A telltale signature of consciousness has been detected that takes us a step closer to disentangling the brain activity underlying conscious and unconscious brain processes.

It turns out that there is a similar pattern of neural activity each time we become conscious of the same picture, but not if we process information from the image unconsciously. These contrasting patterns of activity can now be detected via brain scans, and could one day help determine if patients with brain damage are conscious. They might even be used to probe consciousness in animals.

“It’s very exciting work,” says neuroscientist Raphaël Gaillard of the University of Cambridge, who was not involved in the work. “The use of a reproducibility measure to disentangle conscious and non-conscious processes is genuinely new.” Gaillard has previously shown that coordinated activity across the entire brain is one of the signatures of consciousness .

Consistent signals

So far, efforts to find a brain signature of consciousness have focused on the intensity of neural activity, how long it lasts, and whether signals tend to be synchronised across different regions of the brain.


“We were looking for something other than the intensity and duration of the neural activity that characterises conscious neural processing,” says Aaron Schurger of Princeton University in New Jersey, who led the new work.

He and his colleagues hypothesised that when the brain is presented with the same sensory input – a picture, say – time after time, then conscious awareness of the picture should produce similar neural activity each time.

Conversely, if the sensory input did not enter conscious awareness, it should produce different brain activity each time because there would be other subconscious processes going on at the same time.

Invisible pictures

To test this hypothesis, Schurger and colleagues used functional magnetic resonance imaging (fMRI) to measure the brain activity in 12 volunteers who were asked to look at a series of images – some designed to elicit a conscious response, others a subconscious one.

The researchers invoked conscious processing simply by showing volunteers pictures of faces or houses. To invoke subconscious processing, the researchers presented volunteers with so-called “invisible stimuli”.

These consist of two drawings, of either a house or a face, one shown to each eye. Crucially in each pair, one drawing is in pale orange on a pale green background, the other is the same drawing with the colours reversed. When the brain is confronted with such seemingly contradictory visual inputs it reconciles them by creating a yellow patch. So the volunteer consciously sees nothing but yellow, though the brain has subconsciously processed the face or house.

Probing anaesthesia

A set of fMRI recordings of subjects’ temporal lobes backed up the team’s hypothesis: each time a house or face was consciously processed by an individual, the resulting patterns in brain activity were similar. When the same image was processed subconsciously, the researchers found that the patterns of brain activity were much more variable. “Neural patterns were more reproducible when the drawings were seen consciously compared to when they were not,” says Schurger.

The team thinks that reproducibility – the replication of similar neural patterns in the brain each time it becomes conscious of the same sensory input – gives us clues as to what consciousness is.

It could also be used in the future to tell if someone in a coma is conscious, or probe the consciousness of people under anaesthesia, something that also isn’t well understood.

Journal reference: Science, DOI: 10.1126/science.1180029