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See Inside Scientific American Mind Volume 24, Issue 4

What Do Babies Really Know?

A new study finds a possible brain signature of consciousness in infants as young as five months
baby, baby boy, baby girl, babies



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How do you know that your cute five-month-old infant is truly aware, that she is fully sentient, capable of having a phenomenal conscious experience of her mother's face or voice? Let me hasten to add that the question here is not whether or not normal, healthy babies can selectively identify their mom's face or voice; of course, they can turn their head and fixate with their eyes onto the face and eyes of their mother even very soon after birth. The question I am after is whether such visuomotor or audiomotor behavior goes along with the kind of subjective experiences you or I have when we look at our mother or hear her voice. It is a legitimate question for two reasons.

For one, babies can't speak. They can't tell us whether or not they are seeing faces or hearing voices. It is a different matter once they mature enough to be able to talk to us about their inner experiences. So we have to trust our intuitions, which are deeply colored by our biases about when life starts, when consciousness begins, and who is or is not conscious. The second reason the question is valid is that 150 years of psychology experimentation has shown time and again that adults are perfectly capable of carrying out a range of complex tasks unconsciously.

For instance, subjects can distinguish between a face that looks angry or one that has a neutral expression even if those faces are rendered “invisible” by flashing them only very briefly onto a screen and by adding distracting images just before and just after the picture to effectively mask or erase the picture from the mind's eye. People can also unconsciously detect gender, do simple adding problems when “invisible” numbers are flashed onto the screen, or distinguish between depictions of inappropriate and appropriate actions (for example, discerning between an invisible image of an athlete batting a ball with a baseball bat and an image that has been doctored to show the player swatting at the ball using a flower bouquet). Perhaps babies' behaviors also rely on unconscious, rather than on conscious, processes?

So it becomes critical to find ways to distinguish conscious from unconscious processing in preverbal infants. What is a psychologist to do? One answer is to measure the brain's electrical activity using a common tool we call the electroencephalogram (EEG).

Using such tools, a group in Paris led by cognitive neuroscientist Stanislas Dehaene of the Collège de France has argued for several years that a hallmark of conscious visual perception is a particular type of electric wave, called P300, that occurs whenever an adult subject is attending to a consciously perceived picture or a sound. These signals start roughly around 300 milliseconds after the onset of the image or sound, can be long-lasting, are depolarizing (positive) relative to a reference electrode, and are particularly prominent above the frontal lobe. Most important, they are not present when, for instance, the image is flashed on the screen but is not consciously seen because it is masked. Looking at an image produces a host of faster electrical responses, which are thought to relate to the processing of the image that occurs prior to conscious recognition. Assuming that the P300 slow wave is one of the brain signatures of conscious perception, can they be found in young children?

Recording Brain Waves in Infants

Psychologist Sid Kouider of the Laboratory of Cognitive and Psycholinguistic Sciences at the Ecole Normale Supérieure in Paris, together with Dehaene and other French and Danish researchers, undertook the difficult task of measuring brain waves in 80 infants. Difficult because, unlike undergraduate research subjects, very young children (just like puppies) wiggle around, don't pay attention for long and can't easily be instructed. Their head covered by an EEG cap, the infants sat on the lap of their parents, who were blindfolded so that they would not influence their children's responses. They had to look at streams of images, some that contained photographs of a smiling young woman and some that were only random patterns. What varied across experiments was the duration for which the face was exposed, from barely a glimpse—unlikely to be seen at all—to a sizable fraction of a second that, at least in older children, is invariably associated with the conscious sight of a smiling young woman.

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