Study Reveals How the Brain Distinguishes Between Imagination and Reality
Have you ever had an image in your mind that seemed almost real? Understanding how the brain keeps imagination and reality separate is an essential scientific question. A new study sheds light on the brain’s ability to distinguish between the two — and reveals why this process is sometimes imperfect.
Researchers Identify a Sensory Signal That Helps the Brain Monitor Reality
Every day, humans mentally create vivid images — from picturing a beautiful landscape to imagining a familiar face. These mental experiences can feel strikingly real, yet we usually know they do not reflect current reality. How does the brain maintain this distinction?
A new study from University College London, published online in Neuron on June 5, 2025, explores the neural mechanisms that allow us to tell imagination from reality. The researchers, led by Nadine Dijkstra, investigated how the brain monitors sensory activity to support judgments about what is real and what is imagined.
According to the authors, their findings reveal a simple yet powerful mechanism: the brain tracks the combined strength of sensory signals in the fusiform gyrus (FG), a region of the mid-level visual cortex. This signal helps inform explicit reality judgments — and explains why these judgments sometimes go awry.
What the Researchers Investigated
The research aimed to address a fundamental question: how does the brain determine whether a perceptual signal reflects external reality or an internally generated image? Previous work had shown that imagination and perception recruit overlapping neural systems, creating the possibility for confusion between the two. However, the precise neural mechanisms supporting this distinction remained unclear.
The authors proposed a “reality threshold” model in which perceptual and imagined signals are summed to create a reality signal. If this signal exceeds a threshold, the brain classifies the experience as real; if not, it is classified as imagined. The study tested this model by observing both behavior and brain activity during tasks that deliberately created conditions where imagination and reality could be confused.
The study builds on earlier research suggesting that such sensory monitoring may be a key part of “perceptual reality monitoring” — an essential brain function that helps prevent false perceptions.
How the Study Was Conducted
The experiment involved 26 healthy adult volunteers (14 female, mean age 23 years), all with normal or corrected-to-normal vision. Participants completed a visual detection task while undergoing functional magnetic resonance imaging (fMRI).
Each trial began with a brief cue instructing participants to imagine a grating at either 45° or -45°. Participants then viewed dynamic noise (a rapidly changing random visual pattern) in which a grating might or might not be embedded. The grating, when present, matched the cued orientation in 50% of trials. Participants reported whether they perceived a grating and rated the vividness of their mental imagery on a four-point scale.
Stimulus contrast levels were individually calibrated to target near-threshold detection performance. The experiment included separate blocks for congruent (matching imagery and stimulus) and incongruent (mismatched imagery and stimulus) conditions, allowing the researchers to analyze how congruency influenced perception and neural activity.
The researchers also used computational modeling to test competing theories of perceptual reality monitoring. Models were compared against behavioral and neural data to identify the mechanisms best explaining participants’ reality judgments.
What Makes This Study New
The authors highlight that “a key mechanism through which the brain distinguishes imagination from reality is by monitoring the activity of the mid-level visual cortex.”
They further report that activity fluctuations in the fusiform gyrus predicted when participants confused imagined stimuli with real ones. The study also identified a network of frontal brain regions, including the anterior insula and dorsomedial prefrontal cortex, that interacts with this sensory signal to support explicit judgments about what is real.
Compared to previous research, this study provided a more detailed demonstration of how specific brain regions contribute to perceptual reality monitoring. The authors suggest that their findings “lay the foundations for characterizing a generalized perceptual reality monitoring system in the human brain.”
Key Findings from the Study
The study reports several key findings:
- “Judgments of reality are underpinned by the combined strength of sensory activity generated by either imagery or perception in the fusiform gyrus.”
- “Activity fluctuations in this region predict confusions between imagery and perception on a trial-by-trial basis.”
- The authors found that activity in the fusiform gyrus reflected both the strength of perceptual signals and the vividness of mental imagery, consistent with a reality signal.
- According to the authors, frontal brain regions encoded the binary outcomes of reality judgments and showed functional coupling with the fusiform gyrus.
- The study found that activity in the fusiform gyrus predicted when participants confused their imagery for perception, particularly in trials where imagined and perceived stimuli were congruent.
Authors’ Conclusions
The authors conclude that perceptual reality monitoring relies on monitoring activity in the mid-level visual cortex, particularly the fusiform gyrus. They write: “We show that a latent variable sensitive to combinations of both imagery and perceptual signals was uniquely tracked in the activity of the FG, enabling downstream brain regions to efficiently distinguish reality from imagination.”
They also suggest that control signals associated with self-generated imagery are not sufficient by themselves to prevent confusion. “Our results suggest that control signals associated with self-generated perception are not enough to distinguish imagination from reality.”
Additionally, the study highlights that reality judgments involve not only sensory monitoring but also active transformation by frontal brain regions. The anterior insula, in particular, was identified as a key node in translating sensory signals into explicit decisions about what is real.
The Broader Scientific Context
While the study focused on visual perception, the authors note that similar mechanisms may operate across different sensory modalities. They mention that the involvement of frontal regions such as the anterior insula aligns with findings from research on auditory hallucinations, suggesting a possible general role in perceptual reality monitoring.
The findings also contribute to broader efforts to understand how the brain constructs a coherent model of reality. By identifying a specific neural substrate for the reality signal, the study provides a foundation for future research on how this system functions in both healthy and disrupted states.
Limitations and Future Directions
The authors acknowledge that their experimental design focused on relatively simple visual stimuli (tilted gratings) and that future research should examine how perceptual reality monitoring operates with more complex or naturalistic imagery. They also note that their findings are based on a specific object class and suggest that it remains an open question whether the observed mechanisms generalize to other types of stimuli.
In addition, the authors propose that future studies could investigate how volitional control over mental imagery influences perceptual reality monitoring. They emphasize the need to examine whether similar neural mechanisms operate across other sensory modalities, such as audition or touch.
The study also highlights that individual differences in imagery vividness and attentional control may shape how reality judgments are formed. The authors suggest that exploring these factors could offer valuable insights into why some individuals are more prone to reality-monitoring failures.
Finally, they propose that their findings may inform future research into conditions where perceptual reality monitoring breaks down, such as hallucinations, while cautioning that further work is required to generalize the results to clinical populations.
Conclusion
This study provides new insights into how the human brain distinguishes imagination from reality. By identifying a sensory signal in the fusiform gyrus that tracks the combined strength of imagined and perceived signals, the researchers offer a potential neural basis for perceptual reality monitoring. The authors emphasize that further studies are needed to explore the generality of this mechanism and its relevance across different sensory modalities and individual differences.
The original study is available at: https://doi.org/10.1016/j.neuron.2025.05.015
The information in this article is provided for informational purposes only and is not medical advice. For medical advice, please consult your doctor.
