Roughly 4.4 percent of the global adult population experiences a rare condition called synaesthesia, which causes the brain to confuse sensory information and turn smells into sounds, or numbers and words into tastes and colours.
But scientists might have just identified a specific form of synaesthesia that appears to be far more common - in a small experiment, one in five participants discovered that they could 'hear' motion, and it gave them an enhanced ability to identify patterns in flickering light signal.
Synaesthesia is rare but broad neurological phenomenon, where the stimulation of one sensory or neurological pathway automatically stimulates a second sensory pathway.
So while one synaesthete will taste beef whenever they see the word "window", another will visualise the colour blue every time they hear a certain musical cue - all kinds of sensory inputs can get mixed up, depending on how the brain is wired.
And now it looks like those strange, life-altering phenomena might not be the only forms of synaesthesia out there - they might just be the most obvious ones.
As a new study by researchers from City, University of London suggests, that neurological mix-up could be causing a whole lot more of us to experience the effects of having our sensory wires crossed, but it’s so subtle, we don’t even notice it.
"A lot of us go around having senses that we do not even recognise," cognitive neuroscientist and lead researcher, Elliot Freeman, told Hannah Devlin at The Guardian.
Freeman decided to investigate a phenomenon where people appear to accompany visions of motion with some kind of internal sound.
So if you were watching a perfectly silent fan spin around, your brain might produce its own, very subtle whirring noise, or circles whizzing by on a computer screen might be coupled with a tiny "whoosh!"
Here's what motion would look like to a non-synaesthete versus a hearing-motion synaesthete:
Hearing-motion synaesthesia has only ever appeared in the scientific literature once before, so Freeman and his team gathered 40 participants to see if he could replicate the effect.
The participants were asked to watch and listen to two types of Morse code-like clips - one with various sequences of auditory blips, and the other with patterns of flashing lights.
First, they were asked to watch a series of flashing light clips that came as pairs - they had to compare each pair and identify if the patterns were identical or different.
The participants were asked to specify if they heard sounds accompanying the flashing lights, and if they did, they were quizzed on what kinds of sounds they were hearing.
The researchers were surprised to find that 22 percent of the participants said they could hear sounds when watching the light flashes, and this correlated with their success in identifying the patterns.
"My data suggests there are two kinds of people," Freeman told The Guardian. "Those who generate sounds deliberately and those who get the internal sounds without trying."
The researchers suggest that a link exists between 'hearing' the motion of light and an ability to more accurately identify sequences, because when the brain recodes visual signals as sounds, it gives the participants additional information to keep track of the rhythm.
Try it out for yourself using the video below. How much easier would it be to identify the patterns if the flashes beeped in sequence too?
Next, the participants were asked to listen to pairs of clips with patterns of faint auditory blips, and again, they needed to identify which were identical, and which were different.
But the twist was some were accompanied with mismatched flashes of light, and some weren't.
The researchers found that those who had heard motion in the first experiment - and performed better in identifying the light patterns - actually performed worse than the average when faced with auditory blips coupled with irrelevant flashes.
At this stage, it's not clear why that would be the case, but it could be because in the first experiment, the extra information was helpful, but in this one, it was distracting.
To be clear, this is an extremely small sample size, and results of the experiment are by no means definitive when it comes to figuring out how many of us might experience this effect.
More work needs to be done to see if similar trends bear out in larger, more diverse groups of participants, and measures need to be taken to address the limitation of self-reporting when it comes to the internally produced sounds.
But with further research, we could gain a better understanding of how the human brain couples and distinguishes various sensory cues, and the neural processes that make us see, hear, smell, and sense things that aren't really there.
"The finding that this 'hearing-motion' phenomenon seems to be much more prevalent compared to other synaesthesias might occur due to the strength of the natural connection between sound and vision," Freeman suggests.
The study has been published in Consciousness and Cognition.