You might have already heard the discombobulating fact that some people don’t have an internal monologue (or if you’re an abstract thinker, that some people think with words). To add to the confusing list of things some people’s brains do that others don’t is a condition called aphantasia - the inability to visualize mental images.

Think of a dog, explain it to me - big or small? Fluffy or short hair? What color is it? If you could see what you were describing in your “mind’s eye”, you don’t have aphantasia. If you’re incapable of picturing things in your head, then it’s possible you could sit within the two-to-five percent of the people who are unable to form mental images.

Previous studies have linked aphantasia to patterns seen in other cognitive processes, like remembering, dreaming and imagining. Most research into the unusual condition has relied on behavioral studies, but new research published in Proceedings of the Royal Society B, used an objective measure of skin conductance to see how the ability to form mental images - or the lack thereof - altered people’s reactions to reading or looking at scary stories or images.

Using 46 participants (22:24 aphantasia vs not), they first investigated the role of mental imagery in fear by strapping electrodes to the participants’ skin. This method is commonly used in psychology research to obtain information about emotional state as skin becomes a better conductor of electricity when people are frightened or in distress. The researchers then switched off the lights and left the room, as the participants read written accounts of scary stories. Traumatic scenarios included falling off a cliff, being chased by a shark or ending up in a plane that’s about to crash.

“Skin conductivity levels quickly started to grow for people who were able to visualise the stories,” said Professor Joel Pearson, senior author on the paper and Director of UNSW Science’s Future Minds Lab, in a statement. “The more the stories went on, the more their skin reacted. But for people with aphantasia, the skin conductivity levels pretty much flatlined.”

To eliminate personal thresholds for fear and distress, they repeated the experiment with the same participants, but this time showed distressing images instead of words – like photos of cadavers or a snake about to strike. There was to be no flatlining in this experiment, as the pictures successfully scared the crap out of everyone regardless of whether or not they had aphantasia.

The results highlight the pivotal role of mental imagery in our emotional response to scary stories or distressing accounts from tragic events. Even the most harrowing of scenarios fail to get much of a rise out of participants with aphantasia compared to those with the capacity to form mental images, but this wasn’t because they as individuals were simply immune to fear.

“We found the strongest evidence yet that mental imagery plays a key role in linking thoughts and emotions,” said Pearson. “In all of our research to date, this is by far the biggest difference we’ve found between people with aphantasia and the general population.

“These two sets of results suggest that aphantasia isn’t linked to reduced emotion in general, but is specific to participants reading scary stories. The emotional fear response was present when participants actually saw the scary material play out in front of them… Imagery is an emotional thought amplifier. We can think all kinds of things, but without imagery, the thoughts aren’t going to have that emotional ‘boom’.”

Think you might be aphantasic? You can find out more about research into the condition here.