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Into the minds of babes Studies of psychoactive stimulants and consciousness can shine a light on how we viewed the world as an infant, finds Anil Ananthaswamy
HAT is it like to be a bat? Philosophers of consciousness love toying with that question. We’re fascinated by the possibility of minds so unlike our own. But there’s a deep mystery far closer to home. Never mind bats – we barely even know what it’s like to be a baby. We’ve all been there, but none of us remember. As we develop into fully self-aware beings, our subjective experience of the world shifts dramatically. Once we leave infanthood behind, that early window on the world – and what it’s like to look through it – is closed to us. But research is prising open the shutters. As we learn more about how drugs can alter our consciousness, we’re learning more about how our brain states relate to subjective experiences. And that’s giving tantalising glimpses into our infancy. For those who want to get inside a baby’s head, Alison Gopnik, a psychologist at the University of California, Berkeley, has a few suggestions: go to Paris, fall in love, smoke four packs of Gauloises cigarettes and down four double espressos. “Which is a fantastic state to be in, but it does mean you wake up at 3 o’clock in the morning crying,” she told a room of philosophers and neuroscientists at the Toward a Science of Consciousness meeting in Tucson, Arizona, in April 2014. And if that wasn’t enough, Gopnik adds another ingredient to the list: psychedelic drugs. Because a baby’s world might be vivid beyond adult imagination. To get a handle on the infant state of mind, we first need to know what goes on in the
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74 | NewScientist:TheCollection|TheHumanBrain
brains of adults – then see how it differs in babies. Fortunately, consciousness seems to have a telltale signature. A team led by Stanislas Dehaene of the French National Institute of Health and Medical Research in Gif-sur-Yvette has found that adult conscious perception of stimuli involves a two-stage process. The first stage involves unconscious processing of, say, an image. If we look long or hard enough, then after about 300 milliseconds, the second stage kicks in, and a network of brain regions starts reverberating. The activity correlates with conscious perception: people are able to
“A baby’s world might be vivid beyond adult imagination” report on what they have seen. It is only when this network of frontal and parietal brain regions, dubbed the global neuronal workspace, becomes active that we have conscious access to information about what we have perceived. Dehaene and his colleagues recently teamed up with Sid Kouider of the Ecole Normale Supérieure in Paris, France, to look for a similar signature in babies who were between 5 and 15 months old. In the first study of its kind, the team spotted clear signs of conscious perception. But there was one important difference. In babies from 12 to 15 months old, the second stage of reverberating neural activity began about 750 milliseconds after the onset of stimulus, rather than after
300 milliseconds. And in 5-month-olds, the lag was even greater. Their brains responded after 900 milliseconds. “Babies have the same mechanism, but are just slower,” says Kouider. So, babies are aware of their environment, but, compared with adults, there’s a lag. The slower reaction could be down to the prefrontal cortex, a hub for brain activity that the studies looked at. “It allows the sharing and transmission of information throughout different regions of the brain,” says Kouider. And it is one of the last brain regions to mature, becoming fully developed only in late adolescence. Another slowing factor might be down to the connections between distant brain regions. In infants, the long-distance axons that carry signals in the brain don’t yet have a fully formed coating of insulation called a myelin sheath. This means signals travel more slowly along the axons than they do in adults. But there’s more to the story. Kouider and Dehaene are investigating something called access consciousness – being aware enough of a stimulus to reflect on it and talk about it. Access consciousness is widely studied because researchers typically depend on subjects being able to monitor and report their experience. But some think access consciousness is just one extreme of a spectrum. Is there middle ground between being fully aware and fully unaware? Gopnik thinks so. And that is where babies find themselves, she says. Philosopher Ned Block of New York University has a term for this middle ground. He calls it phenomenal consciousness – what >