On a blustery day in Glasgow, Alex Campbell and his wife Kerry are taking a walk through town. Both are visually impaired and use white canes as they stride with confidence across roads, broken paving slabs and down flights of stairs. But what makes this couple unusual is that every now and then they emit a short, sharp click with their tongue.
To an onlooker the click is barely noticeable. But for Kerry and Alex, the sound that travels out of their mouths, bounces off the objects around them and returns to their ears in a split-second helps them paint in their minds a portrait of the world around them.
The married couple are two of just a handful of people in Britain that have been taught to use echo-location which - as the name suggests - is the same sort of method bats and dolphins use to get around.
Visually impaired people often use sound to help them navigate. But what makes echo-location different is that those who have refined the art are able to pinpoint obstacles with remarkable detail.
"Over time and with practice you build up a whole library of sounds," explains Alex, 47.
"You start with simple objects like a wall, a ball or a table before moving on to a tree, a car or a distant building."
It may sound like science fiction but those who are best at echo-location are so proficient that they can ride skateboards or bikes through busy streets despite being entirely blind.
If you don't believe it type "Daniel Kish" into YouTube.
Mr Kish is often described as a "human bat" and has pioneered echo-location to such a point that he can make out faraway buildings and happily goes solo hiking in the hills above Los Angeles.
Four years ago the 44-year-old, who lost both eyes to an aggressive form of cancer shortly after birth, came to Britain at the invitation of Visibility, a Glasgow-based charity, to teach his skills to people such as Alex and Kerry.
For Kerry, who has never been able to see and began learning echo-location four years ago, the clicks have given her confidence to explore new places and stray from her regular routes.
"If I ever get lost around town I can make a click and know where I am," she says.
"It suddenly makes you so much more aware of your surroundings, the landmarks across the city, the bus stops, park benches or an overhead footbridge. You are able to find these things before your cane does. That's incredibly liberating."
Even though the technique has been shown to vastly improve some people's mobility, it has yet to gain widespread acceptance among instructors.
"Some of our most staunch opponents are blind people who just don't want to be bothered with raising themselves to a higher standard," claims Mr Kish, whose charity World Access for the Blind has pioneered the teaching of echo-location.
"Any time you have an innovation within any profession it is not quickly or easily embraced."
Echo-location takes constant practice. Those who promote echo-location say their intention is not to replace a cane, but compliment it, to become part of a package of tools that can help the visually impaired become more mobile and independent.
Professor Gordon Dutton, a British ophthalmologist and leading expert on cerebral visual impairments, has become a supporter of echo-location, but says that those who wish to use it must work hard.
"Mastering echo-location takes years of work, you need to put in something like 10,000 man hours to get to the level of someone like Daniel... You have to put huge amounts of work in and entirely reprogramme the way your brain thinks."
Neurologists are paying increasing attention to people like Mr Kish precisely because they appear to have reprogrammed their brain. It is a phenomenon that researchers refer to as neuro-plasticity, and is helping us understand the brain's hidden talents.
Scientists in Canada published a paper last week based on MRI scans of the brains of two proficient echo-locators, one of whom was Mr Kish. Researchers placed microphones inside the ears of the echo-locators and recorded the sounds coming back to them. The subjects were put inside a scanner and the sounds replayed to them, and they were asked to describe the image in their head.
The scan results were remarkable. When the sound was replayed the part of the brain usually associated with sight was activated, rather than the auditory cortex which analyses sounds.
"This suggests that visual brain areas play an important role for echo-location in blind people," said Dr Lore Thaler, from University of Western Ontario.
For Mr Kish, the results of the scans were something he had suspected.
"To a blind person an image without visual elements can still be extraordinarily sophisticated, complex and detailed," he said.