The reaction to neuroscientist Doris Tsao’s latest preprint, she says, has been “surreal”. Tsao, at the University of California, Berkeley, described the study on social media on 17 October. Ten days later, her summary has drawn more than two million views and 11,000 likes.

Posted on bioRxiv, the study¹ probed the brains of macaques (Macaca mulatta) during a visual-perception experiment. Using only the monkeys’ neural activity, Tsao’s team managed to reconstruct with high fidelity not only images that the animals were consciously seeing, but also images that were in front of their eyes but escaped their awareness.

Previous studies in monkeys have detected neural activity in response to content they are not aware of, but only as weak signals. “It’s super surprising to find this strong signal that is not consciously perceived,” Tsao says. Even more unexpected to Tsao, the study showed that brain activity switched between patterns associated with conscious processing and those associated with unconscious processing several times per second.

The authors say that the findings, which have not yet been peer reviewed, could be a small step towards solving one of neuroscience’s biggest mysteries: how brains give rise to consciousness, or the subjective experience of being aware.

Scientists studying consciousness were intrigued by the research and praised its rigorous methods. But some pointed out that the social-media buzz might have been overblown. “It’s an interesting little firestorm of a paper,” says John Sakon, a cognitive neuroscientist at the University of California, Los Angeles. “But some of the findings were not totally unexpected.”

Out of sight, out of mind?

Imagine someone looking through binoculars that show a different image to each eye. The person would perceive only one image at a time, rather than a mix of the two. This phenomenon, known as binocular rivalry, is important for the study of consciousness, because it enables researchers to study what happens in the brain when an image enters someone’s consciousness.

How does the brain control consciousness? This deep-brain structure

Exploring this paradigm with monkeys, Tsao and her team presented one image of a face to one eye of a monkey and, after a couple of seconds, introduced an image of a different face to the other eye. When this experiment is done in humans, people who are shown both images report seeing only the second. Researchers assume that the same thing happens with monkeys.

The scientists recorded activity in two brain regions that are known to be involved in facial recognition: the inferotemporal cortex and the prefrontal cortex.

Researchers first showed both faces to the monkey, one at a time, so that they could identify the distinct patterns of neural activity elicited by each image. Then they began the experiment by introducing the first image to one eye. When they introduced the second image to the monkey’s other eye, its brain activity quickly flickered between two neural patterns 15 times per second.