How the brain tells imagination from reality: key ‘signal threshold’ identified

A new study has uncovered a critical mechanism in the brain that helps us distinguish between real sensory input and imagined experiences.

Researchers have identified a signal threshold within the fusiform gyrus—part of the brain’s visual processing system—that plays a vital role in reality perception.

Their findings, published in the journal Neuron, also offer new insight into psychiatric conditions like schizophrenia, where the boundary between reality and imagination can blur.

Scientists from University College London conducted the study using functional MRI scans to track brain activity in 26 participants. Volunteers were shown a noisy screen containing diagonal lines in half of the trials, while in the other half, they were asked to imagine the lines. Sometimes, participants imagined lines that matched the visual cues; at other times, they didn’t.

The researchers observed that both actual and imagined stimuli activated the fusiform gyrus. However, it was only when activity in this region crossed a certain intensity that participants identified the visual as real.

According to the study, this threshold system in the fusiform gyrus acts as a built-in filter, deciding whether a perception is grounded in reality or generated by imagination.

Interestingly, the anterior insula—a brain region linked with decision-making—was also active during this process. This suggests it may serve as a kind of monitor, interpreting the strength of the fusiform’s signals to help make judgment calls on what is real.

Lead author Nadine Dijkstra explained that while imagination does stimulate the same brain areas as real vision, “the signal is usually too weak to convince the brain of external input.” She emphasised that understanding this mechanism may be key to decoding hallucinations and other perceptual distortions seen in mental illnesses.

The study opens the door to more research, especially involving complex stimuli like faces or animals, and even potential experiments with brain stimulation to enhance the vividness of imagined experiences. Thomas Pace, a neuroscientist not involved in the study, called the findings “a big step toward figuring out how reality monitoring works—and how it fails in disorders like schizophrenia.”