You're standing in a dimly lit room. You glance around and see a few people nearby, but suddenly you feel it - the sensation that someone else is there, close behind you. You turn, but nobody's there. Was it your imagination? A ghost? Or something deeper in the wiring of your brain?
According to new research published in Cerebral Cortex, the human brain has a surprising bias when estimating how many people are around. Not only do we tend to overestimate the size of a group, but that bias can be amplified when the brain is tricked into sensing an invisible presence. The study combines robotics, virtual reality, and electroencephalography (EEG) to explore how our social perception of "number of humans" is shaped - and how hallucinations bleed into what we believe we see.
The Social Counting Instinct
Humans, like many animals, have a remarkable ability to quickly estimate how many objects or individuals are in a scene without counting. This skill, called numerosity estimation, is thought to have deep evolutionary roots. For survival, it matters whether you spot two predators or ten, whether a rival group is small enough to challenge or too large to risk confronting.
But humans don't just estimate dots on a page - we are especially tuned to people. Prior work has shown that when estimating numbers of humans, our brains exhibit a reliable overestimation bias. If seven people are present, we might perceive nine or ten. This "social numerosity system" seems to work differently than our perception of non-social objects, hinting at specialized circuits for detecting and counting other people.
Hallucinating a Presence
To test how this bias might be influenced by hallucinations, researchers from the Swiss Federal Institute of Technology in Lausanne combined virtual reality with a robotic system capable of inducing the eerie feeling that someone else is close by - a phenomenon known as a presence hallucination.
Participants wore a VR headset that briefly showed small groups of human figures, usually seven or eight. Before each trial, they manipulated a robotic device with their hands. At the same time, a second robot behind them touched their backs in synchrony or with a deliberate half-second delay. That small mismatch was enough to create the sensation that another person - invisible but undeniably "there" - was standing nearby.
When participants were asked to report how many people they had just seen in the VR environment, their estimates consistently overshot reality. And the effect was stronger in the presence-hallucination condition. It was as if the brain silently added the ghostly figure to the visible crowd.
The Brain's Signature of Seeing Too Many
EEG recordings captured what was happening in the brain during these trials. The critical activity appeared around 220 milliseconds after stimulus onset, in a wave known as the P2p component. This is a stage of processing linked not to raw sensory input but to the perception of number - the difference between what's physically there and what we believe is there.
The P2p signal was stronger during the hallucination condition, and its amplitude correlated directly with the degree of overestimation. In other words, the bigger the brain wave, the bigger the mistake.
Source localization pointed to the left extrastriate cortex, a region behind the ear associated with body perception and embodiment. This is not a general-purpose counting circuit, but one specialized for human figures and social signals. The invisible presence wasn't just a creepy feeling - it was actively integrated into the brain's social numerosity system.
Fresh Perspective
The findings offer a fresh perspective on how perception and hallucination intertwine. Humans evolved to prioritize social information, often at the expense of accuracy. Overestimating the number of people in a group may have once been adaptive - better to err on the side of caution when facing a crowd of potential allies or enemies.
But the same bias could also help explain why people experience ghostly presences, paranoia, or even crowd-related anxiety. If the brain's social counting system is prone to integrating invisible presences, then the sense of "someone there" might bleed into what we visually perceive.
This has clinical implications too. Presence hallucinations are common in conditions like Parkinson's disease and schizophrenia. Understanding how they distort numerosity estimation could shed light on why patients sometimes misperceive social environments, and how this links to changes in body perception and self-awareness.
The Ghost in the Numbers
What makes this study stand out is its clever experimental design. By combining robotics, virtual reality, and brain recording, the researchers were able to systematically induce a hallucination and then measure its impact on perception in real time. Instead of relying on clinical anecdotes, they created a repeatable lab version of the feeling that someone is behind you.
The results show that hallucinations don't just sit in the background of consciousness. They actively shape how we perceive the world, altering the brain's computations about who and how many are around us.
Looking Forward
The researchers note that the study only tested small groups - seven or eight people - and healthy young adults. Future work could explore larger crowds, other social contexts, and clinical populations. Still, the findings highlight a striking truth: our sense of social reality is not a direct readout of the world but a fragile construction, vulnerable to both illusions and invisible companions.
So next time you feel like there's someone else in the room, even when you're alone, remember: your brain may simply be doing what it has always done - overcounting humans, weaving presences into perception, and reminding us that reality is as much constructed in the mind as it is given by the eyes.
