Question: Just following up from a live chat, what was the summary of the investigation of the Finnish woman who had a supernumerary phantom arm?

David McGonigle answered on 30 Nov 2023:

Really sorry to be so late in answering this.

The paper itself is called ‘Whose arm is it anyway? An fMRI case study of supernumerary phantom limb.’ and while it would probably be dull as dishwater to read, I’m happy to send a pdf to your teacher if need be.

So we used fMRI – functional Magnetic Resonance Imaging – a way to image the brain’s activity in an MRI scanner. We found that when we compared periods when the participant experienced the limb vs when she didn’t, we found only a tiny ‘blob’ (as we call them!) of activity. It was NOT where I thought it would be…

Instead of the blob being in the ‘map’ of the body surface in the brain (so it would be like that area being somehow ‘poked’ by another brain region damaged by the stroke and conjuring up the sensation of her arm), it was in an area of the motor system.

The motor system is responsible for movement, planning, calculating where all the muscles and joints need to be, etc. So this area was essentially holding a past image of where the motor system wanted the arm to go to (the phantom always appeared in the last position of the arm – so if it was at her side, and she moved it somewhere else, after 60-90s it would appear at her side).

A good example of how the data shows you something very different to what you expected!