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We all live in two worlds: a world outside of ourselves containing things that others can also see and hear and touch, and a world inside consisting of our thoughts and imaginations, which only exist in our own mind. Usually, we effortlessly keep these two worlds apart. But a recent study I ran together with metacognition expert Steve Fleming at University College London’s Wellcome Centre for Human Neuroimaging, where I am a senior research fellow, affirms that the 18th-century Scottish philosopher David Hume was right all along—distinguishing reality and imagination is less straightforward than we might think.1

To perceive the outside world, our brain combines signals entering our brains through our eyes with what we expect the world to look like based on our past experiences.2,3 This means that our perception of the outside world is strongly influenced by what we believe. An example of this is the checker shadow illusion shown below in which our knowledge about how shadows influence color makes the B square look lighter than the A square, when in fact they are exactly the same shade.

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ONE SHADE OF GRAY: Our knowledge about how shadows influence color makes the B square look lighter than the A square, when in fact they are the same shade.Wikipedia

To imagine something that is not there, we use our knowledge to generate an image of what we think something looks like. Recent studies have shown that the neural processes supporting pure imagination are highly similar to those involved in perceiving the outside world: When we imagine a cat in our mind’s eye, a similar network of brain areas gets activated as when we see a real cat out in the world.4,5 These observations demonstrate that brain signals reflecting imagination and signals reflecting reality are completely intermixed in our brain, muddling the distinction between our inner world and the outside world, between reality and fantasy. Does this intermixing lead to confusion between what is real and what is imagined?

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At the beginning of last century, the American psychologist Mary Cheves West Perky pioneered experimental testing of confusions between imagination and perception.6 Using an ingenious technical set-up, Perky aimed to “build up a perceptual consciousness under conditions which should seem to the observer to be those of the formation of an imaginative consciousness.” In other words, she tried to get people to mistake perception for imagination. Participants were asked to imagine an object, for instance a tomato, and describe their experience. At the same time, unknown to the participant, Perky projected these same objects at just-visible intensities. In all cases, participants failed to notice that real stimuli had been presented and instead insisted that everything that they saw was the result of their own imagination, saying things like “Feels as if I was making them up in my mind,” and “It is more distinct [than the images I usually have]; but I have never tried much.”

There is no real categorical difference between imagination and reality.

Now, more than a hundred years later, this “Perky effect” has achieved a “classic, even mythic status in the literature on imagery.”7 However, replication attempts with modern set-ups have yielded mixed results.8 One reason for this discrepancy might be that contemporary research standards require that the same task is performed many different times. This is important to estimate variability in behavior and determine statistical significance. But, in the context of monitoring reality, repetition greatly influences results because it allows participants to correct previous confusions. As soon as a participant realizes that a real image is presented, they will look for a real image on all subsequent trials, biasing attributions toward reality.

To address this issue, in our experiment, each participant did only one critical trial. But we collected our data online, allowing us to test hundreds of participants.1 This way, we could combine the essence of Perky’s original experiment, in which participants did not know real images could be presented, with enough statistical power by testing hundreds of participants. We asked participants to imagine a shape and indicate how vivid their experience was. At the same time, unknown to the participant, we also presented a just-visible shape that was either the same or different to the one they were imagining. After that we asked participants whether they thought a real shape had been presented or whether anything they saw had been the result of their imagination. Did participants fail to notice the presentation of the real shape when they were also imagining it, thinking it was just their imagination?

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What we found was quite astonishing. In contrast to Perky’s findings, participants were more likely to say they saw a real picture when imagination and reality were matched compared to when they were different. But, when they saw a real picture, they also said their imagination of that same picture was more vivid. Even when we did not show a real picture, participants who nonetheless reported seeing a real picture also said that they imagined that picture more vividly. These results suggest a modification of Perky’s ideas. In line with what she found, imagination was reported as more vivid when the same signals were also perceived, suggesting confusion between the two, but in contrast to what she found, we do not fail to notice things we simultaneously imagine, we are actually more likely to see them in the outside world.

This combination of results can be very neatly explained by the idea that internal and external signals are intermixed to create one conscious experience. When this mixed signal is strong or vivid enough to cross a reality threshold, we think it reflects reality. When the same object is perceived and imagined, the combined signal is stronger, so we are more likely to think this object is really out there. This also fits well with our daily experience. When we are waiting on the bus, we do not fail to see it because we are thinking about what it looks like—that would be very inconvenient, making you miss the bus because you are waiting for it. Instead, we are more likely to see the bus when we are thinking of it, and sometimes, this also leads to confusion; for example, when we mistake the truck coming around the corner for the bus we are waiting for.

This means that there is no real categorical difference between imagination and reality, but that they are subjectively intermixed. When this combination of internal and external signals is strong enough, we believe it reflects reality. This is in line with what Hume wrote, in 1739, in Book I of his Treatise of Human Nature: “The idea [imagination] of red which we form in the dark, differs only in degrees of intensity, not in nature, from the impression [perception] of red that strikes our eyes in sunshine.”9 When the things that we imagine are not really there, nor anything that looks like it, signals are unlikely to be strong enough to lead us to believe they reflect reality. However, when, for some reason, internally generated signals are strong enough, they will be indistinguishable from reality.

As it turns out, reality and imagination are completely intermixed in our brain which means that the separation between our inner world and the outside world is not as clear as we might like to think. If our imagination is vivid enough, we will think it is real and we use our imagination to create our perception of reality, which means, “We do not see things as they are, we see them as we are.”10

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Nadine Dijkstra is a senior research fellow at the Wellcome Centre of Human Neuroimaging at University College London. Her research focuses on the neural overlap between imagination and perception and how our brain can keep them apart (or not).


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1. Dijkstra, N. & Fleming, S. Fundamental constraints on distinguishing reality from imagination. PsyArXiv (2021). Retrieved from: doi:10.31234/OSF.IO/BW872

2. Friston, K., Kilner, J., & Harrison, L. A free energy principle for the brain. Journal of Physiology – Paris 100, 70–87 (2006).

3. Parr, T. & Friston, K.J. The anatomy of inference: Generative models and brain structure. Frontiers in Computational Neuroscience 12, (2018).

4. Pearson, J. The human imagination: the cognitive neuroscience of visual mental imagery. Nature Reviews Neuroscience 20, 624-634 (2019).

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5. Dijkstra, N., Bosch, S.E., & van Gerven, M.A.J. Shared neural mechanisms of visual perception and imagery. Trends in Cognitive Sciences 23, 18–29 (2019).

6. Perky, C.W. An experimental study of imagination. The American Journal of Psychology 21, 422–452 (1910).

7. Thomas, N.J.T. The Perky Experiment Stanford Encyclopedia of Philosophy (2014). Retrieved from:

8. Waller, D., Schweitzer, J.R., Brunton, J.R., & Knudson, R.M. A century of imagery research: Reflections on cheves perky’s contribution to our understanding of mental imagery. The American Journal of Psychology 125, 291–305 (2012).

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9. Hume, D. A Treatise of Human Nature: Vol 1. Of the understanding (1739).

10. Nin, A. Seduction of the Minatour Swallow Press, Chicago (1961).

Lead image: Jorm S / Shutterstock

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