Sensations are units of information received from the environment, such as a visual feature or a sound. Perceptions are organized and interpreted sensations, such as recognizing a face or interpreting a sequence of sounds as a familiar song. These concepts represent more of a historical distinction than a functional distinction; nevertheless, the distinction between sensation and perception continues to be made by researchers and textbook writers. Perceptual development is the emerging capacity to detect information from the environment and from internal sources to adapt to and function within the world. Knowledge about the perceptual development of infants has expanded more rapidly than that of older children. This discrepancy has occurred because the earliest appearance of various perceptual capacities has been emphasized. In addition, knowledge about visual development has expanded more rapidly than knowledge about the development of other perceptual systems. This discrepancy has occurred, in part, because of the belief that the visual system provides the best source of external information.
In educational psychology, a quick review of several introductory textbooks reveals that little attention is given to sensation and perception. Most textbooks mention information processing theories of knowledge acquisition and include a three component model of human memory consisting of sensory registers, short-term working memory, and long-term memory. Within this model, the sensory registers receive environmental input. These registers have a large capacity to receive information, but the information quickly fades away unless it is transferred to short-term working memory. The other section of educational psychology textbooks in which sensations and perceptions are mentioned concerns students with special needs, in particular, those with a sensory challenge such as a visual or auditory impairment. Despite this limited coverage, much knowledge has been obtained on perceptual development within the last half-century and a familiarity with this knowledge could facilitate the teaching of students.
This entry is organized in the following way. The development of the five sensory systems that focus on external information and on two sensory systems that provide "internal" information, is presented first. Next is a section on the coordination of information from multiple sensory sources and on the coordination of the perceptual and motor systems. Finally, there is a section on brain development and on the role of experience on perceptual development.
Development of Individual Senses
In 1890, William James described the newborn infant's visual world as a blooming buzzing confusion. This view represents a strong empiricist perspective that perception develops through learning. It is now known that the newborn's view of the world is not this confusing and also that many important perceptual developments occur within the first year of life. Some principles of looking in infancy include opening the eyes when the light is not too bright, making broad eye movements until an edge (an area of high contrast) is found, and then continuing to look in the general vicinity of the edge (while making eye movements across the edge). These principles maximize the firing rate of neurons in the eye and brain, which may facilitate further visual developments. Several functions of vision are presented in the following paragraphs, including kinds of eye movements, pattern detection, image features, and depth perception.
Two eye movement types used to localize objects in the environment include saccades and smooth pursuit. Saccadic eye movements shift the eyes from one position to another. Early in infancy, multiple saccades may be required to get to a distant target. Among adults, a saccade will typically bring the eye at least 90% of the distance to the target. Smooth pursuit allows one to maintain fixation on a moving object. Infants cannot use smooth pursuit to track moving objects in the first month of life but, by 3 months of age, smooth pursuit eye movements are common.
Pattern detection refers to the ability to perceive whether an image has contours or edges. Visual acuity is one of the most basic measures of pattern detection, and it is used to determine the maximum resolving capacity of the eyes. An eye chart is used for letter recognition, and a ratio is reported indicating the distance at which a line of letters can be correctly seen to the farthest distance at which a person with "normal vision" can read the same line. Hence, a ratio of 20/20 indicates that the viewer correctly recognizes letters at 20 feet that one with "normal vision" recognizes at 20 feet. Likewise, a ratio of 20/30 indicates that the viewer recognizes at 20 feet what the average person recognizes at 30 feet. Hence, the person's acuity is poorer than average. Infants cannot recognize letters, but other techniques have been developed to measure their acuity. In the newborn, acuity is 20/600 or worse, but it rapidly improves to about 20/200 by 4 months, 20/100 at 6 months, and 20/50 at one year. One may be "legally blind" with visual acuity of 20/200 or less; however, even very young infants can see nearby people and objects.
Image features that attract an infant's attention have also been examined. In one technique, different pictures are shown to an infant, and the infant is observed to determine which picture he or she looks at for a longer time. Infants look longer at high-contrast images such as stripes and bull's-eyes than at solid-colored objects. They also look at faces for a long time. Facial patterns may receive special attention from infants because of the survival value of connecting with other people.
Depth perception is the process of determining the distance of objects, and it is a crucial ability for determining the spatial layout of the environment. Three sources of information are potentially available for determining depth: binocular information, static monocular information, and dynamic (or kinetic) information. Infants seem to be sensitive to at least some aspects of each of these sources of information in the first year of life.
Two binocular cues to depth are vergence and disparity. Vergence eye movements involve moving the eyes toward or away from each other in the horizontal plane, and this ability enables the eyes to focus on objects of different distances. Binocular disparity is the slightly different image that the two eyes receive because they are set apart by several centimeters. This information can be used to detect the difference in depth between two objects. Humans are very sensitive to this depth information.
Static monocular cues, or pictorial cues, are often used by artists to create the impression of depth on a flat surface. These cues include interposition, in which one object partly conceals another; aerial perspective, in which far away objects are less clear than close objects; linear perspective, in which the size and space of more distant objects decreases; and relative size, which occurs when two identical images of different sizes are viewed and the larger image appears to be closer to the viewer. Sensitivity to some static monocular depth cues has been found by 7 months of age.
Dynamic or kinetic information may also be obtained with only one eye, but this information cannot be represented in a two-dimensional image. Kinetic information is produced by changes in the retinal image over time. These changes can be created by motion of the object in the environment, for example, the expansion of the retinal image in an approaching object. These changes can also be created by movement of the observer. For example, even very slight lateral head movements may create motion parallax in which objects in the visual field that are closer than the fixation point move rapidly in the opposite direction of the observer's head movement whereas objects beyond the fixation point move slowly in the same direction as the head movement.
Despite the rapid development of the visual system during infancy as indicated in several capabilities, visual perception continues to develop and become more efficient throughout childhood. For example, visual skills become better focused, better organized, and more confined to meaningful environmental features as children age.
Like the visual system, the auditory system is also functional at birth and is fast developing. Fetuses hear before birth: They react to sounds by moving around, and their heart rate increases. Newborns recognize stories that their mothers repeatedly read out loud before they were born, and they prefer to listen to their mothers' voices over those of stranger. Newborns also prefer infant-directed speech over adult-directed speech. Infant-directed speech (motherese, parentese) is high in pitch, is slow, and has exaggerated rises and falls. This speech may help infants with language acquisition by directing their attention to particular words.
Infants are sensitive to sounds that seem to help them learn about people. They prefer sounds in the same frequency ranges in which speech occurs and that cover a range of frequencies (like speech) over pure tones; they prefer the language to which they are exposed over other languages; and by 4 months they attend to their own name.
Infants are also sensitive to many contrasting phonemes (speech sounds), such as the distinction between pa and ba, including phonemes that are not meaningful in their native language. During the first year, they lose sensitivity to many of the contrasts that are not used in the language to which they are exposed. Hence, their ability to discriminate sounds not meaningful in their native language decreases as their sensitivity to sound patterns within their native language continues to improve.
Auditory perception continues to develop during childhood. For example, the ability to detect low-frequency tones develops over several years.
Smell, Taste, and Touch
Researchers have given less attention to the senses of smell, taste, and touch than they have to vision and hearing. These senses are functional early on, and they may be adaptive to the survival of the infant. For example, within the first week of life babies recognize and turn to the smell of their own mother's breast pad over that of another woman. Infants also prefer familiar odors over ones to which they have not previously been exposed. Newborns also discriminate among different tastes: They will suck on a sweet solution longer than on sour, salty, or bitter solutions. The sense of touch contains many types of receptors, including ones sensitive to pressure, pain, and temperature. Even the fetus responds to touch, and the newborn is sensitive to pain, for example, by crying following a pin prick. Other aspects of haptic perception such as the detection of shape, texture, hardness, volume, and weight have been found to be functional within the first one to two years of life.
Internal Senses: Orienting and Proprioception
There are other sensory systems besides the five classic senses used for detecting external environmental information. The orienting system is the sensory system that allows a person to detect the position and motion of the body in space. The vestibular organs, consisting of the saccule, the utricle, and the semicircular canals, are primarily responsible for this sense. The orienting system is used to register linear and rotary acceleration. It is also used to sense gravity. Certain types of eye movements show that the vestibular system is functioning. For example, when an individual is rotated, the resulting stimulation of the semicircular canals creates a pattern of eye movements such that the eyes move slowly in the direction opposite of the rotation and then rapidly back. These eye movements are called vestibular nystagmus, and they are easily observable in the newborn infant. With respect to gravity, one interpretation of the frequently observed behavior of infants systematically dropping objects and watching them fall is that it is helping them learn about gravity. Also, the sense of gravity is so strong among 2-year-olds that they search directly under the location where a ball was dropped even in the presence of an opaque tube carrying the ball to a different location. In other words, 2-year-olds do not attend to the local information of the tube; instead they use general information about gravity to attempt to solve these problems.
Proprioception provides information about the position and movement of parts of the body, that is, the status of the muscles, tendons, and joints. Several examples provide evidence for the early emergence of this sensory system. Through ultrasound (high-frequency sound wave reflections used to see an outline of the fetus), fetuses have been observed opening their mouths in anticipation of the arrival of the arm that allows them to suck on a hand or thumb. Hence, they have some rudimentary knowledge of the location of the hand with respect to the mouth. Imitation of mouth movements among newborns, such as opening the mouth or extending the tongue, seem to indicate a mapping between what is observed in others and how such actions feel to the infant. Recall that newborns cannot see their own mouths, so they cannot visually match what they observe in others with what they observe in themselves. At around 2 months of age, infants are frequently observed watching their hands move (also known as visual hand regard or visual capture), which may help them to develop knowledge of where the limbs are located in space. Finally, infants have looked at monitors that show their moving legs in real time from their own perspective and from another perspective. They discriminate between the two perspectives by looking longer at the unfamiliar perspective. This discrimination requires matching how they feel their legs to be moving with the visual consequences of these movements.
The various sensory systems often do not operate alone. Many objects in the environment stimulate multiple senses, for example, one may see and hear a person, see and touch a toy, smell and taste food, and so forth. Two perspectives on the coordination of information from the sensory systems address whether differentiation or integration occurs during development.
One perspective is that perceptual information is initially undifferentiated, such that a newborn is unable to distinguish which sense is being stimulated. Some speculate that this experience may be similar to the perceptual disorder synesthesia, in which two or more body senses are coupled. In infants, early demonstrations of intersensory coordination support this perspective. For example, 4-month-old infants were simultaneously shown two animations with one soundtrack, which originated from a speaker located in between the two animations. The soundtrack matched the events in one of the animations but not in the other. Infants looked longer at the animation that corresponded to the soundtrack than at the other animation. In other examples, infants were allowed to explore an object with their hands or their mouth without looking at the object. After the tactile experience, they were shown two objects. One was the object they had just been familiarized with, and the other was a novel object. Their looking times indicate that they discriminated between the two objects. Hence, they can match how the object felt with what it looked like. With experience and development, the child becomes better able to differentiate among the senses and to recognize which sense is being stimulated.
The second perspective is that the senses are initially separate, develop relatively independently, and become connected as they mature. The famous cognitive developmental theorist Jean Piaget argued from this perspective. Specifically, that much of the knowledge and skill underlying intermodal perception can only be gained through experiences that involve looking at, hearing, smelling, tasting, and touching the external world.
A hybrid of these two perspectives is that the senses are initially undifferentiated, then they become increasingly differentiated, and finally they become coordinated. Regardless of the developmental pathway, the ability to integrate information across sensory systems is important in many ways. It enables one to recognize objects across different modalities and to associate sights and sounds. For example, infants learn to associate a particular voice with a particular face by 3 months of age. The ability to localize sound is important both for guiding visual attention and for locating sound-producing objects. Newborns turn their head in the direction of a sound, though this ability briefly declines before reemerging at approximately 4 months. One explanation offered for this pattern is that localization is controlled in a subcortical way at birth, that the cortex takes over as it matures, that it is not developed enough to be very accurate at first, but that it rapidly gets better at localizing sound.
Perceptual-motor coordination is the linking of perception and self-initiated movement. James Gibson argued that these two processes are inextricably coupled. For example, in evolutionary terms, people must perceive objects and events in the environment to survive, and survival requires that actions be guided by perceptions. Hence, the purpose of perception may be to guide action, and the purpose of action is to generate additional perceptual information.
One example of perceptual-motor coordination concerns the acquisition of visually guided behaviors, such as reaching. The development of reaching is facilitated by the experience of observing the limbs during active movement. This experience helps one to localize the limb in space (proprioception) and link movement with the visual consequences. It is not clear that the limbs must then remain visible during the targeted action. For example, normally developing infants who are experienced with seeing their limbs move in space can reach in the dark for a glowing object without being able to see their hands.
A second demonstration of perceptual-motor coordination concerns knowledge of the location and distance of sound-producing objects. When placed in the dark, infants reach in the correct direction for a sound, and they are more likely to reach for a sound located within reach than for one located beyond their reach.
Calibration and Recalibration
The linking of various sensory systems and of the perceptual-motor system is not a one-time event. A continuous calibration of the systems maintains accuracy and accommodates changing circumstances, such as the growth of the individual or the transition to more powerful vision correction glasses. Many have argued that vision is the most powerful sense for the process of calibration and that it dominates the other sensory systems. One everyday example of calibration comes from experiencing a moving walkway at the airport. Because the floor is moving as one walks, the effort put into self-movement indicates that one is going faster than that same effort would normally suggest. Hence, if one is then immediately placed in a situation without visual feedback, one may underestimate the distance or time it would take to walk from one location to another. A second demonstration of recalibration occurs when distorted stimuli are presented and people adapt other senses and motor movements to the distortion. For example, distorted prisms placed on the visual system leads to adjustments in pointing and reaching behavior as well as to adjustments in sound localization. These distortions persist for a brief period even after the prisms are removed.
Brain Development and Experience
Role of Experience
One theme that has been mentioned in this entry but not explicitly discussed concerns the roles of experience on perceptual development. The two extreme positions are maturation and induction. With maturation, experiences play no role in the development of the trait. For example, young infants can differentiate among different odors regardless (apparently) of their previous experiences. With induction, a trait only develops as the result of specific experiences. For example, in a congenitally and completely sightless individual, the visual cortex may not develop because it never receives any visual input. Other concepts, such as maintenance, facilitation, and attunement, take more of a middle position. Maintenance represents the idea of "use it or lose it." The trait develops without any specific experiences but is only retained with experience. For example, infants are sensitive to phonemes in unfamiliar languages, but as they get older they can only detect phonemic differences in the language(s) they hear. Facilitation means that, although specific experiences may hasten the rate of development, others who did not receive those experiences eventually catch up. For example, an enriched visual environment may speed up the development of some visual abilities, but those in a "normal" visual environment would also develop those abilities. The term attunement is used to describe an increase in the level of development achieved on the basis of specific experiences. For example, some perceptual-motor coordination may be achieved without ever viewing the limbs, but experience seeing the limbs may allow for a higher level of perceptual-motor coordination.
Developmental changes in the brain may contribute to perceptual development in several ways. Two of these ways include developmental changes in the connections among neurons, and changes in the rate of maturation of different brain structures.
"Synaptogenesis" is the formation of synapses or gaps between nerve cells (neurons). Within a neuron, the transmission of information is electrical, as the neuron fires in response to certain stimulation. Between neurons, neurotransmitters (chemicals) flow across a synapse allowing neurons to communicate with each other. In many parts of the brain, there is a distinct course of development consisting of the overproduction and pruning of synapses. The proliferation of synapses early in development means that the toddler brain has far more synaptic connections than the adult brain. During childhood, the number of synapses decreases to adult levels. Experience plays an important role in determining which synapses are maintained and which are pruned. If experiences lead to neurons firing and neurotransmitters being released, then the synapses are maintained. If experiences do not lead to neurons firing and neurotransmitters being released, then the synapses wither.
The process of synaptogenesis is consistent with the hybrid model of the development of inter-sensory coordination that was presented earlier. During the synaptic proliferation phase, the senses may be undifferentiated, allowing inter-sensory coordination abilities to be detectable very early in development. Experiences within particular sensory systems then lead to an increased differentiation of the senses and also to a meaningful integration of the senses.
The relative size and level of activity in different areas of the brain changes during development. The cerebral cortex is immature relative to other parts of the brain throughout much of childhood. The cerebral cortex is involved in the processing of perceptual information, as well as many other functions. Thus, despite the emphasis in the literature on the earliest appearance of perceptual capacities, which is the reason this entry has focused on infancy, it is clear that perceptual development is very likely to continue throughout childhood.
A recent theory of visual processing in the brain has a focus on the cerebral cortex. This theory of normal visual processing was largely developed from research on people who had a variety of visual disorders due to brain damage. The theory itself does not directly address development, though many researchers are applying it to developmental issues. David Milner and Melvyn Goodale introduced this theory of the visual brain. Structures in the cerebral cortex relevant to the theory include primary visual cortex, posteriorparietal cortex, and infero-temporal cortex. Briefly, two streams of visual information leave the primary visual cortex: A dorsal stream that goes to the posterior parietal cortex, and a ventral stream that goes to the infero-temporal cortex. These visual streams serve different purposes.
The ventral stream is involved in the formation of perceptual and cognitive representations of objects and of their significance. Take, for example, identifying an object. The object's identity is independent of any particular viewpoint of the object. Hence, size and shape constancy enable enduring characteristics of the object to be maintained across different viewing perspectives. The identity of objects and their spatial arrangements should also be stored in long-term memory to maximize the efficiency of identification.
The dorsal stream is involved in the control of goal-directed actions. Take, for example, reaching out and grasping an object. The location of the object must be specified in an egocentric manner (that is, with respect to the actor) so that the actor can move the hand to the location of the object. The object's size and shape also need to be specified in terms relative to the actor so that the hand and fingers can be adjusted appropriately for grasping. Because actors and the objects they interact with almost continuously change relative location, only a very short memory is required.
Hence, the requirements for a system involved with forming representations of objects are different from the requirements for a system involved with the visual control of action. The first stream involves the world independent of the observer, whereas the second stream involves the actor's actions within the visual world.
The dorsal stream is consistent with Gibson's theory that perception and action are closely linked. The ventral stream provides object recognition capacities that are important for higher order cognitive tasks, and it is consistent with Piaget's theory of the construction of knowledge.
Perceptual Capabilities Beyond Infancy
Perception is used to attend to stimuli and events worthy of detailed processing; to identify what is being perceived; and to locate objects and events for guiding action. The emphasis in this entry was on the early appearance of perceptual capabilities, often in infancy. In contrast, some of the areas mentioned in perceptual abilities among adults, such as the cerebral cortex, are immature in infancy and slowly mature throughout childhood and adolescence. Hence, there is danger in all-or-none thinking about perceptual capabilities. In other words, just because there is some evidence that infants can use certain features or information does not mean that the processes underlying that ability are fully formed. Perceptual development continues well beyond infancy, though there currently is not much emphasis on specifying how that occurs.
Michael E. McCarty
Autism Spectrum Disorders; Cognitive Development and School Readiness; Disabilities; Dyslexia; Early Intervention Programs; Motor Development; Observational Learning; Special Education; Working Memory
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Source Citation: McCarty, Michael. "Perceptual Development." Encyclopedia of Educational Psychology. Eds. Kristin Rasmussen and Neil Salkind. Vol. 2. Thousand Oaks, CA: Sage Publications Inc., 2008. 774-780. 2 vols. Gale Virtual Reference Library. Gale.