups of participants were repeatedly exposed to sets of “inverted-density” objects, in which it was always the smaller items that were heavier. One session of 1000 such lifts reduced the SWI slightly. Three days, conducting 1000 inverted-density lifts each day, cancelled out the SWI. Eventually, after 11 days of 1000 inverted-density lifts a day, the SWI was actually reversed – that is, 11 days of a topsy-turvy world, in which smaller objects were always heavier than bigger ones, led these participants to experience the larger of two equally weighted objects as heavier.

Also intriguing is the “material weight illusion”. Objects that appear to be made out of heavier-looking material (e.g. metal) feel as though they are lighter than equivalently weighted objects that appear to be made out of light material (e.g. polystyrene). Related illusions include the “brightness-weight illusion” – light-coloured objects feel heavier than darker objects of the same weight; and the “temperature-weight illusion” which is when cold objects feel heavier than warm objects of the same weight. In one of the earliest observations of this effect, which you can easily try at home, Ernst Weber (1795-1878) described how a cold coin placed on the forehead of a supine person feels heavier to them than a warm coin.

Here’s one more illusion that demonstrates elegantly the influence of our expectations on our perceptions. Buckingham describes how researchers adjusted the normal weight of practice golf balls upwards so that their appearance was unchanged, but they weighed the same amount as real golf balls. To expert golfers, these manipulated practice balls felt as though they were heavier than the (equal weight) real balls. By contrast, non-golfers experienced no such effect – adjusted practice balls and real balls felt the same – presumably because they had no expectations that the practice balls would be lighter than the real balls.

Visual illusions get a lot of attention – they’re easy to share and discuss. Buckingham’s paper provides a valuable glimpse (or feel) into the lesser known world of heaviness illusions. It’s surprising to discover that the causes of many of these illusions remain controversial and mysterious. To take just one example, it’s been shown that subjective, conscious expectations can’t fully account for the size-weight illusion. The size of the illusion is the same whether it is performed with large and small metal cubes, or large and small polystyrene cubes – even though people’s expectation “larger = heavier” would be so much stronger for metal than for polystyrene cubes. This tells us that something else is distorting people’s perceptions of the cubes’ true weight, other than their conscious expectations.

“Future work should aim to determine the nature of the bottom-up influences [those pertaining to the nature of the object, or the forces acting on it] in weight perception as a function of the lifting task,” said Buckingham, “in addition to identifying how these bottom-up effects interact with top-down expectations across the various types of weight illusion.”


Buckingham, G. (2014). Getting a grip on heaviness perception: a review of weight illusions and their probable causes Experimental Brain Research, 232 (6), 1623-1629 DOI: 10.1007/s00221-014-3926-9

Post written by Christian Jarrett (@psych_writer) for the BPS Research Digest.

The Alien awakened by a rubber hand

What happens if you administer a tactile illusion to a brain-damaged patient whose hand is out of their control? A team of researchers has done just that, figuring that illusions could offer new insights into complex neuropsychological disorders.

The patient in question was a 69-year-old lady whose left-sided stroke had left her with alien hand syndrome*. Most of the time her right hand was held in a clenched position that she couldn’t open. Occasionally, accompanied by a mild electric sensation, it moved involuntarily, jerking, or even slapping her in the face.

Michael Schaefer and his colleagues at Otto-von-Guericke University Magdeburg tested the lady on two sensorimotor illusions – the traditional rubber hand illusion and the lesser-known somatic rubber hand illusion. The first involved the patient placing one of her arms on the table-top, with the other underneath. A rubber arm was placed alongside her real arm on the table. The researcher then stroked the patient’s hidden arm and the rubber arm in synchrony. When the illusion works it creates the sensation of feeling in the rubber arm, as if it’s a part of the person’s body. In fact the patient experienced no feeling in the rubber arm at all, regardless of whether it was her healthy arm or alien arm that was being stroked under the table. The rubber hand illusion doesn’t work for everyone so this null finding is not particularly surprising.

Things got more interesting when the researchers tested their patient with the somatic rubber hand illusion (see picture, above). This procedure involved the rubber arm being placed between the patient’s two real arms on a table-top. This time, the patient was blindfolded and the researcher (wearing plastic surgical gloves) picked up one of the patient’s hands and used it to tap the rubber hand. At the same time, and in synchrony, the researcher tapped the patient’s other hand. This procedure creates the strong illusion for the participant that they are touching their own hand rather than the rubber hand – a feeling that the patient said she experienced.

But something surprising also happened when the researchers tried out this illusion. Within moments, the patient’s alien hand leapt up off the table and was grabbed by her healthy hand. She said she felt an electric sensation in her alien hand prior to it rousing. The illusory experience seemed to have awakened her alien hand. This effect occurred every time the procedure was repeated. But crucially it only happened when it was the patient’s healthy hand that was used to tap the rubber hand, whilst the patient’s alien hand was simultaneously tapped by the researcher (and not when the illusion was done the other way around). The awakening effect also disappeared when the procedure was repeated with the patient’s blindfold removed, which is known to destroy the illusion.

All this suggests that it wasn’t touching the alien hand per se that roused it, but rather it was the experience of the body illusion. Schaefer and his colleagues think that their patient has a disconnect between the anterior supplementary motor area (SMA) at the front of her brain (involved in inhibitory control) and other brain regions involved in movement. They reckon this impaired motor integration somehow interacted with the illusory feelings of body ownership triggered by the rubber hand trick. Perhaps, they said, the illusion further weakened the SMA’s already compromised control of the alien hand.

“Although our results should be confirmed by further studies, we believe that the examination of experimental-induced illusions in patients with disorders of self-embodiment is promising and might help us to develop treatments for these diseases in the future.”


Michael Schaefer, Hans-Jochen Heinze, and Imke Galazky (2012). Waking up the alien hand: rubber hand illusion interacts with alien hand syndrome. Neurocase: The Neural Basis of Cognition DOI: 10.1080/13554794.2012.667132

Further reading: Sergio Della Sala on the bizarre ‘Dr Strangelove syndrome’ and what it tells us about free will (Psychologist magazine article).
Simulating anarchic hand syndrome in the lab (earlier Digest report).

*Some experts prefer the term anarchic hand syndrome for this patient’s condition, reserving the term alien hand syndrome for a distinct but related condition in which the patient no longer believes the hand is theirs. For consistency I decided to use the terminology adopted by the authors of this paper.

Post written by Christian Jarrett for the BPS Research Digest.

Children with autism are less susceptible to the rubber hand illusion

The ability to tell where our bodies end and the rest of the world begins comes so naturally we tend not to give it much thought. In fact the brain mechanisms underlying bodily-identity are a vital part of basic social functioning. Given that social difficulties are a central part of autism, a team of US researchers led by Carissa Cascio wondered whether autism might be associated with differences in these basic mechanisms underlying body ownership.

To find out, they performed the first ever published test of how children on the autism spectrum experience the rubber hand illusion – a well-known procedure in psychology that exploits the mechanisms that give rise to feelings of body ownership.

Twenty-one children diagnosed with autism spectrum disorder (ASD) and 28 neurotypical controls (aged 8 to 17 years) undertook the illusion with an experimenter who was blind to the aims of the study. Tested one at a time, each participant sat opposite the experimenter and placed their left forearm and hand on the desk, out of sight, within a purpose-built container. To the right of their concealed left hand was visible a realistic rubber left hand.

The experimenter stroked with a cosmetic brush the participant’s hidden left hand between the second and third knuckles of their index finger and at the same time, in full view, stroked the rubber hand in the equivalent location. For two 3-minute phases the stroking was done on the real hand and rubber hand in synchrony – to the person being stroked this often gives rise to the illusory sensation that the rubber hand is their own. For another two 3-minute phases, the stroking was done out of synch, which usually spoils or reduces the experience of the illusion.

The key finding is that, unlike the controls, the children with ASD didn’t experience the illusion after the first 3-minute phase of synchronous stroking; they only experienced it after the second phase. This was tested objectively by having the children close their eyes and indicate with their right index finger where they thought their left index finger was located. Mislocating their finger towards the location of the rubber hand was taken as a sign that they’d experienced the illusion. Children with ASD may be less susceptible to the rubber hand illusion during synchronous stroking because they prioritise proprioceptive (tactile) information over visual information (the sight of the stroking).

The children were also asked to say whether they’d experienced certain sensations during each stroking phase, such as “It seemed as though the touch I felt was caused by the brush touching the rubber hand”. Here another difference emerged between the groups, with some ASD children agreeing more to this statement after the asynchronous stroking. This suggests some of them experienced the stroking as being synchronous when it wasn’t, perhaps because they have a less fine-tuned sense of whether information from different sensory modalities is being experienced in time.

The clinical relevance of the results is hinted at by the fact that ASD children with more impaired empathy scores tended to experience the rubber hand illusion even less strongly (based on their being less likely to mislocate their left index finger towards the rubber hand).

“Our results suggest that the malleability of the sense of body ownership is compromised in ASD, which may correspond to an altered cortical representation of the bodily self,” the researchers said. “This in turn may give rise to diminished capability for perspective-taking and empathy, as is seen in ASD.”
Cascio, C., Foss-Feig, J., Burnette, C., Heacock, J., and Cosby, A. (2012). The rubber hand illusion in children with autism spectrum disorders: delayed influence of combined tactile and visual input on proprioception. Autism DOI: 10.1177/1362361311430404

Post written by Christian Jarrett for the BPS Research Digest.