Most people can picture images in their heads - the look of an apple, the appearance of their kitchen or the smile of their best friend - but not everyone can.
Those who cannot visualise anything in their mind's eye are probably among 1% of people with extreme aphantasia, according to a review of studies on the phenomenon.
They are also less likely to recognise faces, remember the sound of a piece of music or the feel of sandpaper, and more likely to work in science, maths or computing.
And up to 6% of people may experience some degree of aphantasia.
It is not a disorder and does not imply a lack of imagination but can have subtle effects on everyday life, says Prof Adam Zeman, honorary professor of neurology at the University of Exeter, who came up with the term nearly 10 years ago.
My wife has hella aphantasia. And facial blindness. She jokes that she would make the world's worst eyewitness to a crime - she wouldn't be able to recognize the perp or be able to describe 'em even if she could!
But as it has turned out, she's been having trouble hearing as she's grown older. We keep the subtitles on and avoid dining in even slightly noisy restaurants. Anything besides nearby face to face speech is a crapshoot.
So she saw an audiologist who ran the standard battery of tests and said "Your hearing is fine.... But have you heard of auditory processing disorder?" Turns out that a portion of our auditory processing overlaps with a bit of our visual processing. For my wife, whose visual systems don't all work right, her ability to filter noise and extract meaningful sound has started to degrade a bit. Mechanically her hearing is fine. But she can't process the signal right - possibly related to her aphantasia.
There's a lot we don't know yet about aphantasia. Like synesthesia, it's only been described medically in the very recent past. But if you ever start struggling to understand conversations... Check for auditory processing disorder alongside the rest.
For my wife... well, sge was able to get hearing aids - which look sleek now - and it's been night and day. Instead of shifting frequencies, these are like noise canceling headphones on steroids. She's suddenly able to follow conversations in noisy environments that would have been impossible before! So... if any of that sounds (heh) familiar - there's help available!