Brain quirks

How blind people see

Blind people have revolutionised our view on vision. Biology text books still teach us that vision functions roughly as light hitting the eyes where special cells – rods and cones – turn it into neural signals. These travel to the back of the head, the visual cortex, for brain processing leading to something we experience as ‘seeing’. Some blind people have offered a completely new picture. They see without visual cortex. They see without rods or cones. They see without experiencing ‘seeing’.

Stevie Wonder

Wearing sunglasses might impair vision – in the blind.

The visual cortex lies right at the back of the head and it is – as the name suggests – responsible for vision. If you lose it, you can’t see anymore. This happened to a partially blind patient only known by his initials DB, a man brought to scientific fame in 1974 by an article in the journal Brain. In it, Lawrence Weiskrantz and colleagues describe how DB is asked to say whether he is presented an X or an O in an area of his visual field where he is blind. DB performs more than 80% correct despite only guessing.
What happened when DB was told about his visual abilities? ‘[H]e expressed surprise and insisted several times that he thought he was just “guessing.” [H]e was openly astonished’ (p. 721). This phenomenon has been termed blind-sight and it is very unlike normal vision. It is usually much worse but there are exceptions. For example, DB is actually better at ‘blind-seeing’ very faint lines compared to his intact visual field or normal people’s vision (Trevethan et al., 2007). This rules out all sorts of concerns about blindsight such as the suggestions that DB might be lying or falsely describing degraded vision as no vision at all. Unusually good performance can hardly be faked.
If blind-sight is possible without visual awareness or visual cortex, is it also possible without the eye’s rods and cones which turn light into neural signals? Interestingly, yes. Back in 1995 a team led by Charles Czeisler reported an unusual finding in three blind people whose eyes were damaged due to various diseases. When a bright light was shone in their face, they had less melatonin – a hormone related to the sleep cycle – in their blood. Probably a little known cell type – called intrinsically photosensitive retinal ganglion cells – turned light into neural signals and generally helps us synchronize our sleep-wake cycle with the day-night cycle.
A new article by Vandewalle and collagues shows what the potential of this newly discovered cell type is. They tested three blind people with eye damage and simply asked ‘is there a light or not?’ If a light was on for ten seconds, all three ‘guessed’ significantly differently from chance. This is remarkable as these people reported not seeing anything, electrical brain potentials following light flashes were curiously absent and their eyes were undoubtedly damaged.
When looked at together, these phenomena offer a new picture of the visual system. In the text-books you see a linear picture roughly like this:
light –> rods/cones in the eye –> visual cortex –> rest of brain
A new model is needed because a remarkable range of behaviours can still be performed when the middle elements of this account are removed. Instead of a linear picture we need a collection of parallel pathways all using light to influence the brain. The blind-sight pathway proves that circumventing the visual cortex is possible. People without rods/cones prove that not even these cells are needed to make use of light.
And now imagine that vision is one of the best-understood systems in the brain. If even vision can offer such surprises it is difficult to imagine what other brain systems hide below the surface. However, going ‘below the surface’ also comes with a considerable cost. Ask blind people what they see and they simply say ‘nothing’. Their residual abilities are hidden from them. It takes careful psychological testing to make them aware of what they can do.
So, how do blind people see? Some of them see without even knowing it.

Czeisler CA, Shanahan TL, Klerman EB, Martens H, Brotman DJ, Emens JS, Klein T, & Rizzo JF 3rd (1995). Suppression of melatonin secretion in some blind patients by exposure to bright light. The New England journal of medicine, 332 (1), 6-11 PMID: 7990870

Trevethan CT, Sahraie A, & Weiskrantz L (2007). Can blindsight be superior to ‘sighted-sight’? Cognition, 103 (3), 491-501 PMID: 16764848

Vandewalle G, Collignon O, Hull JT, Daneault V, Albouy G, Lepore F, Phillips C, Doyon J, Czeisler CA, Dumont M, Lockley SW, & Carrier J (2013). Blue Light Stimulates Cognitive Brain Activity in Visually Blind Individuals. Journal of cognitive neuroscience PMID: 23859643

Weiskrantz L, Warrington EK, Sanders MD, & Marshall J (1974). Visual capacity in the hemianopic field following a restricted occipital ablation. Brain : a journal of neurology, 97 (4), 709-28 PMID: 4434190


1) By Antonio Cruz/ABr (Agência Brasil.) [CC-BY-3.0-br (, via Wikimedia Commons

Feeling someone else’s sensation of touch – the neural background, the examples, and you

Touch is the only sensation which we cannot share with another person. The immediacy of touch differentiates it from the distant impressions which sight and audition can give us. However, modern neuroscience is currently revising this picture: you can touch at a distance. One just doesn’t notice it. Can we find people who do?

A very possessive romantic partner may mind when his love interest was looked at. But it is a whole different game if touch was involved. There is something intuitively different about touch which pervades every day culture. It is, arguably, the lack of distance, the necessary intrusion of the touching object into personal space. This makes touching a very personal experience, far more so than seeing or hearing.
Modern neuroscience is currently revising this picture. The first report to challenge the immediacy of touch came out in 2004. A team led by Christian Keysers found that when people saw someone else being touched on the leg they showed activation in the same brain area as when their legs were being touched directly. Curiously, seeing touch from a first person perspective led to similar brain region activity as seeing it from a third person.
What it looks like to touch and being touched in the secondary somatosensory cortex.

What it looks like to see touch and being touched in the secondary somatosensory cortex.

This got several laboratories around the world started on the topic. For example, recently, Schaefer and colleagues showed that when hands are touched or seen to be touched, perspective does actually matter – a first person view-point increases activation more in primary areas than a third person perspective. In any event, the general picture was not a statistical fluke but instead a replicable finding – being touched is represented in a very similar way in the brain as seeing someone else being touched. But this raises two questions: why can’t I feel anything then and why does it happen?
In 2009, Ramachandran and Brang published a paper which may provide an answer to the first question. They studied four amputees who had lost one hand due to accident. When they watched an experimenter being touched on her hand, the lost hand’s phantom ‘felt the touch’ after a few seconds. One anecdote shows the power of this finding:
‘Patient 1 even added that after we had demonstrated this, he had gone home and asked his wife to massage her own hand while he watched, and watching her do so seemed to relieve his phantom pain.’
Importantly, this was not the case for intact hands – whether of control participants or the amputees. The difference, thus, appears to be whether the sensation felt by others is in competition with the own direct input from the skin. If so, the own touch wins the competition and one does not consciously feel someone else’s experience. But without a hand providing direct sensory input – as in the case of amputees – the touch felt by others becomes vivid.
Apes (of the non-human variety) collaborating.

Apes (of the non-human variety) collaborating.

This still leaves the question as to why this happens. The common explanation is that having the capacity to feel the touch of someone else – even if it is so faint as to be below the level of awareness – aids our ability to understand others. As a social species we need a high level of empathy in order to work together efficiently. Evolutionary ancestors who had a touch-empathy link may have been better at collaborating and, thus, were better able to survive and reproduce.
The current account makes an interesting prediction. Next time you have an anaesthetized hand or foot – and thus no own skin-sensation – you might want to check whether you can feel someone else’s touch. Let me know whether it worked. This experiment has not, as far as I can tell, be done, yet. You yourself could disprove the immediacy of touch.

Keysers C, Wicker B, Gazzola V, Anton JL, Fogassi L, & Gallese V (2004). A touching sight: SII/PV activation during the observation and experience of touch. Neuron, 42 (2), 335-46 PMID: 15091347

Ramachandran VS, & Brang D (2009). Sensations evoked in patients with amputation from watching an individual whose corresponding intact limb is being touched. Archives of neurology, 66 (10), 1281-4 PMID: 19822785

Schaefer M, Heinze HJ, & Rotte M (2012). Close to you: embodied simulation for peripersonal space in primary somatosensory cortex. PloS one, 7 (8) PMID: 22912698



1) adapted from Keysers et al., 2004, p. 337

2) By Ikiwaner (Own work)  via Wikimedia Commons

The biological basis of orchestra seating

Many cultural conventions appear like the result of historical accidents. The QWERTY – keyboard is a typical example: the technical requirements of early typewriters still determine the computer keyboard that I write this text on, even though by now technical advances would allow for a far more efficient design. Some culturally accepted oddities, however, appear to reflect the biological requirements of human beings. The way musicians are seated in an orchestra is one such case, but the listener is, surprisingly, not the beneficiary.

When one goes to a concert one typically sees a seating somewhat like the one below: strings in the front, then woodwinds further back, then brass. What is less obvious is that, in general, higher pitched instruments are seated on the left and lower pitched instruments on the right. The strings show this pattern perfectly: from left to right one sees violins, violas, cellos and then basses. Choirs show the same pattern: higher voices (soprano and tenor) stand left of the lower voices (alt and basses). Why is that?


orchestra; seating arrangement; Nijmegen; Nijmegen studenten orkest

An orchestra I have personally performed with.

It turns out that this is not a historical accident but instead a biological requirement. Diana Deutsch has used a series of audio illusions which all showed a curious pattern: when you present two series of tones each to one ear, you have the illusion that the high tones are being played to your right ear and the low ones to the left ear. In case you don’t believe me, listen to this illustration of Deutsch’s scale illusion:
Apparently, there is a right ear advantage for high tones. So, seating the higher instruments on the left side (as seen on the photo) makes complete sense as this way musicians on stage tend to hear higher tones coming from their right. However, from the point of view of the audience this is actually a really bad idea as their right ear advantage is not taken into account. It turns out that orchestra seating arrangements are not favouring the hearing of the audience or the conductor but instead the musicians!
The right ear advantage for high tones is even mirrored in musicians’ brains. We know that the right ear projects mostly to the left auditory cortex and vice versa for the left ear. So, one would expect that people who play high instruments have trained their right ear / left auditory cortex the most when they practiced their craft. These training effects should be mirrored in differences in cortex size. This would mean that people sitting on the left in an orchestra have bigger left auditory cortices. In a fascinating article Schneider and colleagues showed that by and large this is the case: professional musicians who play high instruments or instruments with a sharp attack (e.g., percussionists, piano players) tend to have greater left auditory cortices than right auditory cortices. Their figure says is all.
Schneider; orchestra; seating; brain; Heschl's gyrus; primary auditory cortex; cortical size

How the brains are seated in an orchestra.

The orchestra seating arrangement mirrors not only the listening biases of most human ears but on top of that the brain differences between musicians. By and large, the orchestra is organised according to biological principles. Thus, not all cultural conventions – like the seemingly arbitrary seating arrangement of orchestras – have their roots in historical accidents. Cultural oddities are sometimes merely down to biology.

Deutsch, D. (1999). Grouping Mechanisms in Music The Psychology of Music, Second Edition, 299-348 DOI: 10.1016/B978-012213564-4/50010-X

Schneider P, Sluming V, Roberts N, Bleeck S, & Rupp A (2005). Structural, functional, and perceptual differences in Heschl’s gyrus and musical instrument preference. Annals of the New York Academy of Sciences, 1060, 387-94 PMID: 16597790



1) Nederlands: Symfonieorkest Nijmegen in de grote zaal van de Vereeniging, The SON photo library, via wikimedia

2) as found in Schneider et al., 2005, p. 392

What lies behind the mystery of being born with a phantom penis?

Zan Zig, magic, magician, white rabbit, hat, 1899

This is nothing compared to what the mind does with us every day.

Like a magician our mind tricks us into believing what we see and feel. We only notice that something strange is going on when our expectations are betrayed during the prestige – when the white rabbit is drawn out of the empty hat. Psychology sometimes works in much the same way. After the mind has made us believe in the ordinary, it creates strange cases which point to something bigger going on behind the scenes. One of the most extraordinary illusions is the one of our body. At the final prestige we see people born with phantom penises which no one can see. What was going on behind the scenes?

‘Phantoms’ is what Silas Weir Mitchell called the amputated limbs that their owners could still feel. The most straight-forward explanation simply refers to re-membering. When an amputated limb lives on as a phantom arm one could say that the mind fails to realise the loss and fills in the usual feelings with memories. This re-membering may well explain why some people claim to feel a watch or even clothes on the phantom skin.
It is as if the magician had produced a rabbit out of an ‘empty’ hat and everyone suddenly noticed that the hat was high enough to house it from the start. However, the mind had another trick up its sleeve. Since the initial description of phantom limbs in 19th century amputees, this phenomenon has also been discovered in people who had never been born with limbs to begin with. These so called congenital phantom limbs are very strange because their owners obviously have no memories of limbs. Re-membering cannot explain this.
Perhaps it is time to turn from psychology to neuroscience in our quest to understand this trick. The part of the human brain responsible for limb movements is a well organised bit of cortex which looks very similar across people: the primary motor cortex. When the appropriate bit of my own primary motor cortex once got stimulated with magnetic waves, my index finger twitched. Peter Brugger and colleagues did the same with a woman only known as A.Z.. She was born without arms or legs but reported feeling them nonetheless. Magnetic stimulation of her primary motor cortex made her phantom limbs move. This suggests that the action control mechanism and the brain mechanism responsible for phantom limbs are linked.
Thus, all we know about action control in the human brain can be used to explain away the phantom limb phenomenon. Firstly, the primary motor cortex is at least partly genetically determined, i.e. limb control is part of our genetic make-up whether we’ve got limbs or not.  When trying to control limbs which do not exist, the brain may create the illusion of controlling phantom limbs instead. Secondly, some researchers believe that a muscle activation command is not only sent to the muscles but a copy is also sent to the back of the brain. This allows us to react to expected action outcomes even before they have occurred. Phantom limbs may occur because expected actions get misinterpreted as real ones. Thirdly, mirror neurons code for actions seen and actions done. According to this explanation A.Z. saw many people use their limbs and this made her have the illusion that she could do the same, albeit only with phantom limbs instead of real ones.
Venus de Milo, Louvre, phantom limb, Aphrodite of Milos

A Greek statue depicting phantom limbs.

However, the final prestige defies all these explanations. Something else entirely must be responsible for a phenomenon reported by Vilayanur Ramachandran and Paul McGeoch in 2008: phantom penises. Like phantom limbs they can occur after amputation. Fascinatingly though, they were also reported by female-to-male transsexuals without an artifical penis. Crucially, this cannot simply be put away as ‘wishful thinking’. For one, their phantom penises were not perfect: for some they were shaped in an undesirable way, erected in embarrassing non-erotic situations, or rubbing against the jeans. But more importantly, Western society goes to great lengths to make life as a transsexual seem like an unattractive option. For example, when they were children, two phantom penis owners were taken to a psychiatrist by their puzzled parents to be treated for a penis that did not exist. Why would anyone want to go through this as a child – or indeed through life changing surgery as an adult – if it wasn’t absolutely necessary?
But if being born with a phantom penis cannot be explained by re-membering, brain mechanisms of action control (a penis is obviously not a muscle one can voluntarily control), or wishful thinking – then what lies behind this phenomenon? This final trick of the mind, seemingly the most ordinary sensation of being a man or a woman in a male or female body, defies easy solutions. Ramachandran and McGeoch speculate that hormonal factors before birth could be responsible.
Before any such speculation can be substantiated I can only conclude that this final prestige remains a mystery. Just like an audience member seeing a magician do a trick on a member of the public, I wonder whether I have been tricked as well. Phantom limbs and phantom penises show powerfully that the link between our anatomical body and our body image is a fragile one. The mind is doing all sorts of trickery behind the scenes in order to hide this difference between body felt and body seen. Like with any good magician, one wonders how this trick is actually done.


Brugger P, Kollias SS, Müri RM, Crelier G, Hepp-Reymond MC, & Regard M (2000). Beyond re-membering: phantom sensations of congenitally absent limbs. Proceedings of the National Academy of Sciences of the United States of America, 97 (11), 6167-72 PMID: 10801982

Mitchell, W (1871). Phantom limbs Lippinscott’s Magazine, 8, 563-569

Ramachandran, VS, & McGeoch, PD (2008). Phantom Penises In Transsexuals – Evidence of an Innate Gender-Specific Body Image in the Brain Journal of Consciousness Studies, 15 (1), 5-16


If you like this post, you may also like:
Three fun ways to have three hands – for you at home
1) By Strobridge Litho. Co., Cincinnati & New York  Restoration by trialsanderrors and Morn via Wikimedia Commons
2) By Shawn Lipowski (Shawnlipowski) (Own work) via Wikimedia Commons


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Extreme neural adaptation – how musical ability is lost through focal dystonia

‘There was a question of not having a purpose in life. Just floundering’.

Leon Fleischer was a true musical prodigy. By the age of sixteen he performed with the New York Philharmonic. He was called ‘the pianist find of the century’. Suddenly, in 1964, he lost control over his right hand. His fingers would simply curl up. The end of his career.

The illness which befell Leon Fleischer and about 1% of his fellow musicians is called focal dystonia, the loss of control over muscles involved in a highly trained task. It is a career breaker coming out of the blue. An investigation into the underlying neural problems leads on a journey into the brain’s muscle control circuitry and its ability to learn.
Primary Motor Cortex, M1
The human brain’s motor system which controls muscle movement is well understood. When one stimulates areas in the precentral sulcus (see Figure) one can observe muscle movements. I actually once saw my own finger move when this area was magnetically stimulated. Because the role of this brain area in muscle control is so well understood, it is simply called the primary motor cortex, or M1 for short.
The organisation within the primary motor cortex is such as described in the Figure: inside the brain the leg muscles are controlled, going to the side come hand areas and eventually facial muscles. Localisation of function (where in the brain is x?) doesn’t get much better.
motor cortex

The motor cortex, called: homunculus.

Motor learning is nothing else than changing the brain in order to better perform a task. Roughly, one learns when an intended outcome and sensory feedback about the actual outcome disagree. Focal dystonia is probably an example of how the brain’s ability to learn can be pushed too far. This illness messes up the localisation of function in one of the most clearly organised brain areas.
For example, the primary motor cortex’s finger areas are usually nicely aligned. However, when dystonia affects a finger, its brain area moves away from its allocated place. Furthermore, the amount of brain tissue which only controls the dystonic finger is reduced, likely because adjacent fingers take over some of the finger’s area (Burman et al., 2009). Thus, ineffective control over muscles because of a subtle disorganisation of motor control areas could be the brain basis for focal dystonia.
On the other hand, rather than the outcome of learning – motor control area changes – the process of learning could also be the reason for the illness. Sensory feedback from the fingers arrives on the other side of the ridge that separates the frontal part of the brain (which includes the motor cortex) and the back part beginning with the so called parietal cortex. The area responding to touch is called the somatosensory cortex and – as can be seen in the Figure in blue – it is also very well organised.
Elbert and colleagues (1998) found that dystonic musician’s digit areas were unusually tightly packed. Their MEG study thus shares some of the findings with Burman et al.’s fMRI study. Apparently, movement execution is disorganised, but also feedback is to some degree jumbled up. The brain seems to have lost some of its nice organisation in areas related to dystonic impairments.

Subcortical differences in sufferers of primary focal dystonia. The eyes would be on the left.

Lastly, a recent meta-analysis by Zheng and colleagues (2012) adds two more things to this picture. The aforementioned activation abnormalities in sensorimotor areas are mirrored in unusual structural features. Furthermore, areas deep inside the brain related to motor planning and movement initiation also show such structural abnormalities.
Focal dystonia seems to affect all sorts of parts of the brain’s sensori-motor system both in terms of brain structure and how the structure is used. Which of these effects actually cause the illness and which are just consequences cannot be said based on these findings. Still, the unusual mappings in the motor and the somatosensory cortices together with deep brain abnormalities are an indication that the brains of dystonic musicians may have adapted too much to the demands of professional instrument playing. Neither the brain’s control over the body’s muscles is good enough anymore nor the feedback from the fingers.
There is still no reliable cure for focal dystonia. Some people treat the symptoms with botox to the affected muscles. Otherwise, retraining of the brain’s sensorimotor areas away from the maladaptation is currently being tried.
How did Leon Fleischer deal with focal dystonia? He had to change his involvement with music to one-armed piano pieces, conducting, and music teaching. Later, surgery and some treatment of the symptoms improved his condition. He is by no means cured. Still, he can finally play the piano again with both hands. As you can hear and see in the Academy Award nominated documentary Two Hands, his performance sounds wonderful, but look closely at his right hand’s fingers.
This is what a breakdown in brain organisation looks like.


Burman, D.D., Lie-Nemeth, T., Brandfonbrener, A.G., Parisi, T., & Meyer, J.R. (2009). Altered Finger Representations in Sensorimotor Cortex of Musicians with Focal Dystonia: Precentral Cortex Brain Imaging and Behavior, 3, 10-23 DOI: 10.1007/s11682-008-9046-z
Elbert, T., Candia, V., Altenmüller, E., Rau, H., Sterr, A., Rockstroh, B., Pantev, C., & Taub, E. (1998). Alteration of digital representations in somatosensory cortex in focal hand dystonia. Neuroreport, 9 (16), 3571-3575 PMID: 9858362
Zheng, ZZ., Pan, PL., Wang, W., & Shang, HF. (2012). Neural network of primary focal dystonia by an anatomic likelihood estimation meta-analysis of gray matter abnormalities Journal of the Neurological Sciences, 316, 51-55 DOI: 10.1016/j.jns.2012.01.032


1) By 3D brain data is from Anatomography. (3D brain data is from Anatomography.) [CC-BY-SA-2.1-jp (, via Wikimedia Commons
2) By Maquesta via Wikimedia Commons
3) Found in Zheng et al. (2012, p. 53)




[update 15/8/2014 11:00 new Figure 2 as old one was deleted from wikimedia commons]

Three fun ways to have three hands – for you at home

Tired of having been born with only two hands?

Jealous of Indian goddesses?

Doubtful about Psychology and Neuroscience’s ability to replicate findings?

Then this set of exercises is for you. No need for any technical equipment. If all goes well you will grow* a hand as part of all this. You will have the strong feeling that you have three hands. You will feel and/or care about the illusory third limb. Let’s get started.
1) An additional cheeky hand
Equipment: none
Partners needed: 1
Time: 3 minutes
Success rate: 43% (70% report some sort of self-touch illusion)
Publication: Davies & White (2011)
a) Your partner and you place your hands on the same warm water bottle in order to have equally warm hands.
b) Sit down and close your eyes. Your partner sits opposite you. Her eyes are open.
c) Your partner takes your right hand and makes you stroke and tap yourself lightly on your right cheek. At the same time she herself administers synchronous, identical strokes and taps with her hand to the corresponding location on your face’s left side. Your own left hand simply rests.
d) Vary pressure and frequency of strokes and taps. Mind that on each side of the face timing and pressure have to match. Do so for three minutes.
Davies and White, 2011_cheeky hand illusion

Set-up to add a cheeky hand.

Outcome: The feeling that some sort of third hand, a disconnected one for example, strokes your left cheek.
2) Two rubber hands
Equipment: two rubber/plastic/wooden or otherwise somewhat realistic feeling right hands, a table, one double paint brush (see picture), one normal paint brush
Partners needed: 1
Time: 2 minutes
Success rate: not reported
Ehrsson, 2009_two rubber hands

Set-up to induce two rubber hand illusions simultaneously.

Publication: Ehrsson (2009)
a) Sit at a table. Place your right hand underneath the table, e.g., on your leg.
b) Place the rubber hand models of right hands in front of you over the area where your real right hand is. The models should be 10cm apart.
c) Look at the rubber hands. At the same time you partner uses the double paint brush to stroke the rubber hands and the single paint brush to stroke your real hand. These strokes need to be absolutely in synchrony.
Outcome: The feeling that both rubber hands are your right hands.
3) An additional rubber hand
Equipment: a rubber/plastic/wooden or otherwise somewhat realistic feeling right hand, a table, two paint brushes, a piece of cloth
Partners needed: 1
Time: unknown
Success rate: unknown
Publication: Guterstam, Petkova, & Ehrsson (2011)

Left: Set-up to add rubber hand to one’s own body. Right: Testing how real the ownership of the rubber hand really is (don’t do this test at home).

a) Sit at a table. Put your real right hand on the table in front of you.
b) Place the rubber hand in a similar position slightly to the left of your real hand, about 12cm apart. Cover the space from your real shoulder to the arm-bit of the rubber hand with the cloth.
c) Look at the rubber hand. At the same time your partner uses the two paint brushes to stroke both your real right hand and the rubber hand simultaneously on the index and middle fingers. She needs to do so absolutely synchronously, matching the strokes in time and speed. It is best to stroke irregularly but still synchronously.
Outcome: The feeling that both the real hand and the rubber hand are your right hands.
Did these illusions work for you? Let me know!
As a bonus for all those still in for some more, the following two techniques substitute your real hand for a fake one.
Bonus 1) A rubber hand (with vision)
Equipment: a rubber/plastic/wooden or otherwise somewhat realistic looking hand (it can be a bit bigger – but not smaller – than your real hand and it can also have a different ‘skin’ colour), two identical paint brushes, one standing screen (a big book would do as well), one table
Partners needed: 1
Time: 10 minutes
Success rate: 42%
Publication: Botvinick & Cohen (1998)
a) Sit at a table. Place the screen in front of you and hide your left hand behind it. Be sure you cannot see your left hand.
b) Place the rubber hand model of a left hand in front of you on the table.
c) Look at the rubber hand. At the same time your partner uses the two paint brushes to stroke both your hidden hand and the rubber hand simultaneously. She needs to do so absolutely synchronously, matching the strokes in time.
Outcome: The feeling that the rubber hand is your own hand.
Bonus 2) A rubber hand (without vision)
Equipment: a rubber/plastic/wooden or otherwise somewhat realistic feeling hand, three pairs of rubber gloves, a table
Partners needed: 1
Ehrsson et al., 2005: rubber hand illusion without vision

Top: Set-up for inducing rubber hand illusion without vision. Bottom: same in a MRI scanner.

Time:  60 seconds
Success rate: 78%
Publication: Ehrsson, Holmes, & Passingham (2005)
 a) Both you, your partner and the rubber hand need to wear rubber gloves.
b) Sit at a table. Place the rubber hand model of a right hand in front of you on the table. Close your eyes.
c) Your partner takes your left hand and makes you touch the rubber hand’s index finger’s knuckle. At the same time she herself administers synchronous, identical touches with her hand to the corresponding location on your own right index finger’s knuckle.
Outcome: The feeling that you are touching your own hand even though you are touching the rubber hand.
Finally, if you now wonder whether scientists have also found a way to make you lose a hand, just watch this video. Unfortunately, the techniqual requirements go beyond what is available in most homes and so your own private replication of this illusion will be rather difficult to implement.

Aimola Davies, A.M., & White, R.C. (2011). Touching my face with my supernumerary hand: A cheeky illusion Perception, 40, 1245-1247 DOI: 10.1068/p6956

Botvinick, M., & Cohen, J. (1998). Rubber hands ‘feel’ touch that eyes see. Nature, 391 (6669) PMID: 9486643

Ehrsson, H.H. (2009). How many arms make a pair? Perceptual illusion of having an additional limb. Perception, 38 (2), 310-312 PMID: 19400438

Ehrsson, H.H., Holmes, N.P., & Passingham, R.E. (2005). Touching a rubber hand: feeling of body ownership is associated with activity in multisensory brain areas. The Journal of neuroscience : the official journal of the Society for Neuroscience, 25 (45), 10564-10573 PMID: 16280594

Guterstam, A., Petkova, V.I., & Ehrsson, H.H. (2011). The illusion of owning a third arm. PloS one, 6 (2) PMID: 21383847


images: from respective journal publications

*in a psychological sense

How are invisible colours possible?


Pharell Williams: ‘I could always visualize what I was hearing… Yeah, it was always like weird colors.’

To some people I am half colour blind even though I can see everything from blue to red like most people. For them it is odd that I can only see colours when they are directly presented to me. More than that, I can only see colours for which there are proper words. These people literally see black and white symbols in colour depending on which symbol it is. And the colours they see are sometimes those which I will never see in my life because they are invisible. Incredible? A journey through the visual brain shows how this feat is possible.

The first report of invisible colour perception came from Ramachandran and Hubbard (2001), then at San Diego. They briefly mentioned a man only known as S.S. who suffers from s-cone weakness, i.e. the cells in his eyes which are sensitive to blue are impaired. Therefore, he cannot see the full colour spectrum the way most people can.
S.S. also happens to be a grapheme-colour synaesthete, i.e. he literally sees a certain colour consistently when presented with, for example, a given number. However, these number-evoked colours were special. He described them as ‘weird’ or ‘extraspectral’. He had only ever seen them in his mind’s eye, never in the outside world.
Rather than being an isolated case, regular synaesthetes can see these so called ‘Martian’ colours as well  (Ramachandran & Hubbard, 2003). Furthermore, Oliver Sacks writes about a musician he calls Michael whose synaesthesia links musical keys with colours. Sacks writes that ‘some keys seem to have a strange hue which he can hardly describe, and which he has almost never seen in the world about him’ (2007, p. 182). Pharell Williams could be yet another case.
How can we make sense of this? To understand how the human brain can give rise to Martian colour perception, a quick tour through the visual brain is necessary. When light hits the eye, it is transformed into the electrochemical information currency of the brain. The information travels from the eyes directly to the back of the brain within about 100 milliseconds (a tenth of a second). Because this area in the back of the head is so well understood to be the primary target for visual information, it is simply called the primary visual cortex or V1 for short.
visual pathway

Note how information travels from the front to the back.

After some low level information processing in V1, information is passed on to other visual areas which are specialized in certain jobs, e.g. V5 processes motion, V4 processes colour, and grapheme areas process numbers and letters.
The brain’s left hemisphere. Grapheme area in green. Colour area in red.
When looking at where these different areas lie, one gets a sense for why motion-colour synaesthesia is not found while word/number-colour synaesthesia is so common. The human brain happens to be organised in such a way that V4 (colour) lies very close to the grapheme area. V5, on the other hand, is a lot farther. Research in the last decade (reviewed in Hubbard et al., 2011) has revealed that in synaesthetes colour and grapheme areas are unusually well connected and they show activation of the colour area just after the grapheme area responds when synaesthetes view graphemes.
Martian colours are thought to be so unusual because information has not taken the usual route (eye -> V1 -> V4) but instead went to a grapheme area first and then entered V4 (eye -> V1 -> grapheme area -> V4). Similarly, musically induced Martian colours may look so weird because auditory information recoded in V4 simply isn’t of the same quality as visual information which comes from V1.
So, colour perception appears not only determined by where information is processed but also by where it originates. A good lesson to remember whenever you try to localise function X in brain area Y: information origin matters.
Some people out there see things which are so unusual that there isn’t even a proper word for these experiences. To them I am not only half-colour blind (even though I do not suffer from s-cone weakness like S.S.). I am also unimaginative – as not just my eyes but also my imagination is limited to the colours of the rainbow, a subset of the colours which can be experienced. If you believe that the rainbow is complete, you may well be as colour blind as I am.

Hubbard, E.M., Brang, D., & Ramachandran, V.S. (2011). The cross-activation theory at 10 Journal of Neuropsychology, 5, 152-177 DOI: 10.1111/j.1748-6653.2011.02014.x
Ramachandran, V.S., & Hubbard, E.M. (2001). Psychophysical investigations into the neural basis of synaesthesia Proceedings of the Royal Society B, 268, 979-983 DOI: 10.1098/rspb.2000.1576

Ramachandran, V.S., Hubbard, E.M. (2003). The phenomenology of synaesthesia. Journal of Consciousness Studies, 10, 49-57.

Sacks, O. (2007). Musicophilia. New York: Vintage.

1) By kallerna (Own work) [CC-BY-SA-3.0 ( or GFDL (], via Wikimedia Commons
2) By Wiley (Wikimedia) [CC-BY-SA-3.0 (, via Wikimedia Commons
3) from Ramachandran, V.S., & Hubbard, E.M. (2001). Psychophysical investigations into the neural basis of synaesthesia. Proceedings of the Royal Society of London B, 268, p. 981.