People who are born blind experience pain differently than sighted individuals and are hypersensitive to pain caused by hot or cold stimulation, according to a small study by researchers in Europe.
The results indicate that congenitally blind people are more attentive and more sensitive to external threats — suggesting their brains are "rewired" by their disability. The study is being published in the journal PAIN.
"We have shown that the absence of vision from birth induces a hypersensitivity to painful stimuli, lending new support to a model of sensory integration of vision and pain processing," says lead investigator Ron Kupers, Director of the BRAINlab, Department of Neuroscience and Pharmacology, Faculty of Health and Medical Sciences at the University of Copenhagen, Denmark.
The findings are important because a key biological function of acute pain is to prevent bodily injury. Vision plays a critical role, as it allows a person to immediately detect and avoid potentially hazardous situations.
Previous studies conducted in sighted individuals had already demonstrated the link between vision and pain perception. The research team hypothesized that the absence of visual cues may lead to heightened vigilance for painful stimuli.
Investigators recruited 11 congenitally blind and 15 sighted participants from Italy and a second group of 18 congenitally blind and 18 sighted participants from Denmark.
Researchers used thermal probes on the forearms of each participant to measure their thresholds for pain. The congenitally blind participants were allowed to touch the equipment beforehand and received verbal descriptions to reduce any anxiety. Sighted subjects were blindfolded during the actual testing.
Participants pushed a button whenever the thermal probe was hot or cold enough to cause pain. They also completed a questionnaire on their vigilance and awareness of pain.
The study team found that compared with sighted subjects, congenitally blind people had lower thresholds for pain caused by heat, rated heat pain as more painful, and had increased sensitivity to cold pain.
"The novel finding of pain hypersensitivity in blindness has several important implications for both basic and clinical science," said Flavia Mancini, PhD, Institute of Cognitive Neuroscience, University College in London.
"This study is noteworthy for research on multisensory interactions and plasticity, because it shows a strong link between vision and pain. The next step is to understand the nature of the interaction between visual loss and pain sensitivity. Which aspect of pain processing is involved in the interplay with vision, and what is its neural basis? The hope is that this work will open the door to pain investigations into the world of sensory loss, left unexplained for too long."
Interesting cultural differences also emerged from the study. People in Italy were found to be more emotionally expressive and responsive to pain than people in Denmark.