The beginning of a world
Imagine for a second what it’s like to go to a Broadway show: the colours, movements, lights, music and songs, the snacks eaten during the interval. A sensory overload! But once the applause has died down and the curtain has fallen, we step outside and the “show” goes one. This time, the “show” is the world itself, one we’re barely aware of, although it is one that endlessly calls upon our ability to perceive. Our five senses are the bridge between this world and our brain – but how does these senses start to develop?
The world and our senses
A human baby is born equipped with a general functional ability to comprehend reality. Its peripheral sensors are in place, while its brain is fully wired and ready to receive, distinguish, combine and interpret data. What about a foetus though? Does it exist, as has long been imagined, in a state of relative inertia, waiting to be freed in order to discover and share the fascinating world of its parents, of planet Earth and all its wonders?
In the 1950s, the physiologist Charles Sherrington was amazed by “the miracle of the human eye developing in darkness for seeing the light and the miracle of the human ear developing in silent water for hearing in vibrant air"(1). Today, we have a better understanding of such ‘miracles’, even if a number of questions remain unanswered. First of all, the intrauterine environment is far from being the insulated haven that Sherrington imagined. More importantly however, we now know far more about the nature of the sensory experience available to a foetus throughout the course of its development. Within nine months, in the most challenging conditions imaginable, an unborn infant tirelessly prepares itself for the amazing journey that awaits it. What’s more, it does so according to a precise and fixed sequence – a programme every one of us has followed to the letter and which underlies our identity.
The origins of our senses
Countless anatomical, physiological and behavioural studies have made us aware of the ontogeny of the human senses, or rather the developmental process leading to the maturity of our sensory systems around the age of two or three.
The sequence by which senses are acquired is easy to remember, essentially ranging from the most basic to the most complex. Firstly, a foetus becomes sensitive to touch, a capacity followed closely by the sense of movement. This is in turn followed by chemical-sensory capabilities, which formulate the senses of taste and smell and then the senses of balance (or the vestibular sense) and hearing. Finally, a foetus begins to develop vision, which requires an additional few weeks after birth before it is fully functional(2).
A foetal heart begins beating from the third week of pregnancy onwards, although the first signs of response to the environment have only been documented at the end of the seventh week. At this point, it is possible to record responses to gentle stimulation of the face, followed by the palms and soles of the feet (11-12 weeks), the torso (15 weeks) and gradually the entire body, as part of a process that is complete by around the 32nd week of gestation, but which is largely functional from the 25th week onwards.
Can a foetus feel pain? This is a contentious issue. While it is possible to observe reactions of withdrawal from injections given to extremely premature infants and even those conducted in utero, the cerebral structures for ‘feeling’ pain appear to be lacking, which may suggest that these reactions are in fact simple reflexes. Specialists more or less accept that something such as pain may form part of the experiential repertoire of a foetus between the 23rd and 30th weeks of pregnancy, but most likely not before the 29th week.
The vestibular organs, located in the inner ear, begin to emerge after around five weeks and the first spontaneous movements of a foetus appear between six and ten weeks. However, it is not until the 25th week of pregnancy that clear signs of spatial orientation and a sense of gravity are observed. At this point, the baby becomes sensitive to its mother’s movements, acceleration and deceleration.
Smell and Taste
Smell and taste have been studied via direct intervention on the composition of amniotic fluid, i.e. by incorporating various fragrances. In fact a foetus has no other choice than to ‘inhale’ and swallow the fluid in which it floats. Taste buds begin to grow in the oral cavity between the 7th and 15th weeks. Initial research has suggested that administering saccharin or cumin prompts a respective increase or decrease in the amount of amniotic fluid swallowed by a foetus, indicating that, by the end of pregnancy at the very latest, there is already a preference for sweet tastes over bitter ones(3). Among extremely premature infants, from the sixth month of gestation onwards, it is very easy to identify reactions of pleasure or rejection towards various smells and flavours, with their motor responses and facial expressions leaving little doubt as to their ability to detect and distinguish different chemical substances.
Hearing develops at around the point when the vestibular system appears to become functional, in approximately the 25th week of pregnancy. The intrauterine environment is far from being soundproof: External sounds may be reduced by tissue and amniotic fluid, but low frequencies can easily penetrate these barriers, which, like bones, are also excellent internal conductors of bodily sounds and of the mother’s voice.
The receptor cells in the cochlea finish developing shortly before the 30th week, but almost all mothers witness sharp reactions by the foetus to intense or sudden sounds from the 25th week onwards. Increasingly precise studies are focusing on different types of sounds (varying in frequency and intensity) using microphones placed on the abdomen, loudspeakers or vibration systems and have documented reactions between 16 and 19 weeks(4). It is believed that hearing is largely functional by around the 28th week, with earlier reactions being associated with tactile and vibratory perception, which depend both on the somesthetic and vestibular systems. It should also be noted that, as for studies of other senses, discoveries depend largely on the method and measures used, the intensity and type of stimulation and, above all, the state of alertness of the foetus at the time of testing, the latter being a very difficult variable to control. Repetitive stimulation creates a phenomenon of habituation, which it is possible to take advantage of experimentally. As a result, full-term foetuses may demonstrate sensitivity to different syllables or melodies, or even to different voices and languages and may react to unexpected changes.
It is not completely dark inside a womb. Certain light intensities can in fact pass through the abdominal wall and can, in theory strike the developing retina of a foetus, particularly after 20 weeks when it begins to open its eyes. Although the visual system is not fully developed in newborns and is naturally difficult to study, it has been possible to demonstrate foetal cardiac and motor reactions towards the end of pregnancy, by placing a high-intensity lamp close to the mother’s belly. Newborns and premature babies are certainly not blind. They already have remarkable visual and attentive capabilities and do not appear to be overwhelmed by the extraordinary wealth of stimulations that they are opened up to after their long experience inside the monotonous intrauterine environment.
Luxury, tranquillity and delight, but also noisiness and anger are privileges that become available to us before we have even started counting our age and their primitive nature never really leaves us. We may sometimes say, for example, that something has ‘touched’ us, that we ‘see’ clearly, that a situation ‘feels’ bad, that we will ‘hear’ from one another soon, that we have ‘fallen’ in love or that we could do something ‘with our eyes closed’(5). From the womb to the auditorium, our senses play us a symphony that never ceases to amaze. Our social and verbal worlds are founded largely on our sensory impressions, which are among the very first and most precious that we acquire.