Bodily sensations seem clearer for blind people

Sensing the world around us and sensing our own body are sometimes linked in surprising ways. For example, previous research has shown blind people to have more sensitive hearing and be better at detecting touch and smells than those with typical vision. 

Now new research, published in the Journal of Experimental Psychology: General, shows for the first time that blind people are also better at a form of inner, bodily sensing — heartbeat counting, or 'cardiac interoception'. Previous work has found that the sensing of bodily signals plays a key role in our emotional lives, so this new research could help to explain results from other studies finding that blind people may have superior emotional processing, too.

The James-Lange theory of emotion holds that emotional states arise from physiological changes inside the body. Over the past few decades, a growing number of studies have linked variations in people's ability to sense these changes — including changes in heart rate — to variations in their emotional experiences. People who are better at sensing their own heart rate, without feeling for a pulse, report more intense, and also more nuanced, emotions. 

For this new investigation, Dominika Radziun at the Karolinska Institute and colleagues recruited 36 blind people and 36 sighted matched controls. Thirty-one were congenitally blind, two became blind before the age of two, and three became blind later in life. For each blind participant, the team also recruited an age- and gender-matched sighted control. 

All the participants completed a standard heartbeat counting task, reporting on their number of heartbeats during six trials of different durations, without feeling for a pulse. They were given these instructions: "Without manually checking, can you silently count each heartbeat you feel in your body from the time you hear 'start' to when you hear 'stop'?" The researchers used a pulse oximeter to assess the actual number of heartbeats during the trials, and created a heartbeat counting accuracy score for each person.

Their analysis showed that the blind participants were significantly more accurate than the sighted controls. Where a score of '1' would have indicated complete accuracy and 0 complete inaccuracy, the mean score for the blind participants was 0.779, versus 0.630 for the sighted group. The performance of the sighted group was in line with the results of previous heartbeat counting studies on the general population, the team notes. These results do seem to suggest that blind people perform better.  

The precise mechanisms underpinning the sensory superiorities previously observed in blind people are still debated. Some studies have suggested that superior touch, for example, results not from a lack of vision per se, but a heavier reliance on touch in exploring the world, and this extra use trains that sense. Other studies have suggested that the lack of visual experience causes structural changes in a number of brain areas involved in processing sensory information, as well as alterations to connections between these areas, and it is these changes that drive superior sensory performance. As heartbeat sensing isn't something that most of us train in daily life, the new results fit better within the framework of this latter theory.

Earlier studies have also found that blind people are better at processing emotional information in sounds, for example. Since research shows that the sensing of heartbeats is involved in emotion, this new work suggests that improved heartbeat sensing might contribute to this superior performance. "Our results could provide a missing explanatory link between improved emotional processing and increased sensory acuity in blind individuals," the team writes. 

The validity of the heartbeat counting task used in this study has been criticised, though the evidence on this is mixed. However, the other widely used method for counting heartbeats requires participants to decide whether their heartbeats coincide or not with a series of tones, and as the integration of information from different senses is known to be altered in blind people, this task wouldn't have been appropriate for this study.  

No doubt, the potential impact of heartbeat accuracy on emotion processing in blind people will be explored further in future studies. As authors note, these findings spur further questions about cross-modal plasticity after vision loss, bodily self awareness, and links between bodily sensations and our emotional experiences. This new study will surely prompt new efforts into understanding how various sensory losses and changes — even beyond vision — might affect inner sensing, too. 

Read the paper in full: https://doi.org/10.1037/xge0001366