During early infancy, the motor system undergoes a variety of differentiations as visual, auditory, and tactile motor mechanisms develop. Bodily postures are modified to accommodate these complex sensorimotor acquisitions. In the normal infant, these emerging motor differentiations follow a time schedule prescribed by the maturation of neural connections. Normalcy is expressed by the age at which each of these appears, as shown in Table 27-4. It is also evident from this table that reflex and instinctual motor activities are the most dependable means of evaluating early development. Moreover, in the normally developing infant, some of these activities disappear as others appear. For example, the grasp reflex, extension of the limbs without a flexor phase, Moro response, tonic neck reflexes, and crossed adduction in response to eliciting the knee jerk gradually become less prominent and are usually not elicitable by the sixth month. The absence of these reflexes in the first few months of life and conversely, their persistence beyond this time indicate a defect in cerebral development, as described in more detail further on, under “Delays in Motor Development.” By contrast, neck-righting reflexes, support reactions, the Landau reaction (extending neck and legs when held prone), the parachute maneuver, and the pincer grasp, which are absent in the first 6 months, begin to appear by the 7th to 8th months and are present in all normal infants by the 12th month.
Because many functions that are classified as “mental” at a later period of life have a different anatomic basis than motor functions, it is not surprising that early motor achievements do not correlate closely with childhood intelligence. The converse does not apply, however; delay in the acquisition of motor milestones often correlates with developmental delay. In other words, most cognitively delayed children sit, stand, walk, and run at a later age than normal children, and deviations from this rule occur mainly in specific diseases, such as autism.
In the period of early childhood, the reflexive- instinctual activities are no longer of help in evaluating cerebral development, and one must turn to the examination of language functions and learned sensory and motor skills, which are outlined in Tables 27-5 and 27-6.
Quite apart from the early stage of motor development, in later childhood and adolescence one observes a remarkable variation in levels of muscular activity, strength, and coordination. Motor acquisitions of later childhood such as hopping on one foot, kicking a ball, jumping over a line, walking gracefully, dancing, and certain skills in sports are linked to age but there is wide variability in the finesse with which they are performed. Ozeretzkii has combined these in a scale that putatively discloses arrests in motor development in the developmentally delayed. Also in later childhood, precocity in learning complex motor skills as well as skill in games and the development of an all-around interest in athletic activity becomes evident. By adolescence, high individual physical achievement is well recognized. At the other end of the spectrum are instances of motor underachievement, ineptitude, and intrinsic awkwardness or clumsiness; a member of this group will easily stand out and be designated as “a clumsy child.” Such awkwardness is to be clearly distinguished from the motor impairment associated with a number of cerebral diseases.
Under normal circumstances, sensory development keeps pace with motor development, and at every age sensorimotor interactions are apparent. However, under conditions of disease, this generalization may not hold; that is, motor development may remain relatively normal in the face of a sensory defect, or vice versa. The sense organs are fully formed at birth. The neonate is crudely aware of visual, auditory, tactile, and olfactory stimuli, which elicit only low-level reflex responses. Moreover, any stimulus-related response is only to the immediate situation; there is no evidence that previous experience with the stimulus has influenced the response; that is, that the newborn can learn and remember. The capacity to attend to a stimulus, to fixate on it for any period of time, also comes later. Indeed, the length of fixation time is a quantifiable index of perceptual development in infancy.