New publication in Nature: Human Behaviour

Cutting a nerve of the hand alters the organization of digit maps in primary somatosensory cortex

🔗 https://rdcu.be/fzGHG

About the paper

Peripheral nerves have a remarkable capacity to regenerate. But they do so blindly, without topographical guidance. After hand-nerve transection and surgical repair, the branching architecture of nerves in the hand can therefore change, altering inputs to the CNS.

Our study asked whether this alters the fine-grained organisation of digit maps in primary somatosensory cortex.

🔬 What we did

Using fMRI, we mapped responses to vibrotactile stimulation of each finger in 21 patients with surgically repaired hand nerves and 30 healthy controls.

Rather than relying on the location or overlap of individual digit maps, we measured multivariate representational distances between digit-specific response patterns—capturing both how separable the digits are and the pattern of that separability: their representational geometry. Multivariate representational distances have been shown to provide more precise and reliable estimates of fine-grained digit organisation than conventional spatial mapping measures. And in our healthy controls, they robustly reproduced the canonical organisation in S1.

🔍 What we found

 Nerve repair alters the canonical digit-map structure in S1. The normal pattern of interdigit separability is systematically distorted in the hemisphere contralateral to the repaired hand.

This was not due to weaker or noisier fMRI signals: BOLD responses were increased for the repaired hand, and within-subject reliability of representational geometry was not reduced.

We also found no credible evidence that altered maps were associated with worse functional outcomes. Their clinical significance therefore remains unresolved.

💡 Why it matters

Somatotopic maps are one of the most enduring, pervasive and evolutionarily conserved organisational principles in neuroscience. Yet, remarkably, we still do not fully understand the functional significance of their precise organisation.

Our study shows that these maps can be measurably altered after nerve repair in humans. This gives us a special opportunity to better understand what this organisation is actually for.

Imagine following patients from surgical repair, through nerve regrowth, rehabilitation and recovery—tracking cortical maps alongside peripheral rewiring and behaviour. We may then be able to determine why the maps change, when they change, and whether and how those changes shape function and recovery.

This matters far beyond nerve repair. Any injury or intervention that alters the normal relationship between body and brain raises the same fundamental question: how does the brain adapt when the mappings linking the body to sensation and action change? 

🙏 Huge thanks to the participants who gave their time to take part, and to the fantastic team behind the study: Martin Weber, Andrew Marshall, Ronan Timircan, Francis McGlone, Raffaele Tucciarelli, Obi Onyekwelu, Louise Booth, Susan Francis, Michael A., Edwin Jesudason and Vivien Lees.

October 2019: New publication!

• Valyear KF, Philip B, Cirstea CN, Chen B, Baune N, Marchal N, and Frey SH (2019). Interhemispheric transfer of post-amputation cortical plasticity within the human somatosensory cortex. NeuroImage. DOI: doi.org/10.1016/j.neuroimage.2019.116291

For the first time, we measure brain responses to passive cutaneous stimulation of the intact digits of chronic traumatic upper-limb amputees and discover robust functional reorganisation within the former hand territory. The findings mark a heretofore missing link with existing animal models showing qualitatively similar reorganisational changes following deafferenting forelimb injuries. 

Grant success!

Mapping the consequences of peripheral nerve transection and repair on brain organisation and hand function.

Wellcome Trust Seed Award in Science (215186/Z/19/Z)

Principal Investigator: Ken Valyear

November 2019 – 2021 

£94,063

New PhD studentships!

• We have two new fully-funded PhD studentships available for October 2019: 

• • Project 1: Mapping the consequences of peripheral nerve transection and repair on brain organisation and hand function. 

    • Link to full advert: https://www.bangor.ac.uk/psychology/mapping-phd.php.en

• • Project 2: Movement-science inspired development of upper-limb prostheses for young children. Co-supervised by Dr. Simon Watt of Bangor University, and Ben Ryan of Ambionics (www.ambionics.co.uk).

    • Link to full advert: https://www.bangor.ac.uk/psychology/movement-science-phd.php.en

• Please feel free to contact me directly: k.valyear@bangor.ac.uk