For the last few days of a recent hospital stay one of my roommates was a young women who was blind. She had the ability to recognise doctors, nurses and care workers almost instantly by their voice. One morning, she even knew a care worker who she had met years previously. The constant stream of different staff in hospital is a challenge to people armed with a full set of senses so this ability amazed me. Could this be caused by increased auditory perception in the blind? Once released back to normality, I began researching and the journals did not disappoint. Here’s what I found out!
My first quest was to find out if the blind can hear better than the sighted. This seems probable going on popular evidence such as blind musicians throughout history in classical music (Andrea Bocelli), blues (Ray Charles) and our own Irish music (Turlough Carolan). More support for this theory is that blind piano tuners are commonplace and there is even an Association of Blind Piano Tuners in the UK. In 2004, to answer this question, scientists researched whether “blind people develop superior abilities in auditory perception to compensate for their lack of vision”. One difference in their study compared to previous, was that they categorised types of blindness into early-blind (0-2 years) and late-blind (5-45 years). This would turn out to be key in their results. Early-blind, late-blind and sighted people were tested on their ability to differentiate between two different tones. The early-blind participants performed better at every test compared to late-blind and sighted participants. This makes sense as during early development the brain has more capacity to be flexible (cerebral plasticity). Some re-wiring in the brain of the early-blind could result in their enhanced hearing.
It seems only some blind people have enhanced auditory perception but during my research I discovered something else hearing-related that both the early- and late-blind can do better than sighted people. Echolocation! Animals who find their way around using this method include bats and toothed whales and dolphins. Last year, scientists in Ontario studied this phenomenon in two expert echolocators, Daniel Kish (early-blind) and Brian Bushway (late-blind). They locate objects in this way by producing clicks with their tongues and mouths and listening to the returning echos. Using echolocation they are able to complete tasks such as navigating streets, mountain biking and playing basketball.
In order to study the brain activity during echolocation, the scientists needed to devise a way for participants to listen to the echos inside a fMRI machine. This was a challenge as protective earphones must be worn and limited mouth movements are allowed. To overcome this they recorded the echos in a number of scenarios and then played them back to Kish, Bushway and two sighted people. The expert echolocators and sighted people were able to distinguish control sounds from echolocation sounds. However, only the echolocators were able to identify objects using echos. The most interesting result from this study was that the hearing part of the brain, the auditory cortex, fired in similar ways in both blind and sighted participants. In addition, a part of the brain usually associated with vision, the calcarine cortex, also fired only in the blind echolocators. This fired more intensely in the early-blind echolocator, Kish, who had been using echos to navigate since childhood. The inability of sighted people to use echolocation could be due to lack of practice or because of competition for use of the same ‘vision’ area of the brain. The authors conclude that Kish and Bushway “use echolocation in a way that seems uncannily similar to vision… This has broad practical implications in that echolocation is a trainable skill that can potentially offer powerful and liberating opportunities for blind and vision-impaired people.”
Another area of focus is the use of technology to covert visual information into sound or touch. An article published last week in PLoS ONE, researched one such visual-to-auditory Sensory Substitution Device (SSD). This device consists of a video camera connected to a computer and headphones. The images captured by the camera are converted to “soundscapes” and users who have learnt the variety of sounds can interpret what is in front of them. The scientists trained 9 participants in the use of the SSD and examined them utilizing a standard test used by specialist eye doctors. All participants had improved visual perception and 5 of these exceeded the WHO threshold for blindness using the device. The authors describe some of the completed tasks: “Our participants were able to identify and mimic the body posture of a person standing a few meters away, [and] navigate in crowded corridors while avoiding obstacle.”
The visual-to-auditory device used in this study is low cost ($200) and it proposed as an alternative method to more expensive procedures of restoring “high-resolution functional vision”. From reading the previous paper, perhaps echolocation could be considered as an even lower cost and less obtrusive method. It would be interesting to discover if blind people using these devices were using parts of the visual cortex in a similar way to the echolocators. Something for a future study to consider?
The quest to answer what seemed like a simple question has unearthed lots of fascinating information and even more questions in typical science style. My initial amazement in hospital at my blind room-mates ability to distinguish and recognise voices, has grown to astonishment at the capability of some blind people to echolocate. This is a great example of the flexibility of our bodies and brain to adapt to living with something so potentially disabling as blindness. It seems not only strong-will and determination overcome adversity!.
Gougoux, F., Lepore, F., Lassonde, M., Voss, P., Zatorre, R., & Belin, P. (2004). Neuropsychology: Pitch discrimination in the early blind Nature, 430 (6997), 309-309 DOI: 10.1038/430309a
Thaler L, Arnott SR, & Goodale MA (2011). Neural correlates of natural human echolocation in early and late blind echolocation experts. PloS one, 6 (5) PMID: 21633496
Striem-Amit E, Guendelman M, & Amedi A (2012). ‘Visual’ Acuity of the Congenitally Blind Using Visual-to-Auditory Sensory Substitution. PloS one, 7 (3) PMID: 22438894