Lynne Malcolm: If I ask you to picture an apple in your mind's eye, can you do it? What does it look like? How clearly can you see that mental image?
Adam Zeman: Most of us when we are asked that kind of question have an experience which is somewhat visual. We have an experience which is a bit like seeing. When somebody with aphantasia is asked that question, they can think about an apple but they don't have an experience which is at all visual when they try to see it in their mind's eye.
Sian Thompson: It does seem like a superpower. How do you function in this world with images going through your head all the time? I guess it's just so totally foreign from how my mind seems to work.
Lynne Malcolm: Professor Adam Zeman and Sian Thompson.
Hi, it's All in the Mind on RN, I'm Lynne Malcolm. According to some neuroscientists, about 98% of people can conjure a clear image of an apple in their mind, with details like colour and texture. The other 2%? They say they can't. They don't have any mental imagery or the ability to conjure pictures in their mind's eye. You might even be one of them. This condition has come to be known as aphantasia. It's not a disorder and it appears to exist on a spectrum. Research on aphantasia is gaining pace around the world, including here in Australia. This is partly because of the role mental imagery plays in other conditions, such as PTSD and schizophrenia. ABC science journalist Carl Smith brings us this story about trying to pin down exactly what mental imagery is and what it means to our lives.
Sian Thompson: When I found out that people actually visualise in their minds I was like, what, but they must be aliens, that's a superpower. It does seem like a superpower. It's kind of difficult to get past the idea of it as a superpower and thinking, but why isn't everyone else perfect? Why can't they spell perfectly? Why don't they have photographic memories? And then when you're kind of talking with someone you think they're imagining something, what are they imagining? It's so strange, how can you? How do you function in this world with images going through your head all the time?
Carl Smith: This kind of response appears to be quite common when someone learns they have aphantasia.
Sian Thompson: I am Sian, and I have aphantasia. It is having no visual imagination.
Carl Smith: When Sian tries to picture something in her mind's eye, she says she can't, and she is not alone. Neurologists believe roughly 2% of people have aphantasia. A quick and easy way to try to figure out if you have strong visual imagination is the exact test Lynne gave you at the start of the show; try to picture an apple in your mind's eye, or a peaceful sunset. Rate how vivid or real that image is.
So if I ask you to picture a beach with the sunset over the ocean, what happens in your mind when I say that?
Sian Thompson: Okay, I've got the feeling of the sand, the sunset itself is pretty boring because there's nothing really to think about. I've kind of got the idea of the waves going out to the horizon.
Carl Smith: But you can't see the waves, you can't see the sunset?
Sian Thompson: No. I kind of get the idea of colours but no visual. So people often say to picture a sunset to relax, and for me that's just pretty boring.
Carl Smith: What about when I say picture your mother's face or your father's face?
Sian Thompson: Nope. I'm not very good at faces, I need to meet people a few times before I recognise them. I can kind of imagine their mannerisms or their body language or their expressions, I'm not sure exactly what I'm doing there. I'd normally think about the idea of my mother, and then if you said picture her smiling, there's something happening that before I knew about people visualising I just thought was what you meant when you said picture this. Yes, something's happening, so I can kind of get the idea but I can't see it.
Carl Smith: When I heard this, I immediately wondered whether I might have the same thing. It's easy to doubt whether you can mentally visualise something because it's hard to externally validate, test or quantify. No one else can see into my head to confirm whether I am truly seeing an image or not. But I think part of this is being clear about what mental imagery is. How can someone know what an apple is without a mental picture? To find out I tracked down the scientist who coined the term aphantasia and who has been a pioneer in research into mental imagery, Adam Zeman, Professor of Cognitive and Behavioural Neurology at the University of Exeter Medical School.
Adam Zeman: Most of us when we are asked that kind of question have an experience which is somewhat visual. We probably don't see an apple which is as clear as it would be if we were actually looking at it but we have an experience which is a bit like seeing. It probably will have had a colour, it may or may not have had a stalk. It has some visual specificity, if you like. When somebody with aphantasia is asked that question, they can think about an apple with no problem, they know exactly what an apple is, they will recognise an apple, they can answer questions about apples, but they don't have an experience which is at all visual when they try to see it in their mind's eye. Similarly if they think about their spouse or their front door or their last holiday, they have plenty of knowledge about these things but they have no visual experience when they attempt to summon an image of them.
Carl Smith: So I guess at this point it might be interesting to delineate then between visual memory and visualisation because these people can still remember things, they can still remember the face of their spouse but they just can't visualise it. So could you unpack that differentiation for me?
Adam Zeman: Yes, so that's us a very subtle distinction I think and it's actually one which I had to get my head around when I first started thinking about this. So when I consult my visual memory of what an apple is, it tells me it might be green, it might be red, I know a number of facts about how apples look. That knowledge is somewhat distinct from my ability to call up in my mind's eye an image of an apple. So you can know a good deal about the appearances of things without being able to visualise. Visualisation takes an extra step. You are using your visual knowledge to generate an experience which is somewhat visual.
Carl Smith: Professor Zeman's work on aphantasia started with a letter he received from a GP, explaining that a patient was suddenly unable to imagine any visual image.
Adam Zeman: The doctor was clearly puzzled by this complaint, which had developed following a cardiac procedure. So MX, as we described him, had undergone a coronary angioplasty, a treatment for ischaemic heart disease, and shortly after that he realised that he wasn't able to call to mind the faces of friends and family or places he visited.
Carl Smith: Do we know what proportion of the population might have aphantasia?
Adam Zeman: So I've said that this topic has been neglected, and that's true, but there was one bit of work in the course of the last century by an American psychologist called Bill Faw who gave a questionnaire to a large number of undergrads and he came up with a figure of about 2%. 2% of people have real difficulty visualising, and I think that's roughly right. So we've looked at this in a large community-based study in Exeter, the Extend Study, and in about 2,000 participants we find that just under 1% score right down at floor on the vividness and visual imagery questionnaire, and around 2% fall within a range that one might describe as aphantasic.
Carl Smith: Give us an example of the sorts of questions that you are asking participants in that questionnaire.
Adam Zeman: It's very intuitive, it just asks you to visualise a number of scenes and you rate the vividness of your imagery from no image at all to as vivid as real seeing.
Carl Smith: Sian, very kindly doing much of my work for me, printed out a sample of this vividness of visual imagination questionnaire.
Sian Thompson: Think of a country scene which involves trees, mountains and a lake. How strongly are you visualising this? So there's that kind of scale from perfectly clear and vivid as if I was actually seeing it, down to no image at all. But there's no image. So for that I answer one. A strong wind blows on the trees and the lake, causing waves. But there's no image, so it's one. So kind of going through all these questions and stuff is happening in your mind but every question is just one, one, one, one.
Carl Smith: Tell me about the time when you first realised that you were aphantasic and the shock of…?
Sian Thompson: Well, it was Anzac Day, 2016. I was sitting on the couch in my apartment reading Facebook and my friend had shared the Blake Ross post about aphantasia, I think it's Blind in the Mind…
Carl Smith: So Blake Ross, he was one of the developers who created Mozilla Firefox, the internet browser.
Sian Thompson: Yes, that's right. And he had said something like it's been one of the craziest weeks of his life, and has a very, very long post just describing it all. And as I was reading it I was thinking, really, so people actually see images in their heads, it's not a metaphor? Oh, okay.
Carl Smith: Aside from hearing Sian's story, I've seen this shock, confusion, outrage for myself when I went out for a drink with my friend Tiger and accidentally threw him into the mental imagery deep end.
So when I said to you…when I was talking to the group about aphantasia and I was describing the standard apple test, so visualise a green apple in your mind's eye, what did you think of that when I put you on the spot?
Tiger Webb: I think the first thing that happens is you try and do the thing that everyone else is doing, so you say, oh, the really simple, easy one, the easy one for children, the test we use for children is we ask them to just picture an apple. It's kind of like working out you can't whistle or click, you know that frustration people get when you see a bunch of other people doing what seems to be a very routine, very easy task, and no matter how much you try, you, an adult, cannot perform what seems to be an extremely basic function. So a lot of anger followed.
Carl Smith: So when I said try to picture a green apple in your mind's eye, what happens in your mind then?
Tiger Webb: Nothing. I try and locate it somewhere. I'm looking at you right now and across the table I am trying to…you can see I'm closing my eyes and opening them, trying to produce an image of anything. But then the moment I try and produce any clarity, to even really basic stuff, a circle, my parents' faces, it goes away and it's beyond recall.
Carl Smith: But you know exactly what an apple is.
Tiger Webb: Yes, I do know what an apple is, I know the faces of my parents when I see them. I think it's part of the reason why anger is the prime thing that I think about when I think about this thing I may have or may not have is because I'm going around, I live with someone who I love more than anyone else I've ever known in my life and, like, can she just see my face when she wants? That rules! I want that! Why don't I have that? What's wrong with me that I don't have that? So no, you're right, intellectually I know what all these things are, my memory is a bit disordered I guess but not anything worrying. But yeah, the recall or the imagery is not there.
Carl Smith: Tiger, I'm so sorry to throw you into the deep end and introduce you to the world of aphantasia.
Tiger Webb: You've ruined my life, it's fine, it's fine.
Carl Smith: Now, interestingly, if someone says they don't have mental imagery, like my friend Tiger Webb, it appears they often lean on other internal mechanisms to achieve roughly the same goals. But they don't always use the same tools. Professor Adam Zeman:
Adam Zeman: Using introspection, people with aphantasia often say that they rely heavily on language. Some say that they have vivid imagery in other modalities, so, for example, Oliver Sacks who was aphantasic seemed to have a pretty good auditory memory, he loved music and he was able to a fair degree to replay music in his mind's ear, so to speak. There are other people with aphantasia who say that they don't have imagery in any modality. So I think there is some variation there, but I guess everyone with aphantasia certainly is a highly competent language user. Language is a pretty good way of representing things in their absence.
Carl Smith: When Sian tries to conjure an image in her mind, she describes using an internal process that she calls spatial. As someone who believes they can create a mental images, I found it pretty tricky to understand exactly how this worked, and she found it hard to explain to me, given how fundamentally different our internal tools are.
Sian Thompson: I have this thing called spatial. I should say, everyone is a bit different, you might have different senses, you think in all different ways. And a big one for me is this kind of 3-D representation that's not visual. So if you think of like a 3-D game, like World of Warcraft, you see it on the screen, on the computer, but if you turn the computer screen off, stuff is still happening in the computer, but there is no visual. People also talk about it as kind of things are happening but it's behind the black curtain. There's no visual aspects to it but it's there, but you can't see it, which is difficult to explain to someone who visualises.
Adam Zeman: The inability to visualise is not a bar to imagination or to creativity. So, for example, Oliver Sacks the famous neurologist was aphantasic. Craig Venter, the first person to decode the human genome was one of the first people to get in touch to tell us that he is aphantasic and indeed he contributes some of his scientific strengths to it. So you can clearly achieve great things, despite being aphantasic.
But interestingly, about a third of people with aphantasia do describe having a rather thin autobiographical memory, and I think that makes sense because for most of us visualisation is a rather key part of autobiographical memory.
Carl Smith: How do we know that this actually exists? I think the average person listening, they might think if this is all based on people's self-reporting, how do we know for sure that people are actually aphantasic?
Adam Zeman: That's a very reasonable sceptical question. I think the work we are doing at the moment is an attempt to, if you like, triangulate between people's introspective reports, their performance on psychological tests, tests of various kinds of memory, for example, or of face recognition, and brain imaging. We had some evidence from the original case, from MX, that there was indeed a neural correlate for his subjective reports. So we established that there was a difference in his brain activity compared to control participants. So we got a bit of promising evidence from cases like MX. The next step is clearly to image the brains of people with a lifelong aphantasia and that's something we've been doing over the last year, so it will be very interesting to compare our findings in a few months' time.
Carl Smith: But when it comes to confirming that aphantasia does exist, perhaps the most striking anecdotal evidence that I came across was this:
Sian Thompson: When I'm falling asleep or when I'm dreaming I do see images, so I can compare that and go, well, no, it's not the same.
Carl Smith: So when you're dreaming you can…you do still dream, you see images in your dreams.
Sian Thompson: Yes, it's just got this extra layer to it.
Carl Smith: And you're saying that when you try to visualise something like Harry Potter's face or your mum's face or an apple, you get a very different experience to what you get when you are dreaming and seeing the images during a dream.
Sian Thompson: Yes, no images.
Adam Zeman: Again, there is some variation here. It looks as if about 50%, a little over 50% of those with aphantasia do have visual dreams, whereas some do not. That might well strike someone coming to this for the first time as very odd; how can this be and if they do, how do they know? I think it's perhaps not quite as odd as it sounds because of the conditions in the brain are very different during dreaming from those during wakefulness. So we know, for example, that a whole range of neurochemicals that are involved in maintaining wakefulness essentially disappear from the brain during dream sleep. Another way of thinking about this is that visualisation in dreaming is essentially a bottom-up process, it's driven from the stem of the brain, whereas visualisation during wakefulness when you decide deliberately to see what that apple looks like in your mind's eye, that's a very top-down process, you're making a conscious rational decision to do so. Because of the distinction between the top-down, bottom-up processing, I think it makes sense that dreaming imagery and wakeful imagery might dissociate.
Lynne Malcolm: You're with All in the Mind on RN, I'm Lynne Malcolm. Today, our ability to conjure mental imagery, and what happens when we are not able to. This is known as aphantasia. ABC science journalist Carl Smith sat down with Dr Joel Pearson from the University of New South Wales who believes he has a solution to objectively proving whether someone does have aphantasia or not.
Carl Smith: Dr Joel Pearson has been developing a way to quantitatively test someone's mental imagery strength.
Joel Pearson: This is the question that has really obsessed psychologists and neuroscientists for many years; how do you measure something that's internal, private? It's a fundamental question that we've tried to answer here in the lab. So historically it was questionnaires. Questionnaires up until recently have been the gold standard. Imagine a sunset and give me a number, how vivid is that sunset on 1 to 7, or something like that. Historically there's been a bunch of other methods that have tried to more objectively measure imagery, looking at reaction times or something called mental rotation. A lot of those suffer from lack of clarity in whether they are measuring something called visual attention or whether they are measuring visual imagery. So about 10 years ago we discovered that something called binocular rivalry could be used as a tool to measure imagery. So binocular rivalry is this really interesting illusion where you basically give a red pattern to one eye and a green pattern to the other eye, and you set this up so they are undergoing rivalry, they are in conflict, different images going to each eye.
So we discovered that if you imagine one of these patterns and then flash on binocular rivalry very briefly, what you imagine tends to be what you see. So if I imagine a green pattern, I'll see the illusion as green. If I imagine the red pattern, I'll see it as red. And the stronger my imagery is, the more likely that is to happen. So I do that over and over again and it gives me a measurement of the sensory strength of imagery. So it's a step away from that objectiveness of asking someone's opinion about themselves, and that's really one of the goals of the science we do here, is how do we measure things in the mind, in the brain, objectively and reliably.
Carl Smith: Seeing as I was there in the Pearson lab, I thought it would be worth taking the test too, to learn a little more about how it works.
Joel Pearson: So Carl, this is Rebecca, she is our post doc in the lab here, she has done her PhD here with me studying imagery and memory, and now she is heading up the aphantasia research we're doing.
Rebecca Keogh: Nice to meet you, hi. So we have a computer for the binocular rivalry images to be displayed on. We have a little chin rest for you to rest your chin on so that your head doesn't move around, and we have a whole bunch of old school red/green 3-D glasses for you to wear.
Carl Smith: What am I going to be doing in here once I rest my chin and put my funky red/green glasses on, looking at the computer screen?
Rebecca Keogh: So were going to get you to imagine some images, some red and green images, and we're going to look at how your imagery effects this illusion called binocular rivalry. So essentially the idea is if you just sat there and did nothing, 50% of the time you'd see the red, 50% of the time you'd see the green. Once we ask you to imagine one of the colours, you become more likely to see the one you imagine, and the more often you see the one you imagine, the better your imagery is.
Carl Smith: When someone with aphantasia does one of these tests, what happens?
Joel Pearson: When they try and imagine a green pattern, and often they can't, and we flash on the binocular rivalry, they won't see the green. They might see the red, or more a 50-50 chance there. So they don't get priming. Whereas if I run an experiment or Bec here runs an experiment, and maybe you Carl, we should get priming. That was one of the first we published on aphantasia showing that it does seem to exist at a sensory perceptual level, so it's not just about people's inability to report imagery, there doesn't seem to be any images in the visual cortex.
Carl Smith: Let's sit down and have a go then and see whether my imagery is strong.
This binocular rivalry test might be a little tricky for you to visualise, especially perhaps if you don't have strong visual imagination. But hopefully you get the idea. And it is probably better for the description to be a little vague in case you take this test. It is best for the researchers if you come in cold. Now, it's important to point out here that I didn't really follow the trial's research guidelines when I did the test. I was talking and moving, I adjusted my glasses and didn't stay steady on the chin rest. Plus I'd been told exactly what to expect before I began the test. So Dr Bec Keogh wasn't surprised that my score was pretty low for someone who reports being able to visualise.
Rebecca Keogh: Well, you have 52% priming, so not a huge amount, but I think that is probably due to recording and moving the glasses and things like that. So I'd say if you did it in a more unencumbered kind of way you would get better scores on there, but maybe not, but I would say probably.
Carl Smith: Aside from understanding exactly what aphantasia is, Dr Pearson is also interested in better understanding mental imagery more broadly because it plays a part in many mental disorders.
Joel Pearson: PTSD is one of the big ones, post-traumatic stress disorder. So we know that people who get PTSD have stronger mental imagery. We have published a paper looking at people with Parkinson's disease and visual hallucinations. We found that their imagery is stronger. There's good evidence now also that people with schizophrenia have very strong imagery. So it seems to be that very strong imagery is, one, associated with types of anxiety, but also associated with neurological disorders that bring with them hallucinations, whether they be visual or auditory. So if you don't have imagery, one could build a hypothesis that the prediction is that you may be more immune to PTSD, that you may be less likely to have hallucinations. But then there's just a lot of data looking at moral dilemmas or false memories, some interesting data for creativity, spatial navigation, reading comprehension, that all seems to correlate with imagery strength.
Carl Smith: Dr Joel Pearson has just begun a new project to map the brains of people with aphantasia, with a view to trying to train aphantasics to be able to use mental imagery.
Joel Pearson: It's a multipronged approach, so we are about to launch a large fMRI experiment, that's looking at the anatomy of the brain and the function of the brain, in particular the visual cortex. So we've published papers before just looking at the range of imagery strength in the normal population and we've shown that the size of visual cortex predicts imagery strength.
We've also found, again in the normal population, that people with stronger imagery have a quieter visual cortex. So the question is, are those two things also different in aphantasia, or is it something totally different? So we understand the mechanism and then we can build tools to give aphantasics the experience of imagery, if they want it.
Carl Smith: Since describing and coining the term aphantasia, Professor Adam Zeman from the University of Exeter has been flooded with letters from people who have the condition. He believes all this attention is partly because understanding that some people have aphantasia touches on some broader themes around how we comprehend the human experience.
Adam Zeman: I think what is crucial to being human is the capacity to detach oneself from the here and now and to enter the imaginary worlds of one kind or another, to recollect the past and anticipate the future, enter the world created by a novel or by a scientific theory. And I think one of the reflections that this line of research provokes is that there are actually many ways we can represent the world. I don't think of it as a disorder at all, I think of it as an intriguing variation in human experience. I think lying at one or other end of the spectrum probably has a mix of advantages and disadvantages which we hope to tease out over the next year or two, but I think it's a fascinating variation in human experience, and I think it reminds us that we are all very different and that these differences are often invisible to outsiders but often actually invisible to the persons.
Joel Pearson: People would write about imagery like it was this thing that was crucial for life and without it we can't do this, we can't do that. And it turns out that's not really the case. So it puts imagery in this unusual situation where if you have it it feels like it's crucial for everyday life. I use it personally to predict what I want to do next, remember the past, to help navigate, to do so many things. But we know from data from aphantasics that it's also kind of not necessary, which is really odd. It's like a tool and if you don't have that tool you can use other tools in the toolbox, but if you do have it you rely on it a lot.
Carl Smith: And for Sian, understanding people can be so different in what feels like such a fundamental way has left her wondering just how unknowably diverse our mental experiences might be.
Sian Thompson: You assume everyone else is kind of similar to you. I mean, people are different of course but we're all humans. Then you find something that seems to be such an integral part of most people's experience, and you realise, oh actually no, we're not actually the same in that way, so how many other things are we just kind of assuming everyone is the same, but not actually. Yeah, it's just interesting to think about that.
Lynne Malcolm: Sian Thompson, ending that story from ABC science journalist Carl Smith. If you think you have aphantasia and want to get involved in the Australian studies run by Joel Pearson, or the international studies run by Adam Zeman, there's information on our website.
Thanks to producer Olivia Willis and sound engineer Andrei Shabunov. I'm Lynne Malcolm.
And a little extra now, for those listening to the podcast version of this episode. While hearing this story about aphantasia, you might have wondered whether there are people who have the opposite experience; a richer than usual visual imagination. Carl Smith also asked Adam Zeman about this condition, known as hyperphantasia.
Adam Zeman: The focus of our work was on aphantasia and the majority of the people who got in touch with us have been aphantasic, but we've now heard from quite a large number of people whose experience lies at the opposite end of the spectrum, and I would love to hear from more, and these are people who say that their imagery is extremely vivid, often as vivid as real seeing. This is often a source of enjoyment to them. So people with hyperphantasia enjoy being able to visualise things in detail as they daydream or recollect the past, anticipate what they might do in the future. It can cause difficulties. So, for example, some people with hyperphantasia say that they sometimes get confused about whether they've imagined something or whether it's actually happened because their imaginings are so vivid.
And it is possible that there is a kind of interesting emotional implication of aphantasia versus hyperphantasia. So this is a little impressionistic, but my impression is that people with aphantasia find it a little easier perhaps to live in the present, they are less liable, for example, to nostalgia and craving because they are not so troubled by visual images of people and places from the past, whereas if you are hyperphantasic the opposite is true; you may be a little more vulnerable to the effects of emotionally charged or intrusive imagery. It does look from our data, and I should say this is all a little tentative but we are in the process of writing this up, but it does look from our data as if people with aphantasia are somewhat more likely to work in mathematical and computer related professions, whereas people with hyperphantasia are somewhat more likely to work in what are broadly creative industries.
Carl Smith: And I noticed on your website that you are preparing for the Extreme Imagination Conference of 2019. Tell me what this is all about.
Adam Zeman: I'm delighted that you've brought that up because it relates to a big surprise that I had during this work. So when we were being contacted by the many thousands of people with aphantasia who got in touch, one real surprise to me was that quite a number of aphantasic artists contacted me. And I had assumed that if anything was true of aphantasia it was that people with aphantasia weren't likely to be so interested in the visual world. After all, they can't see anything in their mind's eye. So presumably their interests and their abilities lie elsewhere.
But it turns out that there is actually quite a substantial number of artists who realise during their teens or 20s, like other people with aphantasia, often when they are at art school, that they are a bit different from their peers. Other people at art school seem to have a lot going on in their mind's eye. They have nothing going on in their mind's eye, but nevertheless they love the visual world and they very much enjoy representing it. We thought this was really rather interesting and wondered whether their aphantasia might have any implications for the kind of art they make.
Again, there was some variability. So some of the aphantasic artists said that they really could only work if they had their subject in front of them, which kind of made sense, whereas others said that they actually could use their work as their mind's eye, so to speak, and they could work perfectly creatively but they simply didn't have any strong visual preconception about what they were going to draw or paint, it kind of took shape on the page or on the canvas as they worked.
Similarly we were contacted by hyperphantasic artists, which I guess was a bit less of a surprise, I had assumed that artists would generally have relatively vivid visual imagery, and I think that is true as a general rule. So we thought it would be fun to have an exhibition which contrasted work from artists at the two ends of the spectrum, and the Arts and Humanities Research Council in the UK who had funded our previous work two or three years ago have funded us to put together an exhibition which is going to open in Glasgow at Tramway in January and then move to the Royal Albert Memorial Museum in Exeter in March. And alongside that we are running a conference in early April in Exeter which will be for people with aphantasia and hyperphantasia. So it will be a kind of community building exercise. We are going to have some scientific talks. So in fact Joel Pearson, I'm delighted to say, is coming across to the UK to give a talk at that conference. But as well as the scientific content, it's going to be an opportunity for people with extreme imagery or extreme imagination to come together, to get to know each other, and I hope that it will be a springboard for a further citizen science and research collaborations with our participants.
Lynne Malcolm: And if you want any more information about Adam Zeman's hyperphantasia research, or the Extreme Imagination Conference, that's also on the All in the Mind website. See you next time.
It might surprise you that some people can't conjure an image in their mind's eye. Or, if you have aphantasia, then perhaps you'd be surprised that some people can. Neurologists believe roughly 2 per cent of people have this condition, which affects their ability to create a mental image. But how important is visualising something in your mind?