When the cognitive impairment is less severe than described above, falling into the IQ ranges of 20 to 45, or 45 to 70, there may be somatic neurologic abnormalities. If specific motor defects do not coexist, then sitting, standing, and walking are achieved but not at the expected times. The existence of a mental defect is most clearly evidenced by a delay in psychomotor development and a failure to speak by the second or third postnatal year. The patient does not acquire the usual household and play activities as well as other children. However, delay in speech development must not, by itself, be taken as a mark of developmental delay, for in some children a unique delay in speech can be an isolated abnormality as described in Chap. 27, with subsequent normal development of speech and mental ability. Toilet training may be difficult to accomplish in the developmentally delayed child, but, again, bedwetting may be a problem in an otherwise normal child. Also, the deaf child may have to be considered separately; here the problem becomes apparent by an indifference to noise and reduced vocalization (stereotypy of babbling).
More thorough analyses of cognitive functions in the moderately cognitively impaired child have been undertaken by O’Connor and Hermelin, Pulsifer, and others. These authors measured the efficiency of visual and auditory perception, adequacy of communication, relations between language development and thought, crossmodal sensory encoding, alertness, attention, and memory. It was concluded that none of these functions was specifically impaired. Instead, the child could not properly encode new information because the memory systems and the stock of past assimilated knowledge were insufficient to provide a framework of items and categories with which new information could be integrated. Some patients also seemed unable, perhaps on the basis of this encoding problem, to extract from perceived material selective features that could be interpreted. Furthermore, they were unable to deal with an array of sensory experiences like normal children. The complexity of these mental operations, which we would reduce to a global failure of the normal processes of apperception and integration, was called by Piaget a “failure of assimilation and accommodation.” However, within the spectrum of developmental delay, apart from cognitive impairments, there are curious differences in behavior and personality, even in those with more or less the same IQ. Some individuals with moderately severe retardation are pleasant and amiable and achieve a satisfactory social adjustment. This is particularly notable in patients with Down and Williams syndromes. At the opposite end of the behavioral scale are the syndromes of autism and that of Smith-Magenis and of DeLange, in which the individual fails to manifest normal interpersonal social contact, including communicative language. The phenylketonuric child is usually irritable, unaffectionate, and implacable, and the same is said of certain other forms of retardation, discussed earlier in the section on retardation with dwarfism.
Regarding differences in motor activity levels, many developmentally delayed individuals are slow, clumsy, and relatively akinetic. Others, as many as half, display an incessant hyperactivity characterized by a restless, seemingly inquisitive searching of the environment. When thwarted, they may exhibit a low frustration tolerance. They may be destructive and recklessly fearless and impervious to the risk of injury. Some exhibit a peculiar anhedonia that renders them indifferent to both punishment and reward. Other aberrant types of behaviors, such as violent aggressiveness and self-mutilation, are common.