A newly published study on a common form of synaesthesia suggests as many as one in five of us can 'hear' certain kinds of silent images.

While the mechanisms behind this odd neurological quirk aren't all that clear, this new bit of research suggests synaesthesia might not be all that unusual after all, and possibly emerges from some fairly standard wiring common to most of us.

You might recall last year a number of animated gifs like the one below made their way around social media as people excitedly found they could 'hear' sounds in connection with the image's movements.

Synaesthesia – the weird sensory cross-over that leads some people to visualise noises or feel smells – was the most convenient explanation, yet given how little we know about the condition it still didn't really solve the mystery.

Researchers from City, University of London, surveyed over 4,100 people on their sensory responses to a variety of soundless moving images that included a mixture of 24 abstract and dynamic scenes of various speeds and abruptness.

Their goal was to determine just how many of us experience the phenomenon they referred to as a visually-evoked auditory response – or vEAR for short.

It turns out to be a lot.

"We found that as many as 21 percent of people may experience forms of this phenomenon, which makes it considerably more prevalent than other synaesthesias," says senior researcher Elliot Freeman.

Their results don't definitely explain why some people hear a noise when they see movement, but the prevalence of the experience does indicate it could offer a prime opportunity to study how our brains make sense of stimuli.

More importantly, the fact it's so common might mean we need to scrap assumptions that synaesthesia is the result of some kind of unusual connectivity in our brain's wiring.

Instead these crossed signals could simply be how our brains work normally, arising from fairly standard connections or processes most of us possess.

To learn more, a quarter of the volunteers were asked follow-up questions about their health and past experiences, such as "Do you suffer from tinnitus?" and "When in the dark or falling asleep, do you ever see flashes of light triggered by sudden sounds?"

Their responses show a slight bias towards those who experience ringing in their ears to also hear a sound alongside certain visual cues.

Those who claim to hear sounds when they see moving but silent musical images were also more likely to experience an auditory sensation when they watched the clips.

Put together, it's possible that increased excitability in some parts of the cortex, especially the visual areas, could be leaking into areas devoted to hearing.

So it seems that, rather than having unusual brain wiring, those experiencing vEAR might simply have brains that experience some spill-over in activity between certain areas.

"We think that these sensations may sometimes reflect leakage of information from visual parts of the brain into areas that are more usually devoted to hearing," says Freeman.

"In extreme forms of this crosstalk, any abstract visual motion or flashing may be sufficient to trigger the sensation of hearing sounds."

Interesting as it all is, it isn't the smoking gun explaining why some people hear noises when they see a sudden movement, let alone why we experience other kinds of cross-overs in sensations.

The fact one in five people confess to experiencing something does open the way to conducting more investigations, so we might have better answers soon.

This research was published in Cortex.