Senses in the dark

Hearing

This is a review of interesting scientific facts on hearing and blindness.  It is not exhaustive and might be expanded in the future.  We keep learning new things.

Do blind people hear better?

BrainIt’s an often repeated idea that blind people’s other senses compensate for their lack of sight.

However, there is a difference between having better hearing, and using auditory information more effectively, which some blind people do to an extraordinary degree.

A number of studies demonstrate that blind people have brains that are rewired to allow their visual cortex, which would otherwise be used for the processing of visual information, to improve their ability to process auditory information.  By using their visual cortex, the blind are better than the sighted at extracting more spatial information from sounds.

Interestingly, while many sounds in the blind person’s brain are processed both by the auditory and visual cortex, voices are, like in sighted people, processed in a specialized area, called the superior temporal sulcus. With just one spoken word the STS can infer the sex, age, emotional state and social standing of the speaker. The blind still use the STS to decipher human voices and they use it to a higher extent than the sighted

In general, there are many ways in which sensory deprivation leads to a redistribution of sensory processing functions across the brain.  They depend on many factors, an important one being the аge at which a person looses one of their senses.  Not everything is understood in detail, but one thing is for sure – the human brain is extremely adaptive.

Stevie Wonder, Source Agência Brasil

Stevie Wonder, Source Agência Brasil

Do blind people distinguish musical notes better?

The idea that blindness can aid musical development is an old one. However, previous studies have not been able to quantify this, possibly because they did not take into account the age at which subjects went blind. A study in 2004 confirmed that blind people who had been born blind or lost their sight during the first to years of life indeed do recognize changes in pitch more precisely.  However, there were no significant differences in performance between sighted people and people who had lost their sight after their first two years of life.

These findings reveal the brain’s capacity to reorganize itself early in life. At birth, the brain’s centres for vision, hearing and other senses are all connected. These connections which are gradually eliminated in normal development, might be preserved and used in the early blind to process sounds.

Why do blind people rewire to their ‘visual’ cortex?

As Alvaro Pascual-Leone, a neurologist in the Harvard Medical School, has suggested that the brain’s organization is ‘metamodal,’ not so much linked to a particular sensory modality (vision, hearing, touch) but rather to the kind of information that these senses may convey.

 Finding one’s way around through echolocation

A depiction of the ultrasound signals emitted by a bat, and the echo from a nearby object.

A depiction of the ultrasound signals emitted by a bat, and the echo from a nearby object.

In recent years a number of studies demonstrate how blind and visually impaired people have the potential to use echolocation, similar to that used by bats and dolphins, to determine the location of an object.

It has been found that hearing high-frequency sounds (above 2 kHz) is required for good performance in echolocation, and so common forms of hearing impairment will probably cause major problems.

Dr Daniel Rowan from the University of Southamptom, who has been researching this topic, adds: “Some people are better at this than others, and being blind doesn’t automatically confer good echolocation ability, though we don’t yet know why.” Nevertheless, ability gets better with extensive experience and feedback.

Again, blind individuals who can echolocate do not really have better ‘hearing’: on normal tests of hearing acuity, they score the same as sighted subjects who can not echolocate.  However, when blind echolocation experts hear echoes, parts of the brain associated with visual perception in sighted individuals became extremely active. This means that echolocators are able to extract more information from the echoes themselves.

In the video below you can see how this works, and what one of the best echolocators known to the world, Ben Underwood could do.

A full documentary on Ben Underwood, who, unfortunately, passed away at the age of 16 by the same cancer that took his eyes, can be seen here.

“Facial Vision”

Until the 1940s, blind individuals and psychologists alike were not sure how the blind were able to get around as well as they did.  Many blind people reported what psychologists came to call ‘facial vision,’ a sensation supposedly of pressure on the face that let them know that they were approaching an obstacle when waking.

Early experiments led by Karl Dallenbach on ‘facial vision’ found that the channel for perception was not touch or a mysterious kind of facial ‘sight,’ but instead linked to the sound emanating from the subjects and reflecting off the target obstacles.

How does echolocation work?

Echolocation relies upon the fact that sound travels around 300 meters/second, so if you produce a noise close to your ears, and that sound reflects back off a solid surface, a slight time delay — called the ‘pulse-to-echo’ gap — will separate the original source arriving in the ear from its echo. The time lag between the original sound and the echo can be so small that you may not realize that any pulse-to-echo gap exists. Early research by Kellogg (1962) found that, in fact, some blind subjects were able to detect the distance to an obstacle between 30 and 120 cm to within 10 cm; in other words they were sensitive to pulse-to-echo delays of around .0003 seconds (that’s .3 millisecond—no kidding).

The pulse-to-echo gap, however, is not the only acoustic property that can give some impression of space as volume pitch, interference and timbre of the echo can all be affected by reflection, or that can give information about natural settings.

Echolocation isn’t just in your head

Capacities that are ‘neurological’ also generally need the correct setting and social interactions in order to manifest (take, for example, language). Simply depriving the calcarine cortex of other stimuli does not automatically cause this part of the cortex to convert into a fully realized ‘echolocation cortex. The ability requires active realization and exploration. You’ve got to start clicking ‘click – (echo), click – (echo),’ then walk over and explore the space.

The exploratory process requires, or is at least facilitated if, people around the blind individual allow, even encourage, these sorts of exploratory actions. According to.Daniel Kish, president of “World Access for the Blind”, an American NGO which teaches blind people to echolocate: ‘“Running into a pole is a drag, but never being allowed to run into a pole is a disaster. Pain is part of the price of freedom.” This attitude, however is not wildly popular, especially in a safety-first nation like the United States’

Can you learn echolocation?

We all likely have some sense of space from sound (the reason that an acoustically odd space can surprise us, that we can hear a bottle filling, and that we can get a sense of a concert hall in a sound recording from that space). So we all do it at some level, but can we learn to do more of it?

In the experiment, shown on the silent movie above, sighted subjects learned to orient themselves better after 30 trials. Subjects in another study, who underwent a week living blindfolded, reported better orientation through hearing and even through echolocation, while fMRI studies demonstrated that their visual cortex had started to be activated by auditory and tactile stimulation.  However, all of these effects disappeared soon after the subjects removed their blindfolds.

Or, as journalist Ed Young discusses in his blog, sight and expert echolocation may compete for the same neural resources and perhaps ‘both senses cannot coexist easily with one another.’

Sources

Daniel Rowan, Timos Papadopoulos, David Edwards, Hannah Holmes, Anna Hollingdale, Leah Evans, Robert Allen. Identification of the lateral position of a virtual object based on echoes by humans. Hearing Research, 2013; 300: 56 DOI:

Hopkin, M. Tone task proves blind hear better, Nature News, 2004; DOI: 10.1038/news040712-9

Blind people use both visual and auditory cortices to hear, NewsRelease, University of Montreal, 2010 (http://www.nouvelles.umontreal.ca/udem-news/news/blind-people-use-both-visual-and-auditory-cortices-to-hear.html)

Downey G, Getting around by sound: Human Echolocation, PlosBlogs, 2011

Bavelier D, Neville HJ. Cross-modal plasticity: where and how? Nat Rev Neurosci, 2011

Maya-Vetencourt J.F., Origlia N., Visual Cortex Plasticity: A complex interplay of genetic and environmental influences, Neural Plast, 2012

Young, E., The brain on sonar – how blind people find their way around with echoes, Not Exactly Brain Science.