The scientists then subtracted the EEG signals taken in response to a face sequence from those of a pattern-only sequence to extract the unique signature associated with the face stimulus and tracked how this electric signal evolved over time. Segregating these signals according to the age of the infant into groups of five, 12 and 15 months old, and expressed in terms of statistical significance, yields the colored plots, overlaid onto an outline of the head.
All the kids showed the expected early response that develops in brain regions located at the back of the head, above the visual cortex. This response is proportional to the visual contrast and other image parameters, reflecting neuronal processing of the actual stimulus, whether or not the stimulus was actually consciously perceived. Subsequently, a sustained depolarization (relative to a reference electrode) develops over the front of the brain, in particular in infants 12 months or older. This component of the signal has a more all-or-none character, reflecting the all-or-none character of conscious experience. The data reveal that one-year-old children, at least, do have a brain signature similar to that associated with conscious perception in adults. The electrical signal is perhaps a third of the speed it is in an adult, reflecting the delayed myelination (myelin is the covering of the axon that speeds up transmission of long-distance electrical communication) and immaturity of the young brain.
Of course, the extent to which they truly do have a subjective experience of a smiling face is difficult to ascertain for now. Clever scientists in the future will likely develop some fancy tech-nique to read out the content of these young minds.
The evidence for an even further delayed slow potential is less compelling in very young infants. This finding raises the general question of when does conscious sensation begin? In the infant's first year of life, at birth, in its last trimester in the womb or even earlier? Research on animal and human fetuses suggests that the baby in the womb is partially sedated, even though it can move around, as mothers can certainly attest to [see “When Does Consciousness Arise?” Consciousness Redux; Scientific American Mind, September/October 2009].
Indeed, it may well be that the fetus feels as much as we do when we are in a deep, dreamless sleep. It may be that the dramatic events attending birth, including drawing its first breath, are the triggers for its first conscious experience of life. This, too, we shall know one day.