3.3: Sensory Development in Infants and Toddlers

Learning Objectives

By the end of this section, you will be able to:

• Explain how sensation and perception typically develop in infants and toddlers
• Define the relationship between sensation and perception in infants and toddlers
• Describe influences on sensation and perception among infants and toddlers

At first Amy is surprised that their two-week-old daughter Karina turns her head when she hears Amy’s voice. But as Amy thinks about this behavior, they realize that since hearing develops before birth, Karina has been listening to their voice for many months now. Amy mentions this observation to a friend who is an audiologist, who tells them that hearing is one of a newborn’s most highly developed senses.

The development of the senses is a crucial achievement in the first two years of life. Healthy infants are born with the ability to detect visual, auditory, tactile, gustatory (taste), and olfactory (smell) sensations. As they grow and develop, they become more adept at decoding these stimuli, a skill that helps them make sense of the world around them. Sensory development relies on both sensation, the detection of sensory stimuli, and perception, the interpretation of that sensory information. In this section, you’ll explore sensory development in infants and toddlers.

Vision

Although newborns can see, vision is the sense that is least developed at birth. The womb is a dark place, offering limited opportunity to use this sense. At birth, the anatomical visual structures are present but still very immature, resulting in blurry vision and uncoordinated eye movements, and an infant’s vision is best at viewing close-up objects, such as a face or breast when being held. Newborns have low visual acuity, which is their ability to see precise detail (Figure 3.14). Over the first year of life, babies develop the same visual acuity of adults (Johnson, 2011; Kellman & Arterberry, 2006). Their contrast sensitivity, the ability to see differences in shades of light or outlines, is also quite weak at birth but improves quickly (Johnson, 2011; Kellman & Arteberry, 2006). For example, an infant would perceive a black-white stripe contrast but may not distinguish between two shades of gray.

Figure 3.14 Newborns' visual acuity increases as their vision develops over the first months of life. Seeing the shapes and contrasts of a caregiver's face is an important part of this growth. (credit top: modification of work "Mini headshot" by Diana Riser/Flickr, CC BY 4.0; credit bottom; modification of work "Headshot" by Zenobia Rice/Flickr, CC BY 4.0)

One common misconception is that newborns can see only black and white. However, at birth, infants do have some basic color vision, specifically for bright colors like blue, green, and red, with some research indicating infants often prefer blue (Skelton & Franklin, 2020). Color vision develops steadily during the first year of life due to the strengthening of cones, the cells in the retina of the eyes that are sensitive to color and detail. Around four to six months of age, infants can discriminate between colors almost as well as adults do (Clifford et al., 2009).

Young infants also lack the eye motor control and neural development to track moving objects efficiently, which plays a role in their scanning of faces. While you might first look at someone’s eyes to gather social cues and other information, a one-month-old infant will focus on the outside of the face, the hairline and chin, but by three to four months of age, infants are looking at both the mouth and eyes when scanning faces. By nine months, infants look at the eyes more than the mouth, showing development in vision and early social interaction cues (Wilcox et al., 2013). As the visual system develops over the first two to three months of life, infants begin to seek more detail and prefer images of faces over other geometric patterns (Ichikawa et al., 2013).

Depth Perception

Depth perception, the ability to tell whether objects are nearer or farther than other objects, is not present at birth. At about two months of age, as infants acquire binocular vision or the ability of the eyes to work together, they begin to perceive depth. By six months, infants can distinguish depth perception in pictures too (Kavšek et al., 2012).

The visual cliff developed by Eleanor J. Gibson and R.D. Walk (1960) (Figure 3.15) made a major contribution to our understanding of depth perception. It consists of a table with a glass top that covers a checkered pattern. Half the pattern appears to be directly under the glass, while the other half appears to be several feet below it. Infants who were able to crawl were placed on the center of the table to see whether they’d crawl to the shallow side, and then to the deep side that appeared to have a steep drop-off. Few of the six- to fourteen-month-old infants would cross, indicating that depth perception was present by six months of age.

Figure 3.15 An infant is placed on the center of the visual cliff as the mother observes the child’s depth perception and willingness to crawl to her. (credit: modification of work “F9: A mother urging her child from across the deep side of the visual cliff. Despite a transparent surface covering the cliff, the child hesitates to move forward. Source: From Gibson and Walk (1960). Copyright 1960 Nature Publishing Group; reprinted by permission.” by National Library of Medicine/CC BY 4.0)

Infants with more locomotor experience (crawling and exploring) are more sensitive to visual depth cues. For example, those who have just begun walking will avoid the deep side of the cliff even more than infants who are crawling (Adolph & Kretch, 2012; Dahl et al., 2013). Since the original study, psychologists now note how many different internal systems are involved in this task. To make a choice to crawl across or not, infants need to have the binocular vision and eye muscle coordination to sense depth (sensation), the ability to perceive or understand that cliff (perception), and the motor experience and learning to make a choice based on the environment.

Link to Learning

Some developmental psychologists have used the visual cliff experiment to demonstrate that infants develop a fear of heights as they begin to crawl. However, as this video demonstrates, over the last twenty-five years, Karen Adolph from NYU’s Infant Action Lab has conducted a series of experiments showing instead that babies learn from their experiences and adapt their movement as they grow and learn.

Visual Preferences

Infants have clear visual preferences. When given two visual stimuli to look at, they will often look longer at one than at the other. Even as newborns, infants show a preference for curved over straight lines, human faces and face shapes, and three-dimensional objects over two-dimensional ones (Johnson, 2011). They also prefer new images to familiar ones, faces to patterns, and patterns to solids. By three months of age, infants will look more at female than male faces when their primary caregiver is female, but they do not show this preference when their primary caregiver is male (Bayet et al., 2015).

Link to Learning

Infants are born with clear visual preferences for certain types of visual stimuli. Watch this video showing how researchers have been able to identify these visual preferences using creative studies to measure their preferences.

Hearing

Infants develop a sense of hearing around the fifth month of prenatal development. Before birth, they have many opportunities to hear the biological mother’s heartbeat, voice, and other internal sounds, so hearing is already an active sense at birth. Verifying an infant’s hearing is an important first step in identifying hearing difficulties and providing the unique support all babies need to acquire language and communication skills, learn from the environment, and have early social interactions (Bower et al., 2023). Some infants may need surgery to ensure the ear is functioning properly, while others can begin sign language or other interventions such as cochlear implants as early as possible.

Research has shown that newborns prefer speech sounds over nonspeech sounds (May et al., 2018). They also prefer to hear their native language even though they can discriminate most sounds in other human languages (Jusczyk et al., 1994; Kisilevsky, 2016). We also know that infants prefer books the biological mother read aloud when seven months pregnant to a story not previously heard (DeCasper & Spence, 1986). Although early research found that infants prefer their biological mother’s voice over other voices when both were saying the same things (DeCasper & Fifer, 1980), recent research suggests that infants are responding to familiarity rather than to a particular person (Kragness et al., 2022).

How do we know infants have these abilities? After all, they can’t tell us their preferences. In one method, called the high-amplitude sucking technique, infants are given a special pacifier that sends information about their sucking intensity and frequency to a computer. Whenever the infants produce a strong (high-amplitude) suck, they hear a sound. They quickly learn that sucking controls the sounds and will suck more strongly and more frequently to hear the sounds they like the most (DeCasper & Fifer, 1980). During one early study using this technique (DeCasper & Spence, 1986), some mothers read The Cat in the Hat aloud during pregnancy (an average of sixty-seven times for 3.5 hours of exposure overall) and others didn’t. The infants who had heard the story in utero sucked more when hearing it after birth, indicating they preferred it, while those in the control group who had no prior exposure did not have a preference.

Link to Learning

When does learning begin? Before we are born. This TED conference video presents science writer Annie Murphy Paul talking about new research that shows how much we learn in the womb.

Touch

Touch is the most highly developed sense at birth and makes a critical contribution to our physical, cognitive, and socioemotional development. It is part of the somatosensory system and starts developing around week eight of pregnancy, beginning with touch receptors, the neurons that respond to touch. By approximately week thirty, the fetus can feel pain, pressure, and differences in temperature (hot versus cold). This means newborns are sensitive to pain during procedures like blood tests and surgeries (Garcia-Rodriguez et al., 2021).

Touch can also promote positive experiences with the world from birth. Newborns have sensitive skin and will respond to touch with reflexes like the rooting, sucking, and Moro reflexes discussed earlier. Being touched helps infants build relationships with others when they are held and cared for (Carozza & Leong, 2021). In fact, skin-to-skin contact between parents and babies (sometimes called “kangaroo care”) is encouraged right after birth to begin the bonding process, and it has also been shown to have real health benefits for the newborn (Mehrpisheh et al., 2022; Yildirim et al., 2023). Gentle touch also improves mood, sleep, growth, and health in premature infants (Fadlalmola et al., 2023; Wang et al., 2015). Finally, touch provides sensory experience with the world, supporting spatial awareness, body awareness, motor development, and learning (Della Longa et al., 2020; Seidl et al., 2023).

Taste and Smell

Taste and smell help us identify flavors and thus directly influence the foods we choose and thereby our nutrition. Taste is the sensation of flavor—sweet, salty, bitter, sour, or savory—that we receive from receptors in the mouth and on the tongue. Smell is our perception of odors via the nose; it’s a function of the olfactory system (nose and mouth). Both taste and smell begin developing prenatally.

Infants prefer the familiar scents of their biological mothers. For example, six-day-old infants turn more often toward a breast milk pad scented with their biological mother’s milk than one scented by a donor (Loos et al., 2019; Porter et al., 1992). As early as one day after birth, infants are also soothed by the smell of their mother (André et al., 2020; Sullivan et al., 2011).

As a first taste experience, the fetus starts swallowing amniotic fluid by around week twelve and can perceive distinct flavors based on the biological mother’s diet. A newborn will show signs of recognizing those flavors (Mennella, 2014), and many will exhibit a preference for flavors and scents to which they were exposed via amniotic fluid (Lipchock et al., 2011). For example, the newborns of biological mothers who consumed more garlicky food in pregnancy were more accepting of garlic odors, as indicated by mouthing and head orientation (Forestell, 2020, 2024; Hepper, 1988). This finding held true for anise, carrot, mint, vanilla, blue cheese, fruits, and vegetables (Bakalar, 2012; Gabriel & Mennella, 2023).

Newborns enter the world able to distinguish between sweet, sour, bitter, and salty flavors, and within hours of being born, they show different facial expressions for different flavors. For bitter, they might wrinkle their nose or frown; for sweet or savory, they make rhythmic tongue protrusions, elevate the corners of the mouth, or smack their lips (Lipchock et al., 2011). Infants continue to acquire taste preferences through flavors in human milk or formula. The flavor of human milk changes based on the mother’ diet. Infants and young children who are fed human milk as infants are more accepting of new foods that were a part of their mother’s diet (Dunn & Lessen, 2017). Formula-fed infants have also been shown to prefer the flavor of the formula they consume, but because there is no variation in the flavor, they are often less accepting of new foods at first than infants fed human milk (Forestell, 2017).

Intermodal Perception

If you eat an ice cream sundae, you simultaneously see the dripping hot fudge, feel the cold sensation of the first spoonful, and taste the sweetness of the chocolate flavor. An awareness of the world gained through multiple senses at the same time is known as intermodal perception.

The integration of auditory and visual sensory information is one of the first intermodal pairings we make. For example, one study showed that four-month-olds can connect the sound of a ball to the height of its bounce (Lewkowitz, 1999). Infants as young as two months can connect phonetic information from the lips and from the voice. In other words, they can tell whether spoken words match the movement of the lips (Choi et al., 2021; Edgar et al., 2022). Motor skills can help facilitate infants’ ability to integrate intermodal perception. For example, the ability to turn the head and reach for a toy allows the infant to see, touch, and process information about the toy at the same time. Similarly, parental lip smacking during mealtime also serves as an important signal for the infant about the process of eating and can encourage infants to start feeding (Wiggins & Keevallik, 2021). Thus, intermodal connections allow infants to link perceptual experiences and information in their environment, an important task in cognitive development (Streri & de Havia, 2023).

We have seen that newborns can use all their senses to gain information about their world. These sensory and perceptual abilities are still immature but quickly develop over the first year of life to adult levels (Johnson & Hannon, 2015). Understanding the sensory abilities of infants along with their perceptual preferences allows us to interact with them in more effective ways and to use the appropriate stimuli to support their development.

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