Perceptual Development: Taste and Olfaction

Development of Taste Perception

Taste buds, which contain clusters of receptors that will ultimately be sensitive to the basic tastes of sweet, salty, sour, bitter, and umami, begin to emerge with the formation of the mouth and palate just eight weeks after conception. By 13 weeks, the taste buds begin to resemble those of the adult and are found throughout the oral cavity. It is probably not a coincidence that the timing of these developmental changes corresponds to the emergence of fetal sucking and swallowing behavior. By term, infants are actively swallowing between 500 to 1,000 milliliters (ml) per day of amniotic fluid. This not only represents a major route of amniotic fluid absorption, but it also serves to stimulate the taste buds and influence their synaptic connections. Because the chemosensory composition of the amniotic fluid is in constant flux throughout pregnancy, especially once the fetus begins to urinate, the newly developed taste buds are exposed to varying concentrations of a variety of chemosensory stimuli, including sugars, sodium and potassium salts, and various acids. Behavioral studies that have tested preterm infants between 33 and 40 weeks postconception have shown that they produce more frequent and stronger sucking responses to a sucrose-sweetened nipple than to an untreated nipple. Thus by the last trimester, taste buds are capable of detecting sweet tastes and communicating information to structures that are responsible for organizing and controlling affective behaviors within the central nervous system.

More extensive testing of full-term newborn infants has revealed that they not only have heightened preferences for sweet tastes, but as indicated by consumption tests, they are also able to discriminate between various concentrations of dilute sucrose solutions and between different types of sugars. Additionally, as originally described by Jacob Steiner in 1973 and replicated by others, within two hours after birth infants express positive affective reactions, such as facial relaxation, sucking responses, and lip retractions that resemble [Page 763]smiles, in response to a drop of sweet solution placed on their tongue, which are interpreted as hedonic (i.e., liking) responses. Although individual differences in children's sweet taste preferences exist largely due to the interplay between genetics and experience, heightened preferences for sweet taste have been observed in children throughout the world. These findings suggest that infants have an innate preference for sweet tastes. This biological predisposition may have evolved to attract infants to sources of calories such as breast milk, which is predominantly sweet.

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