Sensory and Motor development
Sensory and Motor development
Sensory and motor functions are basic to all behavior. In its simplest form, the stimulus-response unit of behavior is composed of a receptor (sense organ), neural impulses traveling over afferent, central, and efferent pathways, and some resultant form of motor (muscular) response. The sense organs respond selectively to various stimuli (visual, auditory, olfactory, thermal, tactual, proprioceptive, chemical, and gravitational), and the resulting responses most often involve some form of muscular reaction appropriate to the nature of the stimuli received. Developmentally, the earliest forms of behavior are simple sensorimotor reflexes. With growth, maturation, and differentiation, the senses become more acute, and the muscles become stronger and function more smoothly. At the same time, the central nervous system matures, with its increasingly meaningful accompanying sensory perceptions, and the motor responses become better organized, while many of the reflexes give way to behaviors under voluntary control.
During the period of earliest development, when the changes are most rapid, there is a close coordination between structure and function. Gradually the functions become relatively independent of the structures. As the rate of growth slows, the structures differentiate, and the functional processes become increasingly complex; that is, once the basic structures are formed, there is little or no correlation between their normal variations in structural complexity or maturity and the increasing complexity and diversification of motor coordinations, perceptions, and other mental processes.
Neural and cortical development. Conel’s studies of the postnatal development of the human cerebral cortex (1939–1963) have been well summarized by Eichorn (1963) and by Eichorn and Jones (1958), who also point out that changes in the histologic structure coincide with developmental changes in neural function as expressed in the electroencephalogram. At birth and even at one month the cortex is very immature, with fragile cell processes, no Nissl bodies, and very few neurofibrils. The greatest cortical development occurs in the primary motor area of the upper trunk, leg, hand, and head, followed in order by primary somesthetic, visual, rhinencephalon (olfactory), and auditory areas, with other parts still very immature. By three months there are marked advances in the number of nerve fibers, both exogenous and associational, with greatest development in the motor area of the hand, frontal eye fields, and striate cortex (Gruner 1962). There is also over this period a rapid advance in myelinization of the neural fibers. This myelinization serves to channel the neural impulses along fibers and to reduce random spread of impulses across neurons. Again, at six months there is marked development, particularly in motor areas controlling the hand and upper trunk, leg, and head, while visual and somesthetic sensory growth is accelerated. Between 6 and 15 months the motor areas of the brain show less marked growth, with the order of maturity being hand, upper trunk, head, and leg. The primary visual area by now is second to the motor, with the visual association areas more developed than the somesthetic association areas.
Parallel with these histological changes, Eichorn and Jones (1958) point out that at birth the electrical activity of the cortex is very slow and irregular, with the greatest regularity in the region of the most mature cortical structure. It is possible, however, to induce some rhythmic EEG patterns in the neonate and in the month-old infant, while between one and three months there is a shift of the EEC from random activity to some patterned slow activity in the visual and auditory sensory areas of the brain.
Fetal development. We find, too, a definite parallel between early behavior and the neural histology and electrical functions in these early months. The very first actions of the fetus, according to Hooker (1943), are muscular: the rhythmic beating of the heart in the third week of gestation. This, however, is a preneural action of the heart muscle. A response (presumably neural) to stimulation was first observed at eight weeks and consisted of a lateral bending of the neck which moved the head away from a hair touching the area of the cheek. Carmichael (1946) has given an excellent account of this early fetal development. He points out the gradual involvement of the entire body and the appearance of reflexes until, by 26 weeks of gestation, the reflexes necessary to life are usually present. These reflexes include functioning of the respiratory, circulatory, and digestive systems as well as the sense organs that respond to light, sound, touch, body position, and so on.
Because the infant’s repertoire of responses is so limited, it is difficult to obtain exact information about sensory acuities. However, it is possible to observe and record such behaviors as visual regard, pupillary reflexes to light, startle, and changes in activity level to sounds and tactile stimulation. More recently, sensory reactivity has been recorded by observing changes in EEC and in heart rate and by such devices as observing eye nystagmus to moving striped patterns (Eichorn 1963; Fantz & Ordy 1959).
It is evident that the intact full-term newborn in some degree sees, hears, and responds to pressure, touch, taste, and change in temperature. There is evidence from his behavior and from the structures of the nervous system that of his various senses, vision is most developed.
Vision. Changes in visual acuity during the first month appear to be very slight. As observed in a standard test of infant development, soon after birth the infant will briefly regard a large moving object (such as a person) nearby and directly in his line of vision. A little less often he will regard a small bright red object in motion, when it is held about eight inches above his eyes (Bayley 1933; White, Castle, & Held 1964). At about two weeks his gaze may follow this moving object (a red plastic ring) across his visual field—right to left or the reverse (Bayley 1933). At three weeks his eyes may follow a moving person two or three feet away. At about one month he follows the red ring with up and down eye movements and, a little later, as it is moved slowly in a circle (18 to 24 inches in diameter). At six or seven weeks the infant appears to inspect his surroundings when carried in an upright position, and he turns his eyes toward the red ring at a thirty-degree angle when it is moved into his field of vision from the side. By the fourth month the infant’s retina is able to accommodate to objects at varying distances in an almost adult fashion (Haynes, White, & Held 1965).
In experimental situations several investigators have found very early evidences of differentiation of visual stimuli. Several studies have shown (Berlyne 1958; Fantz 1958) that infants three to four months of age indicate preference for (that is, spend more time looking at) patterned stimuli as contrasted with plain ones. Fantz and Ordy (1959) have shown that infants under five days of age will look more at black and white patterns than at plain-colored surfaces. Doris and Cooper (1964, p. 456) have reported a clear correlation between age and brightness discrimination among 16 infants 4 to 69 days of age. They tested this by observing nystagmic eye movements to a moving field of black and white stripes. [SeePerception, article On Perceptual Development; Vision, article On Eye Movements.]
These findings are in general agreement with the responses to visual stimulation observed in the infant mental scales. Continuing with the Bayley Scale (Bayley 1933), at around two months the baby blinks at the shadow of a hand passed quickly across his eyes, he visually recognizes his mother, and his eyes follow a moving pencil; at two and one-half months he searches with his eyes for a sound, and he regards a one-inch red cube on a table when he is held upright; at three and one-half months his eyes follow small objects, such as the red ring, a teaspoon, and a ball, as they move across the table before which he is held in a sitting position. A typical four-month-old’s occupation is to inspect his own hands; at four and one-half months he regards a pellet one-quarter inch in diameter; at five months he discriminates between strangers and familiar persons (largely visually, it would appear, from the expressive nature of his gaze). This evidence of visual discrimination of patterned objects shows advancement when at twelve months he looks with interest at colored pictures in a book. Many of his behaviors in the second year give evidence of his utilization of visual discriminations as he imitates motions, builds towers of cubes, adjusts round, square, and triangular blocks into their appropriate form-board holes, and goes on to more complex operations.
Increasing visual acuity in the first few months of life for premature and full-term infants has been assessed by Brown (1961).
Another source of information on visual development comes from the studies of ophthalmologists. Keeney (1951) has tabulated functional development of vision and binocularity for a series of ages from the third fetal month to nine years. Many of his items are identical with, or closely similar to, those already noted. We may add sensitivity to light at the seventh fetal month and a series of visual acuity fractions starting at one year, when visual acuity is about 6/60 with imperfect fusion. At two years it is at least 6/12. At two and one-half years more mature mechanisms of accommodation result in improved acuity. At three years vision is about 6/9, at three and one-half years fusion capacity is improving, at four years vision is near 6/6. At five years ocular pursuit is inferior to fixation, and at five and one-half years fusion is well established and accurate. By six to six and one-half years ocular pursuit is accurate, and the average child can discriminate letters and word symbols and begin to read. Thereafter up to the age of nine, ability to tolerate prism vergences develops and continues to increase. [SeeVision.]
Thus, we see that even though vision is present at birth and, relative to the other senses, advanced, acuity in one aspect or another continues to increase, at least up to nine years. The changes are more rapid at first and become slower with advancing age.
The eye is the most highly developed and complex of the sense organs, and we find, accordingly, that the development of visual acuity is a function of several variables, including the simple ones, brightness and hue; patterned vision, which is related to degree of complexity of both qualitative and quantitative variables; and depth discrimination, both monocular and binocular, together with the development of accommodation and convergence. Much remains to be done in clarifying and identifying the developmental aspects of all of these.
In the senses generally, and most acutely in vision, the pure sensory aspects of development are confounded with perception and the meaningful and adaptive responses to the stimuli which are presented for the study of sensory discrimination.
Hearing. The developmental pattern of auditory acuity is in many ways similar to that of vision. The normal newborn infant responds, by reflex startling, to a sharp, loud clack or the ringing of a bell near his ear (Bayley 1933). Ten days after birth he reacts to the lesser sound of a rattle and at twenty days to the sound of a softly speaking voice. The localization of the source of a sound is incipient in the two-and-a-half-month-old who may be seen to search with his eyes for the bell or rattle when sounded outside his field of vision. By three months his eyes will turn from the bell to the rattle and back, when they are sounded alternately while held eight inches apart in his field of vision. The four-month-old will turn to the right and the left to see the bell which has been rung opposite first one ear and then the other.
There is evidence of rapid cortical development in the auditory area of the cortex in the first four months. Normal acuity appears to be well developed by this age (Wever 1949). Further changes in hearing appear to be primarily perceptual. The sixto seven-month-old is interested in producing sound. He bangs his hand or a toy on the high-chair and listens. He babbles and repetitiously tries out syllables. At eight to nine months he listens selectively to familiar words and begins to respond appropriately to simple commands.
Auditory acuity varies according to the pitch of the sound (Wever 1949, p. 364). However, this variation in pitch sensitivity appears to be a function of individual differences rather than development in the infant and child. After thirty years there is some decrement in auditory acuities, and this is greater for increasingly higher pitches (Wever 1949; Sommers, Meyer, & Fenton 1961). [SeeHearing.]
Tactual and pain sensitivity. There is evidence of increasing sensitivity to pain in the first four days of life (Lipsitt & Levy 1959) and probably for a somewhat longer period of infancy. Schludermann and Zubek (1962) found no changes in pain sensitivity from age 12 through 50, though decrements occurred after that age.
The young infant clearly reacts to tactual stimulation. However, skin sensitivity also appears to increase with age. For example, Ghent (1961) studied tactual thresholds in the hands of children 5 to 11 years old and found that sensitivity increased over this age range. She also found a sex difference, with girls showing greater sensitivity and approaching the adult level of sensitivity at an earlier age. [SeePain; Skin Senses And Kinesthesis.]
The development of motor coordinations, evidenced first in simple reflexes, appears to depend on the interactions of muscular response to stimulation, growth and increasing strength of the muscles, and the development of coordination through practice. All of these are interdependent. Practice strengthens the muscles and stimulates their growth. It also promotes learning, for example, through the simultaneous stimulation of visual and muscle senses in the eye-hand coordinations involved in reaching, grasping, and manipulating small objects. In newborn infants these coordinations are seen in such reflex responses as head lifting, various postural adjustments to body position, crawling, and reflex grasping. Soon, between one and two months, we observe playful bursts of activity in the form of arm and leg thrusts. As the muscles grow stronger, the infant is able to hold his head erect, to push his chest up by his arms, to turn from his back to his side at four months, and to sit, at first with support and by six months, alone (momentarily). By three months his hands are no longer tightly fisted, and he holds a small toy with a grasp which is no longer entirely reflex. The six-month-old will reach for a toy with one hand. (Earlier he tends to “close in on” an object, using both hands simultaneously.) He shows early manual coordination in rotating his wrist, in partially using his thumb in opposition to his fingers, in grasping, and in trying to pick up pea-sized pellets. The eight-month-old sits alone steadily, may be starting to crawl or creep, and picks up small objects with complete thumb opposition. By nine months he can get himself into a sitting position and pull to a standing one by his crib rail. The ten-month-old creeps with agility and can often walk with help, sit down, and bring his hands together for games like pat-a-cake.
The one-year-old can take a few steps alone. In the next six months he will be able to throw a ball, walk backwards, and walk up and down stairs with help. The two-year-old walks up and down the stairs without holding on, and by three years he jumps from small heights, runs, walks on tiptoe. The four-year-old can walk a line and can hop a few steps on one foot.
Individual variability. There are of course, large individual differences in the age at which children become able to do these things, as well as differences in the skill and smoothness of motor coordinations. Motor skills after early infancy are very largely determined by practice. Furthermore, there is great specificity in skills. Evidently each motor function must be practiced in order for it to be performed with skill. Ability to catch a ball cannot be used to predict ability at the high jump or the broad jump. Bayley (1951a), for example, found that for ages 4½ to 12 years, scores on a battery of ten tests of manual dexterity were unrelated to strength and showed correlations of about .40 with scores on jumping (sum of three tests) and of .28 with scores on balance (sum of five tests). Scores on jumping, balance, and strength tended to correlate with each other at around .30 for most ages. Similarly, Espenschade (1940) found for children of 13 to 17 years no relation between gross and fine motor skills, though similar gross motor activities were moderately related. For example, scores on the dash usually correlated near .60 with those for the broad jump, near .40 with the jump-and-reach, and near .40 with the distance throw.
Consistency over time. Correlations showing the degree to which scores are consistent over time, on total motor tests, are only moderate in young children. For example, in the Berkeley growth study Bayley (1935) found that correlations of scores at 27 and 30 months with scores at six younger age levels in the first two years are, with two exceptions, below .40. At later ages, between 4 and 12 years, these Berkeley children’s scores on the manual dexterity tests again show only moderate consistency over time. Scores on the 1J year test showed correlations of .29 with scores at 4ǀ years, .50 at 5J years, .49 at 6J, .55 at 7-J, and .49 at 8$ years. Espenschade (1940) gave a series of motor tests to 160 children tested at six-month intervals between the ages of 13 and 17 years. She found a fair degree of consistency over a four-year period for most individual children. Glassow and Kruse (1960) found similar stability in relative scores for girls aged 6 to 14 years, with the running and jumping scores more stable than scores for throwing. Inconsistency in these cases may be attributed to the fact that scores in adolescents tend to be related to the degree of physical maturity and strength.
As measured by scores on standard tests of motor abilities, motor skills are seen to increase continuously through infancy and childhood. The increases are greatest in the first 18 months, after which the rate appears to decelerate gradually (Bayley 1951a).
Sex differences. There is no sex difference in motor-test scores during the first 12 years (Bayley 1939). However, after this age the girls.’ scores tend to stabilize while the boys.’ scores continue to increase (Espenschade 1947). This continued increase in boys.’ scores is correlated with their continuing growth in strength (Govatos 1959). Furthermore, both strength and scores in gross motor abilities are correlated in boys with their degree of physical maturation (Jones 1944; Clarke & Harrison 1962). Those who are accelerated in puberal development are stronger and more skilled in gross motor coordinations than their more slowly maturing age peers. It is also true that the more muscular boys, with strongly masculine physiques, are stronger than those with less masculine builds (Bayley 1951b). [SeeIndividual Differences, article on Sex Differences.]
Specificity. In general, after the first 15 months of age, motor skills evidence much specificity (Bayley 1951b; Espenschade 1947; Letter 1961). It is evident also that motor skills are very responsive to practice and training (Clarke & Henry 1961; Clarke & Petersen 1961). This appears to be evident even in the very young (Holt 1960). Within normal limits and the limits of muscular strength, it should be possible to increase specific motor skills considerably through practice.
[See alsoDevelopmental Psychology; Infancy; Senses. Related material on child development may be found inIntellectual Development; Language, article onLanguage Development; Moral Development; Personality, article On Personality Development.]
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