Apr 09 2009

Imaginary Phantom Limb

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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.

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