The skills necessary for speech communication develop in the first years of life. Some steps in the pre-verbal period of infancy are essential for the ability to use verbal communication later on. A rule of thumb says that a child that has not learned to chew is, from a sensomotor point of view, not ready to start using articulated speech. During development sudden changes will occur in perception, comprehension, exploring, as well as in social-interactive skills. Such moments mark a transition from an elementary to a more complex level of cognitive functioning. Some of these transitions have been discussed in Chapter 8.
There is continuity in the biological and mental phases of growth. Maturation of the neuronal system makes learning possible. This continuity has been visualized in Map 9.5.1: the horizontal axis from the left to the right. Maturation gradually succeeds growth, and where maturation ends learning begins. All three phases of development can be studied at the levels that are displayed along the vertical axis. Cells can be observed singly or in connection with each other in tissues, such as brain tissue. At a more complex level we contemplate the physiology of organ-systems and their interactions, or the whole individual in his/her transactions with the environment. Many researchers spend a lifetime in one tiny compartment on the surface of this map. The Map can help them to survey the whole area.
Map 9.5.1 Development of form, function and behaviour
Growth, maturation and learning are successive phases of development in all biological processes, and mental processes follow the pattern.
Developmental disorders can have their origin in all three phases:
(1) during growth when the foetus or the infant is undernourished or is supplied with a damaging dose of toxic substances. Examples: consumption of alcohol by the mother or infection during pregnancy by the Rubella virus or Toxoplasmosis. Because in this phase the cells and organs are the primary targets of damage, the emphasis on development in the growth phase is at the bottom of the Map.
(2) the process of maturation may be deranged by failed or dis-coordinated timing. When the clocks of growth processes are out of tune, vital moments of coupling may be missed. Thus a lag in mesenchymal growth in embryonic tissue can cause some parts to develop at a slower rate which prevents the meeting and fusing with their counterparts. The result may be a congenital defect of the septum of the heart or a cleft palate, or both at the same time. The cause may be partly genetic and partly external. In the neural system deviating patterns may develop when migration of early neurons in the foetal brain is influenced by external factors. This can result in an abnormal distribution of brain-functions over the right and left hemispheres. The damage takes place in an organ or organic system: the emphasis is on the middle area.
(3) learning is disturbed. This affects the neuronal system and the entire personality as it interacts with the environment. The shaded band calls the attention to the upper area of the right column. Before cognitive and mental development is at all possible the brain tissues which serve as matrix for the relevant circuits, must have matured sufficiently. Only then the neuronal tissue will have reached a phase that it is receptive to learning (the vertical column on the right).
After organic maturation has taken place, an emotional or cognitive function is susceptible to stimulation only during a critical period. From the beginning of and during that period, the individual can "turn on" a function by using it. It is called priming: an actual exchange between stimulation by the environment and response by the individual is required to start a function. If the function is not called upon, either because of sensory deprivation or because of indisposition of the individual, its potential will not be realized. An unused response system will be shut down and remains so forever. This may seem an inefficient strategy. It nevertheless serves a purposeful economy in adjusting to individual circumstances. For instance "lost" children that grow up in a herd of gazelles or a family of wolves do not acquire speech, but they do develop the use of the sound and sign language of their companions.
After the period of receptiveness has passed, it becomes difficult or impossible to reopen that particular learning channel. If no learning has occurred in the critical period the expected "normal" neuronal patterns will not develop. Without stimulation at the right time discrimination and recognition of distinctive features of speech-sounds cannot cross the border from a potential to a real function (Map 9.6.1, the right column).