When falling asleep, it's not unusual to imagine a calm, provincial scene, like a flock of sheep jumping over a fence.
Some people with a rare and poorly understood condition find that task nearly impossible.
While they can describe what a sheep is and remember what it looks like, they cannot visualise something in their mind's eye without actually seeing it.
This is known as aphantasia, the inability to summon a mental picture of something from memory, whether it be a loved one's face or a familiar place.
First described in the 19th century, the unusual condition was only properly defined in 2015. Today, only a handful of published studies exist, and many of these are based on self-reports.
Niel Kenmuir was among the first to speak up about their experience with aphantasia after the term was coined, and in 2015, he told the BBC he knew he was different from a young age. Counting sheep was a big part of that.
"I couldn't see any sheep jumping over fences, there was nothing to count."
Just this year, Serena Puang described a similar childhood experience for The New York Times: "I never saw anything – just black. I've been counting silently into the darkness for years."
While aphantasia might make counting sheep difficult, the condition has no apparent effect on a person's creativity or imagination, and only some report problems with their visual memory.
Most people with aphantasia live wholly functional and ordinary lives, and many do not realise they are different until adulthood. Some even have visual dreams, although not all. People with the condition can still describe and recognise what faces and places look like, which suggests their verbal imagination and spatial memory are still very much intact.
The latest study on the condition definitely supports this idea. In the experiment, 103 participants with and without aphantasia were shown photographs of three living rooms and asked to draw them on paper, once while looking at the photo and another time from memory. Afterward, the drawings were assessed by 2,795 online scorers for object and spatial details.
"Importantly, we observe no significant differences between control and aphantasic participants when drawing directly from an image, indicating these differences are specific to memory and not driven by differences in effort, drawing ability, or perceptual processing," the authors write.
"Indeed, aphantasic participants reported an equal confidence in their art abilities compared to controls, and many had experience with art classes and art-based careers."
When the picture was available for reference, the two groups scored the same. But when asked to recall the scene without a prompt, those with aphantasia had a harder time drawing the room.
Altogether, the 61 participants with aphantasia recalled significantly fewer visual details, and their drawings contained less colour and more words. One person, for instance, wrote 'window' instead of actually drawing the details.
That said, those with aphantasia showed just as much spatial accuracy as the 52 control participants, positioning objects in their correct locations with their correct size. The group also showed fewer memory errors compared to controls.
"One possible explanation could be that because aphantasics have trouble with this task, they rely on other strategies like verbal-coding of the space," says psychologist Wilma Bainbridge from the University of Chicago.
"Their verbal representations and other compensatory strategies might actually make them better at avoiding false memories."
Those with more typical visual memories, on the other hand, might be conflating mental images of other living rooms they know. One participant without aphantasia, for instance, added a piano where there was none.
In the end, the results suggest people with aphantasia lack visual imagery but have an intact spatial memory unconnected to the mind's eye. Other recent studies have found spatial memory is similarly intact.
Far more research is needed to figure out what is going on at a neurological level, but researchers think those with congenital aphantasia might experience something similar to those who are congenitally blind, and who can still describe and navigate the layout of a room, even though they can't 'see' it as such.
"These individuals have a unique mental experience that can provide essential insights into the nature of imagery, memory, and perception," the authors write.
"The drawings provided by aphantasic participants reveal a complex, nuanced story that show impaired object memory, but intact verbal and spatial memory during recall of real world scene images."
The scientific community is finally seeing aphantasia for the treasure it is.
The study was published in Cortex.