Emerging consciousness glimpsed in babies
A glimpse of consciousness emerging in the brains of babies has been recorded for the first time. Insights gleaned from the work may aid the monitoring of babies under anaesthesia, and give a better understanding of awareness in people in vegetative states – and possibly even in animals.
The human brain develops dramatically in a baby's first year, transforming the baby from being unaware to being fully engaged with its surroundings. To capture this change, Sid Kouider at the Ecole Normale Supérieure in Paris, France, and colleagues used electroencephalography (EEG) to record electrical activity in the brains of 80 infants while they were briefly shown pictures of faces.
In adults, awareness of a stimulus is known to be linked to a two-stage pattern of brain activity. Immediately after a visual stimulus is presented, areas of the visual cortex fire. About 300 milliseconds later other areas light up, including the prefrontal cortex, which deals with higher-level cognition. Conscious awareness kicks in only after the second stage of neural activity reaches a specific threshold. "It's an all-or-nothing response," says Kouider.
Adults can verbally describe being aware of a stimulus, but a baby is a closed book. "We have learned a lot about consciousness in people who can talk about it, but very little in those who cannot," says Tristan Bekinschtein at the University of Cambridge, who was not involved in the work.
Signature of awareness
Kouider and his colleagues wanted to see if a similar signature of conscious awareness was present in pre-verbal infants. They put EEG caps on groups of babies aged 5, 12 and 15 months, recording brain activity as the babies were shown a series of rapidly changing images. Most of the images were randomly patterned ovals, but among them was a face, fleetingly displayed for between 17 and 300 milliseconds.
Each group responded to the face with the expected two-stage pattern. But the second stage – the activity linked to conscious awareness – was a much slower and less distinct response in the 5-month-old babies than in the older groups.
In 12-month-old babies the second stage of activity arrived 800 to 900 milliseconds after the image was displayed. The 15-month-old group showed a very similar response. In the youngest infants, there was a delay of more than one second before the second pattern appeared. In adults, the second pattern shows up after 300 milliseconds, on average.
"Babies have the same mechanisms as adults but they are very slow," says Kouider. "There are things happening in the brain but they are unable to deal with the information."
Kouider is careful to note that the results do not offer direct evidence of subjective experience. Although the babies' changing brain activity highlights the development of visual perception, it is not yet clear when the second-stage timings become short enough for awareness to kick in. "I don't know what proof would look like," says Natasha Sigala at the Brighton and Sussex Medical School, UK. "This is as good as it gets for the moment."
The observed brain activity is consistent with what we know from anatomical studies. In young infants, neurons are not yet fully covered with myelin – a fatty insulating sheath. This physiological immaturity explains the delayed signals seen in 5-month-old babies, says Kouider. Developing brains also have more connections than mature ones ( these connections get pruned as different regions become dedicated to certain activities), and the early glut of connections would also disrupt brain signals, he says.
"The results give a really good handle on visual awareness in infancy," says Sigala. It may also help explain why we are unable to form memories at a very young age, she says. "My personal view would be we cannot have memory without perception in place."
Ron Chrisley at the Sackler Centre for Consciousness Science at the University of Sussex, UK, suggests that if similar patterns of brain activity were found in animals, then it would be a good basis for attributing consciousness to non-humans – though lacking that pattern should not count against them. "There might be more than one way in this universe to be conscious," he says.
Journal reference: Science, DOI: 10.1126/science.1232509
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