Category: Illusions

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.

Visual illusion could help prevent falls

Visual illusions are not only fun, they also help show how the brain works by exploiting its shortcomings. But what about using visual illusions for practical benefit? By making a step look taller than it really is, David Elliott and colleagues have demonstrated a way of doing just that.

Trips on steps are nasty for anyone, but for the elderly they can be fatal. Two thousand elderly people die in the UK every year following a fall, with the majority of these falls happening on stairs.

Elliott’s team asked twenty-one students to judge the height of two steps, one of which was decorated with horizontal bars on its leading face, thus making it look shorter; the other was decorated with vertical bars, thus making it look taller (the right-hand step on the image above). Both steps were actually the same height. Asked to estimate the height of the steps, the students guessed the height of the vertically decorated step to be just over 5 mm higher than the other step.

Most importantly, an eight-camera motion capture system showed that when the students stepped onto the steps, they lifted their foot higher for the vertically decorated step compared with the horizontally decorated step by a distance of about 5 mm. This was true whether the students looked with both eyes, or just one.

Most people trip on stairs because their toes clip the edge of the step, so an illusion that leads people to exaggerate the clearance they give to a step, even by only a small amount, could have significant benefits in terms of reducing falls. Ideally the researchers ought to have included a ‘control’ step that didn’t feature any decoration. However, this was a preliminary study and the researchers anticipate the illusion will be enhanced through future tests, increasing foot clearance still further.

As well as having practical implications, this study also has theoretical importance. An influential account of visual processing posits that there are two pathways in the brain: the dorsal “where” pathway and the ventral “what” pathway, with only the latter being prone to visual illusions. In support of this account, some experiments have shown that people’s perception can be tricked by an illusion (such as the size of an object) while their motor system is unaffected, as demonstrated by the person using an appropriate grip size. The current observation that both perception and action were tricked by the design of the steps, challenges this dual pathway account.

“The most parsimonious explanation of our results is that visuomotor actions are directed by the visual system without the need to invoke two wholly separate pathways for action and perception in the dorsal and ventral streams respectively,” the researchers said.

ResearchBlogging.orgDavid B. Elliott, Anna Vale, David Whitaker, John G. Buckley (2009). Does My Step Look Big In This? A Visual Illusion Leads To Safer Stepping Behaviour. PLoS ONE, 4 (2) DOI: 10.1371/journal.pone.0004577

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

Shadow illusion casts light on psychotic experience

A 22-year-old epilepsy sufferer with no known psychiatric problems has described the eerie feeling that a shadow-like person is mimicking her actions, when really no-one is there. She had the experience when, prior to surgery, Swiss researchers applied electrical probes to the left temperoparietal junction region of her brain. This area is known to be involved in multisensory integration and in distinguishing the self from others.

When the patient was lying down and the probe was applied, she felt as though a figure was behind her. “He is behind me, almost at my body, but I do not feel it”, she said. When she sat upright and embraced her knees, she described the unpleasant sensation that the shadow-like man was now also sitting and was clasping her arms. During a language-task in which she was asked to read out words on cards, she said “He wants to take the card; he doesn’t want me to read”.

The woman’s perceptions resemble those reported by some psychiatric and neurological patients – in particular she didn’t realise the figure was an illusion of her own body – and Shahar Arzy and colleagues concluded their findings may help understand the mechanisms behind experiences like paranoia and alien control. “It is notable that hyperactivity in the temporoparietal junction of patients with schizophrenia may lead to the misattribution of their own actions to other people”, they said.

Incidentally, the practice of exploring brain function in epilepsy sufferers prior to surgery is hardly new – the celebrated Canadian neurosurgeon Wilder Penfield famously charted some of the first somatosensory maps by observing patients’ responses when he stimulated parts of their exposed brain with an electric probe.

Arzy, S., Seeck, M., Ortigue, S., Spinelli, L. & Blanke, O. (2006). Induction of an illusory shadow person. Nature, 443, 287.

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

Link to related Digest item.
Link to discussion of issues raised by this research.

Rabbit illusion tricks the brain too

Like a kind of neural Voodoo doll, there’s a representation of our body in our brain, so that when we’re touched on, say, our left arm, the brain’s representation of our left arm is activated. So what do you think would happen in a tactile illusion when being touched on one part of the body leads to the sensation of having been touched somewhere else? Would it be the brain’s representation of the body part that was touched that was activated, or would it be the brain’s representation of where the touch was ‘felt’ to have occurred, that was activated.

That’s what Felix Blankenberg and colleagues have investigated using a brain scanner and the Rabbit illusion. In this illusion a person’s wrist is tapped several times followed by tapping of their elbow. Under optimal conditions it can lead to the sensation that the tapping continued up the arm from the wrist to the elbow, like a ‘rabbit’ hopping up the arm.

Blankenberg placed electrodes along the arms of thirteen participants and compared the brain activity that occurred when pulses were delivered all the way up their arm; when six pulses on the wrist were followed by three at the elbow (inducing the Rabbit illusion, as confirmed by participants’ reports); and in a control condition, in which three pulses were given at the wrist, three at the elbow, followed by three at the wrist again – a pattern that does not induce the illusion.

They found the brain’s representation of the middle part of the forearm (in the primary somatosensory cortex) was activated when pulses were actually delivered there, and crucially, also when, during the illusion, sensations were felt there even though no pulses were actually delivered there. By contrast, the region was not activated during the control condition.

“The intervening hops of the rabbit that get mislocalised and filled-in for conscious phenomenology evidently also get filled in and appropriately re-localised within human primary somatosensory cortex”, the researchers concluded.

Blankenburg, F., Ruff, C.C., Deichmann, Rees, G. & Driver, J. (2006). The cutaneous rabbit illusion affects human primary sensory cortex somatotopically. PLoS Biology, 4, e69.

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

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