A premature baby holding a cylinder

Even premature babies at 33 weeks post-conceptional age, about 2 months before term (40 gestational weeks), are capable of recognizing and distinguishing two objects of different shapes (a prism and a cylinder) with their right or left hands. This is the first demonstration of fully efficient manual perception in preterm human infants.

The phenomenon was discovered by researchers at two laboratories: the Laboratoire de psychologie et neurocognition (CNRS / University of Grenoble 2 / University of Chambéry) and the Laboratoire de psychologie de la perception (CNRS / University of Paris Descartes) in cooperation with a team from the Neonatology Department of the Grenoble University Hospitals. The findings have been published on the PLoS One website.

The source of all perceptual knowledge, the sense organs and sensory systems of premature babies are less efficient than those of full-term babies, even though the latter are also not yet fully developed. Starting in the very first minutes after birth, a full-term infant is subjected to extensive tactile stimulation: it is washed, held on its mother’s stomach, nursed, diapered, etc. Its body almost immediately experiences contact with skin other than its own, with towels, sheets, nipples — in short, with objects of different textures, shapes and consistencies. It is common knowledge that a baby will flex its fingers tightly if its palm is touched by a finger, but this grasping reaction is not just a simple reflex. Even in the first hours of its life, a full-term newborn already has effective manual perception, a tactile capacity that enables it to make sense of its environment. But what about the premature infant, whose neurological functions are even less developed due to its early birth?

To find out, the researchers conducted an experiment with 24 premature babies aged 33 to 34+6 gestational weeks (GW), approximately 2 weeks after their birth. Their average gestational age (age at birth) was 31 GW (which corresponds to about 7 months of pregnancy) and their average weight at birth was 1500 g. The research team adopted an experimental method based on habituation (first phase) and reaction to novelty (second phase), similar to that used for full-term newborns. This method relies on a simple universal principle: the gradual loss of interest that all humans experience in relation to a familiar object and the renewed attention elicited by a new, unfamiliar object. In the first phase, the researcher places a small object (a prism for half of the babies and a cylinder for the other half) in one of the baby’s hands (the right hand for half of the group and the left for the other half). As soon as the infant lets go of the object, the experimenter places it back in the same hand and measures how long the baby holds the object each time. The researchers observed that the holding time decreased over the course of the trials, indicating that the baby had become “habituated” to the shape of the object.

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In the second phase, once the babies are habituated to their first objects, the researchers present an object with a new shape to half of the group and a familiar object (the same as in the habituation phase) to the other half. The result: the holding time is longer for the new object (reaction to novelty) than for the familiar object. This proves that the decrease in holding time (observed in the first phase) is not due to the babies’ simply growing tired, because otherwise they would not be more interested in something new.

This experiment reveals for the first time that preterm infants are capable of recognizing an object with their hands (tactile habituation) and that they show a preference for a novel object, reflecting their capacity to differentiate between two objects of different shapes (tactile discrimination). In other words, each time they hold an object, premature babies, like those born at term, are capable of extracting information tactilely on its shape, temporarily storing this information in their memory and comparing it with new tactile input. If the object is the same they soon stop holding it, but if it is different they show greater interest. Therefore, preterm infants, like full-term newborns, are receptive to tactile information and are already learning.

These findings improve our understanding of the perceptual capacities of premature babies and should help neonatology professionals optimize the handling and treatment of their preterm charges, in particular for the purpose of reducing their stress and offering them optimal conditions for their development.

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