A touch from a phantom third arm

A 64 year old woman developed a phantom third arm after a stroke, but unusually, the patient was able to see and feel the illusory limb. A study just published online in the journal Annals of Neurology used brain scans to examine the patient. They established that the phantom sensations were accompanied by similar sorts of brain activity as you’d get from a real arm.

Unlike a classic ‘phantom limb‘, where a patient feels sensations as if they’re coming from the previously amputated body part, a ‘supernumerary phantom limb’ is where a phantom seems to appear additional to the already existing arm or leg.

The condition is rare but has been reported before and is known as the ‘supernumerary phantom limb’ in the medical literature. As we discussed last year, it is usually associated with strokes that affect the subcortical areas of the brain.

One of the reasons this new case is so interesting is because not only could the patient feel their additional limb, but they claimed to be able to see it and feel touches from it as well.

Tactile sensations in the SPL [supernumerary phantom limb] happened when she clenched her hand (she could then feel her phantom palm with her phantom fingers) and when she ‘touched’ certain parts of her body (in which case, the sensation was felt both in the phantom and the touched body part).

She could touch parts of her head, as well as her right shoulder. She claimed to be able to use the SPL to scratch an itch on her head (with an actual sense of relief). Moreover, she reported that the phantom could not penetrate solid obstacles (see supplementary materials for more details).

While a handful of cases of ‘visible’ supernumerary phantom limbs have been reported, this combination of seeing and feeling the touch of one is unique.

Importantly, the patient was not delusional – they didn’t believe they had an extra limb – they knew the sensations were unrealistic, but the experience was still there.

The limb was also not permanently felt – the patient could trigger it at will – and it appeared “pale,” “milk-white,” and “transparent.”

The researchers were keen to see if these sensations were reflected in the activity of the brain by using fMRI scans.

They found that ‘moving’ the phantom limb in front of the line of sight caused increased activation in the visual cortex of the brain.

Most strikingly, they found that when asked to ‘touch’ her cheek with the illusory hand, activity in brain areas representing cheek sensation increased.

There is always the chance that someone with very bizarre symptoms could be lying, but it is also the case that brain disturbance can cause all sort of confusions and distortions – so in some cases a patient’s description of what’s happening may not always be a reliable guide to exactly what they’re experiencing.

In this case, the brain imaging suggests that the ‘supernumerary phantom limb’ was genuinely being perceived as a visible additional arm and that its ‘touches’ were being processed by the sensory system in a similar way to touches from existing limbs.

Because the condition is so rare, and so conceptually bizarre, there is no good explanation of why it occurs except that it may be linked to the disturbance of our already established body and action ‘maps’ in the brain.

Apparently, there is more information about the case in supplementary material which can be found ‘in the online version of this article’, but the additional information doesn’t seem to be online. Ironically, the study seems to have a phantom of its own.

Link to study.
Link to PubMed entry for same.

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