Picture an apple, any apple.
As long as you don’t have aphantasia—the inability to visualize things in your mind’s eye—this suggestion triggers brain activity that’s surprisingly similar to what happens when you see a real-world apple with your eyes. Such neural overlap is economical because both cases require the brain’s visual system to carry out many of the same tasks. But it also raises a question: How does our brain tell reality and imagination apart?
Neuroscientists are now beginning to understand the brain circuit that handles this distinction. In a recent study in Neuron, researchers identified a brain region that generates what they call a “reality signal.” This signal is then evaluated by another region—one that, when it functions abnormally, has been linked to schizophrenia. Understanding this reality-monitoring circuit could help scientists understand, and possibly even treat, schizophrenia and other disorders that impair the ability to discern reality.
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We tend to think that we perceive reality as it is, with cameralike eyes that objectively log the light that hits them. But as information from the eyes flows into the brain, it becomes more abstract and subjective; lines and colors, discerned in the brain’s early visual-processing regions, transform into concepts and meaning when they reach higher-level cortical networks.
This “bottom-up” processing is only one part of the story. Information can also flow “top-down,” with higher-level cognition influencing perception. You can see this in action with ambiguous illusions, such as the Rubin vase or Yanny-Laurel audio clip, where your expectations determine what you see or hear. Or maybe you’ve been eagerly awaiting a visitor and keep thinking you hear a knock even when there’s no one at the door.
“Perception isn’t just a passive process,” says Philip Corlett, an associate professor of psychiatry at the Yale School of Medicine, who was not involved in the new study. “Stuff coming down from cognition sculpts what we perceive, and then stuff that comes up from perception might change what we believe.”
Bottom-up and top-down processes both contribute to our visual experiences—but bottom-up is arguably more important for perceiving the external world, while conjuring mental images is usually controlled by top-down commands. But given that both processes generate activity in the same regions, how does the brain keep reality straight? To investigate this question, researchers showed hard-to-see patterns to participants while they were in a brain scanner. The participants viewed a screen with a staticlike background, which sometimes had a faint pattern of diagonal stripes overlaid on it. The stripes could be angled either to the right or to the left.
Participants were asked to imagine either a left-sloping or right-sloping pattern in their mind’s eye while viewing the screen and were then asked to indicate whether one of the patterns was actually being displayed (which was true exactly half the time). Sometimes the screen showed the same pattern they were imagining, and sometimes it showed that pattern’s opposite or nothing at all.
When participants were looking for and imagining the same pattern, they were more likely to say they saw it—even if it wasn’t there—suggesting they mistook imagination for reality. Participants also said their mental imagery was more vivid when a pattern was present, providing it matched what they were imagining. This suggests perception can also influence imagination.
The functional magnetic resonance imaging (fMRI) brain scans revealed a region that was more active when participants reported seeing a pattern, either real or imagined. “There’s a brain region close to your temples, the fusiform gyrus, which is active both when you see something and when you imagine something,” says neuroscientist Nadine Dijkstra of University College London, who led the study. “Most surprisingly, we found that activation in that region predicts whether you think something’s real, even when it’s imagined.”
The researchers call this activity in the fusiform gyrus a “reality signal,” and their findings suggest it is formed from the sum of activity from both mental imagery and perception. The researchers think this signal is then evaluated by another region, the anterior insula, which was active while participants were completing the tasks. The study results suggest the anterior insula evaluates the reality signal and makes a “yes or no” decision; activity above a certain threshold feels real, while activity below it feels imagined.
This arrangement should work fine most of the time, as perception generates stronger activity than imagery, so it usually produces signals above threshold; imagery typically doesn’t. This is thought to be because the lack of bottom-up sensory input while forming imagery means certain groups of sensory neurons are not activated, producing less activity overall. This would explain why mental images don’t produce signals that cross the reality threshold—as long as everything is working properly.
But operating in this way still leaves open the possibility that imagery and perception could be confused. It’s not hard to imagine how dysfunction in parts of this circuit could result in faulty judgements about reality. For instance, if the signal produced by the fusiform gyrus during imagery is too strong, or if the threshold in the anterior insula is set too low, imagination could be mistaken for reality. “Schizophrenia has been associated with abnormalities in the prefrontal cortex and anterior insula, which suggests hallucinations in schizophrenia may be due to issues with reality threshold setting,” Dijkstra says.
Something similar may happen with auditory hallucinations, which often take the form of hearing voices. A 2016 study asked healthy participants, who had varying tendency to experience hallucinations, to listen for sentences masked by noise while silently producing the same words in their mind as inner speech. Hallucination-prone participants were more likely to report hearing a voice when doing this, regardless of whether a voice was present. This is strikingly similar to Dijkstra and her colleagues’ findings, Corlett says, because it suggests auditory hallucinations are caused by people mistaking their inner voice for an external one.
While hallucinations are often thought of as a symptom of psychosis in disorders such as schizophrenia, many other neurological conditions involve them as well. Around 40 percent of Parkinson’s disease patients experience visual hallucinations, for example. Future studies with people prone to visual hallucinations could test “whether the imagery levels at which they falsely detect something are different” than non-hallucination-prone controls, says neuroscientist Lars Muckli of the University of Glasgow.
If so, the reality-monitoring circuit could be targeted in potential treatments for hallucinations. “It might eventually be possible to recalibrate a person’s reality threshold through training or via neurofeedback or brain stimulation,” Dijkstra says. (Neurofeedback techniques present people with real-time readouts of brain activity, which can teach them to modulate their brain activation over time.)
First, though, to demonstrate causality, Dijkstra and other researchers need to test if stimulating the fusiform gyrus causes people to think something is more real or less real. “That’s technically very difficult, so it’s a long-term project, but we’re planning on doing that,” Dijkstra says.
The researchers also want to understand whether a mental image’s real-world plausibility affects activity in this brain circuit. “We have a model of what’s likely in the world, which probably also goes into this decision-making process,” Dijkstra says. “If you imagine a pink elephant really, really vividly, you’re probably still not going to think it’s real because pink elephants don’t exist.” Context is critical. An elephant in your living room is pretty unlikely, whether it’s pink or not. “That’s actually an experiment we’re running right now—looking at the influence of context on this,” Dijkstra says. “It’s definitely an important factor.”