MOTHERS will want to crucify me for this seemingly cruel question, but it needs to be posed: How do we know that a newly born and healthy infant is conscious? There is no question that the baby is awake. Its eyes are wide open, it wriggles and grimaces, and, most important, it cries. But all that is not the same as being conscious, of experiencing pain, seeing red or smelling Mom’s milk.
It is well recognized that infants have no awareness of their own state, emotions and motivations. Even older children who can speak have very limited insight into their own actions. Anybody who has raised a boy is familiar with the blank look on your teenager’s face when you ask him why he did something particularly rash. A shrug and “I dunno—it seemed like a good idea at the time” is the most you’ll hear.
Although a newborn lacks self-awareness, the baby processes complex visual stimuli and attends to sounds and sights in its world, preferentially looking at faces. The infant’s visual acuity permits it to see only blobs, but the basic thalamo-cortical circuitry necessary to support simple visual and other conscious percepts is in place. And linguistic capacities in babies are shaped by the environment they grow up in. Exposure to maternal speech sounds in the muffled confines of the womb enables the fetus to pick up statistical regularities so that the newborn can distinguish its mother’s voice and even her language from others. A more complex behavior is imitation: if Dad sticks out his tongue and waggles it, the infant mimics his gesture by combining visual information with proprioceptive feedback from its own movements. It is therefore likely that the baby has some basic level of unreflective, present-oriented consciousness.
The Road to Awareness
But when does the magical journey of consciousness begin? Consciousness requires a sophisticated network of highly interconnected components, nerve cells. Its physical substrate, the thalamo-cortical complex that provides consciousness with its highly elaborate content, begins to be in place between the 24th and 28th week of gestation. Roughly two months later synchrony of the electroencephalographic (EEG) rhythm across both cortical hemispheres signals the onset of global neuronal integration. Thus, many of the circuit elements necessary for consciousness are in place by the third trimester. By this time, preterm infants can survive outside the womb under proper medical care. And as it is so much easier to observe and interact with a preterm baby than with a fetus of the same gestational age in the womb, the fetus is often considered to be like a preterm baby, like an unborn newborn. But this notion disregards the unique uterine environment: suspended in a warm and dark cave, connected to the placenta that pumps blood, nutrients and hormones into its growing body and brain, the fetus is asleep.
Invasive experiments in rat and lamb pups and observational studies using ultrasound and electrical recordings in humans show that the third-trimester fetus is almost always in one of two sleep states. Called active and quiet sleep, these states can be distinguished using electroencephalography. Their different EEG signatures go hand in hand with distinct behaviors: breathing, swallowing, licking, and moving the eyes but no large-scale body movements in active sleep; no breathing, no eye movements and tonic muscle activity in quiet sleep. These stages correspond to rapid-eye-movement (REM) and slow-wave sleep common to all mammals. In late gestation the fetus is in one of these two sleep states 95 percent of the time, separated by brief transitions.
What is fascinating is the discovery that the fetus is actively sedated by the low oxygen pressure (equivalent to that at the top of Mount Everest), the warm and cushioned uterine environment and a range of neuroinhibitory and sleep-inducing substances produced by the placenta and the fetus itself: adenosine; two steroidal anesthetics, allopregnanolone and pregnanolone; one potent hormone, prostaglandin D2; and others. The role of the placenta in maintaining sedation is revealed when the umbilical cord is closed off while keeping the fetus adequately supplied with oxygen. The lamb embryo now moves and breathes continuously. From all this evidence, neonatologists conclude that the fetus is asleep while its brain matures.