How Does the Brain Differentiate Between Reality and Imagination?

Our ability to imagine helps us plan for the future and relive the past. Interestingly, the same brain structures involved in perceiving reality also play a role in imagination. So how does the brain tell them apart?

The key lies in the direction of information flow, which can be explained by two concepts: Top-Down (from top to bottom) and Bottom-Up (from bottom to top).

Top-Down and Bottom-Up: What Do They Mean?

Top-Down processing is where our past knowledge and expectations shape our perception. This flow of information starts in higher cognitive areas, like the prefrontal cortex, and is crucial for imagining. When we imagine a scene, the brain uses past experiences and knowledge to create that mental image.

In contrast, Bottom-Up processing occurs when sensory information from the environment travels up to the brain’s higher structures, helping us build an accurate picture of reality based on external data, not memory or expectations.

Top-Down and Bottom-Up in Everyday Life

To better understand these processes, let’s look at a real-life example. In a study, it was found that women’s tears reduced male aggression by 44% (Agron et al., 2023). Here, the Bottom-Up effect is the unconscious, neurophysiological response to tears, while the Top-Down effect would be a person’s decision to ignore this response due to past experiences or expectations.

And What About Hallucinations?

If distinguishing between reality and imagination were only a matter of information direction, how do hallucinations happen? Conditions like schizophrenia or Charles Bonnet syndrome—a condition where people with vision loss experience vivid hallucinations—show that it’s not always that simple. For example, people with Charles Bonnet syndrome might see dragons emerging from campfire flames, experiencing this as vividly as anyone else would see real flames.

Imagination vs. Reality: The Perky Hypothesis and Sensory Intensity

Recently, scientists have proposed two main theories to explain how we distinguish reality from imagination:

1. The Perky Hypothesis suggests that imagination always overlaps with real perception.

2. Another theory proposes that we distinguish reality from imagination depending on the intensity of the sensory experience.
   

An Experiment to Measure Imagination

To test these theories, researchers designed an experiment in which participants were asked to look at a screen displaying static noise while imagining moving diagonal lines. At the end of certain trials, the lines they were imagining briefly appeared on the screen. After each trial, participants reported how vivid their mental image was and whether they actually saw the lines on the screen.

The results were surprising. Participants were more likely to report seeing the lines when they were actually present, but some also reported seeing them when they were not. Those who reported especially vivid mental images were more likely to confuse the imagined image with reality.

What the Brain Says: MRI Results

Functional MRI scans during the task revealed that the intensity of both real and imagined images correlated with increased activity in the visual cortex and prefrontal cortex (Dijkstra & Fleming, 2023). This suggests that when internal signal strength—or “vividness”—reaches a certain threshold, the brain can interpret it as real. This could explain why we sometimes experience sensory hallucinations, like seeing dragons in the fire.

Final Thoughts

Can you imagine living without knowing if what you see is real or imagined? Research into how we distinguish between reality and imagination not only helps us understand conditions like hallucinations but also pushes us to explore the boundaries of our own perception.

References

Dijkstra, N., Fleming, S.M. (2023). Subjective signal strength distinguishes reality from imagination. Nature Communications, 14, 1627. https://doi.org/10.1038/s41467-023-37322-1

Agron, S., de March, C. A., Weissgross, R., Mishor, E., Gorodisky, L., Weiss, T., Furman-Haran, E., Matsunami, H., & Sobel, N. (2023). A chemical signal in human female tears lowers aggression in males. PLoS biology, 21(12), e3002442. https://doi.org/10.1371/journal.pbio.3002442