medwireNews: People who undergo upper limb amputation show a significant increase in gray matter in brain regions involved in vision, research findings show.
Interestingly, this increase was not seen in patients suffering from phantom limb pain (PLP), suggesting that visual adaptation mechanisms may be important to compensate for the lack of sensorimotor feedback from the missing hand.
Such a process "may therefore function as a protection mechanism against PLP development," write Thomas Weiss (Friedrich Schiller University, Jena, Germany) and team in Cerebral Cortex.
Weiss's team used magnetic resonance imaging to evaluate gray matter volume in 21 people who had undergone upper limb amputation and 14 healthy volunteers (controls). Of the amputees, 11 had PLP of moderate-to-severe intensity while 10 had no or only mild PLP.
Imaging revealed various differences in brain morphology between the groups. The overall volume of grey matter in both right and left hemispheres was significantly lower in amputees than in controls, report Weiss et al.
Also, the volume of gray matter in the primary motor cortex was significantly lower in amputees than in controls.
However, amputees had a significantly higher volume of gray matter in regions of the temporal lobe (the left temporal pole, left dorsolateral prefrontal cortex, left fusiform cortex, right middle temporal cortex) and in the right superior parietal cortex.
These areas of the brain are involved in the visual stream, including processing spatial orientation of objects, object perception, and the detection of movement, the researchers remark.
When the amputee group was stratified according to the severity of PLP, Weiss et al found that those with more severe pain had reduced gray matter in brain areas involved in pain processing, while only those with mild or no pain had significantly increased gray matter in regions of the visual stream.
"Results indicate that all patients may have an enhanced need for visual control to compensate for the lack of sensory feedback of the missing limb," write Weiss et al. "As we found these alterations primarily in the mild PLP patient group, successful compensation may have an impact on PLP development."
The researchers conclude: "In summary, the observed regional increase of gray matter in parts of the visual stream in the patient group as compared to healthy controls points to the increased importance of the visual system in upper limb amputees with a possible use-dependent increase of gray matter volume both in the dorsal and ventral visual streams."
Noting that patients in this study had typically undergone amputation many years earlier, the researchers call for further longitudinal studies to track PLP development and cortical volume beginning at the time of amputation.
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