Human experience is a mixture of what we perceive from the outside and what we create on the inside. Street noises, the warmth of sunshine These are based on sensory experience. But we can shut our eyes and vividly imagine faraway lands, faces of loved ones, or creatures that don’t exist. Both perception of the real and imagination engage corresponding brain areas, so here is a complicated task: separating the imagined from the real.
1. The Brain’s Balancing Act: Distinguishing Perception from Imagination
Thomas Naselaris illustrates how he is able to vividly imagine a unicorn strolling down a street and recognize it as not being real. It’s not the same as seeing a real horse, although the thought might be very present in his mind. This capacity to distinguish fantasy from fact, far from being automatic, engages complex neural systems. Philosophers have long wondered why we do not always get confused with internal thoughts and the outside world. One solution may be in how the brain determines the “strength” or clarity of what it senses and what it imagines.
Recent studies indicate that the brain employs internal thresholds to filter out images an imagined image typically doesn’t achieve the level of intensity necessary to be experienced as real. But when it does, the brain may mistake imagination for reality. This mechanism explains why hallucinations or intense dreams seem real. It’s not that the brain is failing it’s that the signal being processed registers beyond the threshold typically reserved for real sensory input.
2. The Reality Threshold: A New Understanding of Mental Imagery
Nadine Dijkstra and colleagues suggested the idea of a “reality threshold” in a Nature Communications study. The hypothesis suggests that mental images need to be of sufficient strength or clarity to be acted upon as real. Weak signal from imagination remains comfortably within the domain of thought. Sufficiently strong, it can cross over into what the brain interprets as perception.
Their experiments tested whether this boundary could be manipulated or interpreted. Participants were asked to imagine lines on a diagonal in a noisy background and rate the vividness of their mental pictures. In some of the trials, very weak visual stimuli corresponding to or conflicting with their imagined lines were projected. Surprisingly, where the projected lines corresponded with people’s imagined direction, people were more inclined to report seeing a real picture despite the line being very weak. This indicates that perception is impacted by internal imagery, particularly when mental images are sensed as vivid.
In a second experiment phase with no actual image participants also reported the following as “real” stronger imagery. These results imply that vivid imagination is sufficient to deceive the brain into perceiving thoughts as external stimuli. This provides an interesting explanation for some mental phenomena, from daydreams felt to the powerful visions some individuals experience in psychosis or before sleep.
3. A Century of Insight: From the Perky Effect to Modern Experiments
The connection between perception and imagination has captivated researchers for more than a century. Dijkstra’s pandemic work led her to reinvent the original 1910 experiment by psychologist Mary Perky. In it, subjects visualized fruits while faintly projected images barely perceived showed up on a white wall. They reported not seeing any real images even though they technically existed. Rather, they thought the images were purely imagined, depicting what would come to be referred to as the “Perky effect”: when outside observation confirms what one is imagining, the brain tends to fall back on guessing it’s an image in one’s mind.
Perky’s experiment showed that the brain is biased toward imagination when information is ambiguous. Bence Nanay referred to this as a cognitive science classic a must-read when researching the overlap of imagination and perception.
In the 1970s, psychologist Sydney Segal built on this with the projection of unrelated images (such as a tomato) while subjects created alternate scenes (such as the skyline of New York City) in their imaginations. The outcome? Merged perceptions. One subject might report the scene imagined with an unanticipated red color. These experiments provided initial evidence that the brain does not always make a clear distinction between internal and external input at times, it combines both into a single perception.
Although some replication experiments faced methodological problems, such as multiple testing that led participants to guess the purpose of the experiment, Dijkstra’s team planned theirs with care. By having participants tested but once, they hoped to maintain the spontaneous character of perception and minimize response bias. Their method yielded greater accuracy about how perception and imagination play off each other in the moment.
4. Brain Scans, Thresholds, and Individual Differences
To find out how mental images and actual seeing are handled by the brain, Dijkstra and her team reanalysed brain scan data from a previous study with 35 volunteers. Volunteers were shown or imagined pictures of objects such as roosters and watering cans. Activations in the visual cortex appeared very similar whether a person was actually seeing something or imagining it. This lent further evidence to the theory that the same neural processes tend to underlie the two actions.
However, mental images tended to be weaker providing neural evidence for the surmised threshold. Dijkstra surmised that a sufficiently powerful mental image may bridge the line and become indistinguishable from reality. This could explain phenomena such as lucid dreaming, hallucinations, or drug-induced imagery. Yet, it is not known if the brain works on a binary threshold (“imagined or real”) or a sliding scale.
The frontal cortex presumably is also responsible for evaluating the strength of images and eliminating signals. Neurons, connectivity within the brain, and even certain populations of neurons might be responsible for how imagination is translated. Muckli suggested that various layers of the cortex could deal with real versus imagined input. He appreciated Dijkstra’s research, although it partially contradicted his own suppositions. Both views are in agreement that one thing: knowing these mechanisms makes us appreciate how thin the line between inner thought and outer reality is.
Peter Tse further contributed that imagination is not merely about pictures it encompasses higher-level reasoning, such as meal planning or designing machines. Such mental operations extend well beyond visual stimulation and demonstrate the brain’s capacity to recreate complex situations without any external input.
5. Imagination in the Transforming World: From Psychiatric Illness to Virtual Reality
The environment in which our brains operate has changed dramatically since Perky’s time. Today, we’re constantly exposed to simulated images through screens, projections, and digital environments. This visual saturation may have lowered our reality threshold, meaning our brains may now require stronger sensory cues to label something as “real.” Dijkstra and her colleagues speculate that modern brains have adapted to this new landscape, adjusting the way they evaluate vividness.
These results raise significant issues for mental health. For instance, patients with illnesses such as schizophrenia could hallucinate because their imagination is so robust or because the brain’s threshold for designating something real is set too low. Karolina Lempert suggested that in such scenarios, even a normal mental image could be a perception of the outside world. This is consistent with the view that minor imbalances in brain calibration can have spectacular perceptual consequences.
Scientists also identify other populations that are worthy of investigation, including those with hyperphantasia (very vivid imagination) or aphantasia (being unable to imagine). Their lives provide natural windows into the operation of reality-checking systems in people differently. Lucid dreaming, psychedelic experiences, and flashbacks in PTSD all indicate that perception and imagination can merge under some circumstances sometimes powerfully and painfully.
Technology places an additional layer on this discussion. As artificial intelligence and virtual reality mature, the brain’s capacity to distinguish digital simulations from actual life can be challenged further. Researchers already are conducting research studies employing brain scanners to watch for the threshold mechanism at work. Unraveling and even controlling this system might have utilitarian applications assisting people with visual disability, supporting mental illness treatments, or increasing immersion in virtual environments.
At the center of it all is a potent understanding: our experience of reality is not absolute. It’s influenced by memory, surroundings, brain activity, and the intensity of our imagination. What becomes real isn’t always outside us, and what’s imagined isn’t always comfortably internal. The brain builds our experience filtering, mixing, and brokering every picture we see or imagine.
