The findings, published in Nature Human Behaviour, could ultimately help researchers understand why recovery from serious nerve injuries is often incomplete, and how changes in the brain might interact with rehabilitation.
The brain represents different parts of the body, including individual fingers, in an orderly arrangement, often described as a ‘map’. Each finger has a characteristic place within this map, in a remarkably consistent arrangement across people. This organisation is thought to reflect both the structure of sensory information arriving from the hand and the way we use our hands in everyday life.
When one of the major nerves is cut, communication between the hand and brain is disrupted, and movement and sensation can be lost. Surgeons can restore continuity by reconnecting the severed ends of the nerve. Remarkably, nerve fibres can then grow back into the hand, gradually restoring communication with the brain and allowing movement and sensation to return.
This process, however, unfolds without a precise guide. Regenerating nerve fibres do not necessarily reconnect to the same parts of the hand they supplied before the injury. The communication lines between the hand and brain are restored, but their organisation can be altered. It is as if the hand now speaks a different language: communication returns, but the code reaching the brain has changed.
Researchers have long suspected that this altered wiring could change the brain’s maps of the hand, based largely on studies in animals. Research led by Dr Ken Valyear, Senior Lecturer in Psychology at Bangor University, has now tested this idea in humans.
Using functional MRI at the Bangor Imaging Unit, Dr Valyear and his PhD student at the time, Dr Martin Weber, mapped brain responses to touch in 21 people who had undergone surgical repair of one or more major nerves in the hand, comparing them with 30 people without nerve injuries.
They found that the characteristic pattern of finger representations in the brain was altered following nerve repair. These differences could not be explained by weaker brain responses to touch in the patients. In fact, the opposite was true: touch to the repaired hand produced unusually strong responses in the same area of the brain in which the altered finger maps were found.
Dr Valyear explained, “When a severed nerve in the hand is surgically repaired, the communication lines between the hand and brain are restored, but the new connections are not necessarily organised in the same way as before.
“The map alterations we observe in patients may reflect altered connections in the hand and, in turn, a changed pattern of information reaching the brain. Although this is a compelling possibility, we did not directly measure how the nerves had rewired in the hand, and it may not be the whole story. We also found unusually strong brain responses to touch of the repaired hand, suggesting that the brain itself is undergoing functional changes following the injury.”
Understanding these changes could ultimately be important for rehabilitation. Recovery following nerve injury is often incomplete, with patients experiencing lasting difficulties with sensation and movement and, for many, persistent pain.
Dr Valyear added, “A major aim of our work is to understand whether changes in these brain maps help explain patients’ recovery. We found clear changes in the brain, but no clear relationship between the extent of those changes and patients’ sensory or motor impairments. So we now know that the maps change, but we still do not know what those changes mean for recovery.
“To answer that, we need to follow patients over time — ideally beginning soon after surgical repair and continuing through nerve regrowth, rehabilitation and recovery. Combining brain imaging with more direct measures of how the nerves have rewired, and of how patients use their hands in daily life, could help reveal why the maps change and whether those changes are important for recovery.
“Ultimately, the major clinical question is whether and how these brain changes — including the unusually strong responses we see after nerve repair — interact with rehabilitation and contribute to meaningful functional recovery.”
The work addresses a fundamental question about how the brain adapts when the usual relationships between sensation, movement and the body are changed — including after injuries to the body or brain that disrupt those relationships, or when we learn to use new tools and technologies.