Subjective signal strength distinguishes reality from imagination was published in Nature Communications this year by Nadine Dijkstra and Stephen Fleming, and has been featured in Quanta Magazine’s top three breakthroughs in biology and neuroscience in 2023. The study discovered that the brain has a “reality threshold” against which it constantly evaluates processed signals.

The process behind our ability to distinguish between perception and imagination has been investigated for many years. In 1910, psychologist Mary Perky attempted to develop a scientific definition of imagination. In her experiment, people were asked to imagine objects like bananas, while a faint image of a similar shape and colour was projected on a screen. Surprisingly, participants thought they were imagining the objects, but were actually seeing the projections. Even when the brightness increased gradually, they did not realise they were perceiving real images.

This finding, known as the Perky Effect, highlights the connection between mental imagery and what we actually see. Perky found that when what we see aligns with what we’re imagining, we tend to think we have conjured it up in our minds.

In a setup resembling Perky’s experiment, participants were asked to imagine specific diagonal lines while staring at a static screen. Contrary to the anticipated Perky effect, where imagined and real perceptions blend, results from the 2023 publication revealed a nuanced picture. When participants were questioned about the reality of the lines they saw, responses varied. When faint images of the lines were shown, participants were more likely to report seeing them. When participants were not shown the lines, some still reported seeing them, suggesting vivid imaginations at work.

The study analysed neuroimaging data from the participants, which indicated that there may be a direct involvement of visual and prefrontal cortices in distinguishing imagination from reality. The results of the study suggest that reality monitoring–distinguishing imagination from real perception–could be done simply on the basis of signal strength, suggesting that very vivid imagination can be mistaken for reality, even in healthy participants. The findings impact our understanding of how our sense of reality is created and what might go awry in reality monitoring disorders such as psychosis.

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