Amputation rates in France

According to an epidemiological study published in 2023, 116,866 major lower limb amputations and 1,616 major upper limb amputations were recorded in France between 2011 and 2020. Most are related to chronic diseases such as diabetes or serious accidents.

Phantom limb sensations common

The amputees do not all wear a prosthesis, but according to a study, nearly 90% of them share the same experience: that of the phantom limb, that is, the sensation that the lost limb is still present. In many cases, this impression is accompanied by unpleasant manifestations such as tingling, burning, cramps or even more intense pain.

Phantom limb symptoms and treatments

These symptoms often appear immediately after the amputation, but can persist for several years. In addition to medical treatments and physiotherapy, methods such as mirror therapy or neuromodulation can help alleviate the symptoms of amputees.

Understanding phantom limb through neuroscience

For many years, researchers have been trying to understand the phenomenon of the phantom limb from a neurological point of view, particularly the role of primary sensory-motor regions. Among them, the somatosensory cortex processes sensations and the motor cortex commands movements.

Cortical reorganization debated

The ability of the adult brain to reorganize after an amputation remains hotly debated today. Does the cortex really reorganize or does it abandon the circuits that have become inactive? A study published on August 21 in Nature Neuroscience by a team from the University College London seems to provide new elements of response.

Researcher Angela Sirigu's insights

Angela Sirigu, a research director at CNRS, has conducted numerous studies within her team at the Marc Jeannerod Institute of Cognitive Sciences (Lyon) on the mechanisms of cortical reorganization in amputees. Today affiliated with the Institute of Neuroscience of Timone (Aix-Marseille University), she recalls that "most of the people who experience a phantom limb are those who have suffered a sudden accident."

Phantom limb's emotional and memory aspects

The perception of the phantom limb is not only a neurological phenomenon but can also be rooted in an affective and memorial dimension linked to the trauma of the accident. "Some, for example, remember even the position [of their limb] at the moment of the accident. It is as if an image was frozen in their brain," she specifies.

Pain and brain signals

The cognitive neuroscientist explains that "most, but not all, phantom limbs are painful, [sometimes] to the point that people have to take morphine." As she explains, after an amputation, "the brain still sends signals to the muscles [of the amputated limb], but these signals cannot be executed since the limb no longer exists. An error signal is [then] sent to the brain: 'the limb is no longer there.' The brain reacts to this signal, perhaps by producing a pain signal."

Prosthetics and phantom limb pain

However, Angela Sirigu notes that in people who wear a prosthesis, phantom pains appear at first and then decrease. This can be explained by the brain's interpretation of these error signals. "When you have a [prosthesis], even if there is no somesthetic feedback, from a visual point of view, it is as if the brain is saying: 'ok, everything is fine, my limb is there'," she points out.

Cortical reorganization and phantom limb pain

The specialist recalls that many studies, notably those of Herta Flor, a German neuropsychologist, have shown that the intensity of phantom limb pain was correlated with the degree of cortical reorganization in the brain. However, the ability of the adult brain to reorganize at the cortical level remains largely debated within the scientific community.

MRI study on cortical reorganization

The study published in Nature Neuroscience, conducted using brain MRI scans on three adults before their arm amputation and up to five years after the procedure, shows that amputation does not cause large-scale cortical reorganization and that the representation of the hand remains stable in the primary sensory-motor regions.

Implications for neuroprosthetics

The researchers estimate that this discovery could favor the development of neuroprosthetics, whose control would be facilitated by the maintenance of brain activity related to the amputated limb.

Brain plasticity and stability

Angela Sirigu provides a nuanced perspective on these results. On the one hand, the specialist acknowledges the brain's plasticity. "When we observe the brains of amputees, we notice cortical reorganization." On the other hand, she points out that body representations remain stable in the brain.

Phantom sensations as evidence

"Phantom sensations are evidence that the representation [of the amputated limb] is still there." Within the Marc Jeannerod Institute of Cognitive Sciences, which she previously headed, "we have always said that the representation of the hand remains present and is not replaced by that of the arm," she assures the specialist.

TMS study on cortical reorganization

Indeed, in a study published in 2006, the Lyon researchers examined three above-the-elbow arm amputees using transcranial magnetic stimulation (TMS) while recording the activity of the stump muscles. "In our motor cortex, each muscle is represented in great detail," recalls Angela Sirigu.

Brain area responses and phantom sensations

"The area of the hand in the motor cortex is clearly identifiable. It is part of what we call the 'hand knobs', a horseshoe-shaped area where all activities related to hand movements are concentrated." During this study, "when we performed the TMS and stimulated the area corresponding to the hand, we obtained responses [muscular] at the level of the stump, that is, in the remaining muscles, which normally do not control the hand," explains the researcher.

Stump muscles and brain activity

The experiment shows that the stump muscles can thus occupy the brain area of the hand, a sign of cortical reorganization. "But what is interesting is that when we asked the subjects what they had felt, [they described a sensation] at the level of the phantom hand. This response shows that the representation of hand movements is still there, it has never disappeared," she continues.

Neuronal circuits and phantom limb

Thus, the neuronal circuit that controls hand movements persists after an amputation and is not replaced by that of another limb.

Hand transplants and brain representation

With her team, Angela Sirigu had also studied patients who benefited from hand transplants in Lyon under the impetus of Professor Jean-Michel Dubernard, a pioneer in allotransplantation. Before the transplantation, the researchers had already observed cortical reorganization, but stimulating the area corresponding to the hand still triggered sensations in the phantom hand.

Transplant and brain reactivation

After the transplant, the same brain area activated again, this time in connection with the transplanted hand, proof that the representation of the hand had never disappeared and could be re-invested by the brain. According to Angela Sirigu, the authors of the British study thus confirm a finding already well established in the scientific literature.