Apr 09 2009

Imaginary Phantom Limb

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Comments: 31

The brain can be a freaky thing. We live in this illusion of reality created for us by our brains. This is not to say there is no objective external physical reality – there is as far as anyone can tell – but our experience of ourselves and the world is a neurologically generated illusion.

The brain processes sensory information so that it is a useful, and not necessarily accurate, depiction of the world. This sensory input is also highly selective, giving us that slice of reality that proved to be most evolutionarily adaptive. That part of our brain that pays attention then attends to a tiny slice of that highly processed selective sensory information and mostly ignores the rest.

The sliver of information that makes it to our awareness is combined with information from our memories – information about how we think the world works. The sensory data is therefore interpreted, mostly subconsciously, to fit our internal models of reality. Meanwhile, other bits of gray matter encode our mood and emotions, while others make us feel as if we are inside our bodies, which are in turn separate from the rest of reality. We have vast memory stores, organized as overlapping networks of pattern recognition. But can only hold a few bits (2-7) in our working memory where we can actively manipulate data.

This neurogenic cacophony of data and information processing are woven together into a seamless narrative we experience as our waking consciousness. It is a useful and adaptive approximation of reality – but it is not reality.

The experience becomes freaky when the process breaks down in some specific way, so that the seamless narrative is internally inconsistent, or bizarrely out of step with reality.  Neurologists all have their favorite stories about patients who exhibit such strange malfunctions. The common ones include neglect syndromes, where a patient’s internal model of reality loses one half of their body and the universe. They then struggle to make sense of their half-world, but have no awareness that they are doing so. If you hold up their neglected arm in front of their face and ask them whose arm it is, they will say that it is yours.

Geneva University neurologist, Asaid Khateb, reports on a 64 year old woman patient of his who presents with a very rare manifestation of the freaky breakdown in the neurological narrative. The woman suffered a stroke which paralyzed her left arm. Afterwards she reported having a new third arm, which she could see, feel, and move.

This phenomenon is known as a supernumerary phantom limb (SPL).  It is one of several types of disordered that have been described after a stroke leading to neglect of the paralyzed limb – the lack of recognition the limb. These can include simple lack of recognition that the weak limb belongs to them, the belief that the limb belongs to someone else, or the false belief that the limb is normal and functional. One study found that 92% of patients with neglect of a weak limb have some disordered sense of ownership of the limb. SPL just seems to be the most rare of such phenomena.

This patient feels as if they can move their third phantom limb. Of course, it has no physical existence, so they cannot manipulate external reality with it. However, if they scratch an itch with their phantom limb, the itch is relieved.

Khateb studied this patient with fMRI and he compared the brain activation of the patient while moving their intact right arm, while imagining moving their right arm without moving it, imagining moving their paralyzed left arm, and then moving their phantom limb. He found the expected pattern of brain activation while moving the good right arm, and similar but much decreased activity while imagining moving either the right or left arm.

When the patient moved their imaginary phantom limb, the brain activation looked similar to the pattern seen when actually moving the normal limb. This suggests that the patient is not just imagining that they have an extra limb – their brain is functioning as if the limb is real.

So what does this tell us about the relevant brain function, and what is happening in this patient? As I discussed above, there are specific bits of the brain that combine sensory information with motor intention and control to create the sensation that not only is there a limb attached to us, but we own it. This limb ownership seems to localize to the insular cortex – a part of the brain near the motor strip where voluntary control is located.

This sense of ownership is part of a loop of neural activity that includes proprioception – feeling where a part of our body is in three-dimensional space. This sensory modality is what enables you to know where your limbs are even when you cannot see them. But this loop also includes visual information. In fact, it can get confused if the visual information does not agree with the proprioceptive information. There are experiments underway, in fact, to trick the brain with visual information into thinking that a person occupies a virtual representation of themselves.

The loop of limb ownership also includes motor intention – that part of the brain that plans and initiates movement, and also the motor cortex that carries out the movement.

What is likely happening in SPL patients is that there is damage to some but not all of the structures in the loop of ownership. This loop becomes disconnected from the neglected and paralyzed limb, which results in the creation of a new phantom limb onto which the ownership is projected. In fact in some cases the SPL comes into existence when the patient tries to move their paralyzed limb – it comes into existence (the patient feels as if they are pulling it out) by intention.

This process is probably not dissimilar to the techniques that are being developed to trick the brain into creating a sense of ownership over a virtual simulation. This suggests that it is very possible to be made to actually feel and manipulate a virtual avatar in a video game or other virtual application. In this case a virutal limb is substituted for a phantom limb.

Such cases are not only extremely cool and fascinating, they provide key insights into how the brain works. The lesson here is that everything you feel and experience – even the most basic aspects of your existence – are generated by parts of the brain that evolved to generate that specific experience. This includes things most people take for granted, such as the sensation that you exist, that you are inside your body (and not floating outside of it), that you are separate from the world around you, and that you own and control your own body parts.

Further, this means that when people have bizarre experiences that are far outside what they are used to, especially when they involve features that are now known to be neurological, it must first be considered that their experiences result from altered brain states – not external reality.  However, we seem to have evolved to make the opposite assumption – that whatever we experience is real. This disconnect is one of the primary fuels for belief in the paranormal.

31 responses so far

31 thoughts on “Imaginary Phantom Limb”

  1. brianegan says:

    “There are experiments underway, in fact, to trick the brain with visual information into thinking that a person occupies a virtual representation of themselves.”

    I KNEW it! You’re all working for the robots, aren’t you? You’ll never get the Zion mainframe access codes!

  2. tmac57 says:

    I wonder if this explains why I can search my kitchen cupboard for five minutes for a can of green beans and swear that ” there aren’t any”, and my wife can look in there for 3 seconds and find them. Witchcraft I say!!!

  3. Puppet_Master says:

    I constantly reminded of how the brain creates the illusion of the world around us. Here’s a good example:

    I was on my way to school and I put my book on the passenger seat. My girlfriend got into the car and sat on top of my book, so I assumed my book was under her butt. When getting out of the car, I was reaching under her to get my book and couldn’t find it. She asked what I was doing and I asked her to get up because she was sitting on my book. She has the confused look on her face and I soon found out why – the entire time I was searching for my book, it was on her lap. My eyes where directed at the book and although the photons were hitting my retina, my brain was not seeing the book that was directly in front of me.

    I’m fascinated by that situation because it’s indicative of just how much the brain constructs our perception of reality – which in many cases does not correspond to actual reality.

  4. Enzo says:

    Amazing stuff. The brain is definitely the new biological frontier in the post-genomic age.

    “This process is probably not dissimilar to the techniques that are being developed to trick the brain into creating a sense of ownership over a virtual simulation.”

    I recall a story (forgive the lack of reference) detailing a patient’s problems involving an amputated limb which was still felt. The phantom limb was generating a pain sensation that felt as if it was constantly clenched, as if the fingers were digging painfully into the palm.

    The physician attached to the case, Dr. V. Ramachandran, devised a mirror contraption to trick the mind into thinking the remaining hand was the phantom hand, and the patient was able to “unclench” to temporarily relieve the pain. Awesome.

    Dr. Ramachandran’s website: http://cbc.ucsd.edu/ramabio.html

  5. CrookedTimber says:

    Very fascinating! Please forgive the naive question as this is far outside my field. I assume since it was the patients left arm that was paralyzed that the stroke occurred in the right hemisphere of the brain. Do similar instances occur if the stroke occurs in the left hemisphere of the brain?

    Also, it seems that this post could be twisted by nonsense peddlers to justify their “everything is an illusion, you can make your own reality” claims. I am sure you have anticipated this and are armed for rebuttal.

  6. tmac57 says:

    CrookedTimber-“Also, it seems that this post could be twisted by nonsense peddlers to justify their “everything is an illusion, you can make your own reality” claims. I am sure you have anticipated this and are armed for rebuttal.”
    Yeah, I am also anticipating something like ” the phantom 3rd arm has quantum shifted into a different dimension through the patient’s intention , and is thus invisible in our dimension, but is being manipulated in the new dimension through the undetectable ‘intentional force’.

  7. CrookedTimber says:

    tmac57 – LOL exactly.

    Or…because existing inside yourself is an illusion created in the brain, if you can train yourself to overcome it and tap into the ‘quantum reality’, you can…[insert bs here] distance heal, law of attraction, psi…

    I forsee another good debate brewing whether it be Chopra, Egnor, or another.

  8. Kudos to Puppet Master for that very creative excuse for copping a feel. (j/k).

    I seem to have a phantom third leg emitting from the middle of my back, the only purpose for which is to kick myself in the ass.

  9. neokortex says:

    Great post, Dr Novella. Good timing as well. By coincidence I am in the process of reading a book on David Hume’s philosophy, in which the current chapter describes his theory of mind. It’ll be nice to have a more contemporary science-based overview (albeit brief) of the subject to compare and contrast. It is interesting to contemplate how this might inform the formation and contours of political (and religious) ideologies–since ideologies often shape a person’s perceptions of reality–somewhat along your mention of paranormal beliefs. A whole different can of worms I know.

  10. artfulD says:

    Can you properly deduce the purposes behind what may simply be their unintended consequences?

  11. CrookedTimber – generally these do not occur with left (actually dominant, which is usually left) hemisphere strokes. The reason is that the right hemisphere has a bilateral map of the world. This is necessary for visuo-spacial processing, which is done by the right (non-dominant) hemisphere. The left hemisphere has only a right-sided map of the world. So with a left hemisphere lesion, the right hemisphere can still map the entire world and therefore there is no neglect, With right hemisphere lesions the bilateral map is gone, only the right-sided map of the left hemisphere remains, so there is neglect of the left side of the world.

  12. HHC says:

    Phantom limbs are real! Cried my alchohol dementia patient. He had no limbs but he insisted they were there and wanted the keys to drive his car off the psychiatirc unit. Look out motorists!

  13. HHC says:

    Dr. Novella, What is the medical explanations for the thinking of the client I described?

  14. HHC says:

    This man’s limbs were lost due to gangrene.

  15. Puppet_Master says:


    Do you know if he had diabetes? I’m curious to what caused the necrosis in the first place.

  16. HHC says:

    Puppet Master, I recall the patient had no legs and severe infections in both hands such that his infected fingers fell off his hands onto the floor of the unit.

  17. HHC says:

    If the gangrene is the result of diabetes, is the medical explanation simpler?

  18. unBeguiled says:

    “The lesson here is that everything you feel and experience – even the most basic aspects of your existence – are generated by parts of the brain that evolved to generate that specific experience.”

    I encountered a patient once with Anton-Babinski syndrome. She was completely blind, but denied it. Most of all I wanted to understand her qualia: what was her experience of “seeing” like?

    Of course, given current technology, all other person’s qualia are a mystery.

  19. Steve Page says:

    Very good post, Steve, and it should come in handy for my forthcoming neuropsychology essay/exam! I actually started my psychology degree because of cases like those that you mentioned; about five years ago, I read “Mapping the Mind” by Rita Carter, which was aimed at showing the lay audience just how amazing the brain was, and it was the first time I’d encountered neglect, extinction, blindsight, phantom limb syndrome, etc. and the first time I realised that we are literally the sum of our parts, inasmuch as damage to specific regions causes specific deficits…from that point on, I was hooked on the brain. It’s a bit of a shame that most people I encounter equate psychology with Freudian psychoanalysis.

  20. daedalus2u says:

    If a person can experience a phantom limb as real, the experiences of some individuals of phantom entities (i.e. the various gods) as real should not be so surprising.

  21. lurchwurm says:

    Kant would’ve loved to have this kind of evidence when he was writing the Transcendental Aesthetic! As a current student of Kant, this article helps me appreciate his genius even more, considering that most of the evidence he had to work with during his life was primitive, i.e. Descartes’ notion of distance making objects appear differently, Hume’s “all things are relations between ideas or matters of fact,” and Newton’s gravitational force replacing the mechanistic view of nature. I would love to see what Kant would have to say about current research in this area.

    Dr. Novella, have you had a chance to read through the Critique of Pure Reason, and if so, does his philosophy creep into your thoughts when you’re doing your research?

  22. empiricalgod2 says:

    Oliver Sacks explores such a phenomenon in one of his books, but cannot remember which one.

  23. HHC says:

    unBeguiled, Drs. Reitan and Wolfson have studied cortical blindness a result of ischemia or infarction of both occipital lobes. Vision is lost bilaterally, but the pupillary responses are intact. Some but not all patients have Anton’s syndrome. They believe they are able to see, deny blindness, and readily provide visual descriptions of their environment.

  24. unBeguiled says:


    “They believe they are able to see, deny blindness, and readily provide visual descriptions of their environment.”

    Right, that was my experience with my patient. But a description of a quale is not the same as the quale. That’s what I was getting at.

  25. kvsherry says:

    empiricalgod2-could you be thinking of “The Man Who Mistook His Wife for a Hat?”

  26. HHC says:

    kvsherry, If the man mistook his wife for a hat, I believe he would see the head of his wife and then recall the construct of hat at the same time. Organic brain damage can lead to talking to parking meters as persons. The top portion of the meter is visualized as the head and the long pole as the body. My recent
    case with a known alcohol/ methamphetamine user showed he would become enraged with parking meters and punch them out.
    Useful pasttime, eh?

  27. Saorsa says:

    I’ve shared a similar experience with some of those listed above. I was paralyzed in an accident and have no sensation in my lower body, but occasionally get the distinct–and exceedingly uncomfortable–feeling that my feet are twisted around backward. I can look at them and see that they are fine, but the sense of dislocation remains.

    Anyway, very fascinating article. As an aside, some of you might be interested in Cognitive Poetics. It theorizes (in simplistic terms) that certain types of poetry “works” by disorienting the brain’s habit of organizing mental phenomenon into hierarchies, which we use to filter information and understand “reality.” By disrupting those hierarchies the poetry offers a feeling of insight that wasn’t previously available.

  28. HHC says:

    Saorsa, Its unfortunate that you are occasionally feeling so uncomfortable. Would repositioning of your body while seated alleviate this sensation? I am wondering if the pressure of sitting compresses the discs in the cervical and thoracic areas where you may have sensation? Do you have disc herniations in the spine?

  29. HHC says:

    Saorsa, Have you had a recent psychological assessment?

  30. Saorsa says:

    HHC, Positioning doesn’t seem to be an issue, as it occurs whether I’m sitting or lying down. It is by no means debilitating, just very uncomfortable, and passes if I focus my attention on something fairly engaging, like a book or a game. I do experience another phantom sensation that I forgot to mention: My hands are also paralyzed and if I try to move them I feel a sensation of movement, but they don’t actually move. It feels almost like there is an invisible glove around my hand, preventing it from moving.

    No psychological assessment, but I’m not particularly concerned about these problems. Having spoken with others enduring SCI, I know phantom sensations are not that uncommon.

  31. HHC says:

    Saorsa, I have a buddy, a Viet Nam vet with SCI. Repositioning him is important whether reclining or sitting tall because he is less likely to lose consciousness. He likes massage therapy. He loves the stimulating talk of pretty ladies. He acts and talks like a therapist whenever you share your problems with him. And of course, he insists you share your problems. He enjoys the sunshine relaxing in his chair, but he gets thirsty and needs something good to drink. Loves a good argument about his care. Demands to talk to his lawyer too. When his medications aren’t properly balanced, he is less coherent and alert. He’ll get angrier too with his friends. I am so glad that I can share my time with him. Perhaps his life as I have described it will be instructive for yours.

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