We have seen that normal development of the brain depends on interaction between genetic inheritance and environmental experience. The genome provides a general structure of the nervous system. Nervous system activity and sensory stimulation refine the mode of operation. This ‘fine-tuning’ doesn’t mean the addition of new components or connections. It is achieved by elimination.
We thus see that the normal development of the brain depends on a critical interaction between genetic inheritance and environmental experience. The genome provides the general structure of the central nervous system, and nervous system activity and sensory stimulation provide the means by which the system is fine-tuned and made operational. But this fine-tuning does not depend on adding new components. It is achieved by discarding much of what was originally present. It is as if the radio arrived on the assembly line with twice as many electrical components and connections as necessary to work.
The process by which the brain connections change over time has been the result of mature animals interacting with their environments. Using sophisticated techniques for determining the numbers and densities of neurons and synapses in specific regions of the rat’s brain, a group of scientists found that, during the first months of the rat’s life, a rapid spurt in the growth of synapses occur, regardless of the amount, or type, of sensory experience.
This is called ‘synaptic blooming’ and it is always followed by a sharp decline in synapses. That is to say, there is an element of pruning, a pruning of synapses that takes place based on the activity and stimulation (sensory) in the brain. It ultimately results in the configuration of characteristic connections of the adult rat’s brain.
This period of synaptic “blooming” is followed by a sharp decline in the number of synapses. That is, an elimination or “pruning” of synapses then takes place based on the activity and sensory stimulation of the brain, and ultimately results in the configuration of connections characteristic of the mature rat’s brain. This initial blooming and ‘pruning’ of synapses are experience-expectant learning; the initial synaptic overproduction appears to be independent of the animal’s experiences. It is as though the mind expects important things to happen during the first week and first few months of life and, prepared for these experiences, possess an overabundance of synapses, to which a fraction of the initial set are selected. It is as though the brain is expecting important things to be happening during the first weeks and months of life, and is prepared for these experiences with an overabundance of synapses, only a fraction of which, however, will be selectively retained.
In synaptic density, and the expansion of cortical volume, leave no doubt that the postnatal period is one of rapid development in the frontal cortex (human, that is.) By the age of two, synaptic density is at its height; and, at the same time, when other components of the cerebral cortex, also cease growing as brain weight approaches that of the adult. Synaptic density declines subsequently, reaching, by adolescence, an adult value of 60%.
This over-abundance of synapses is thought to be responsible for the striking plasticity of the immature brain that leads to the learning of skills that can be, in childhood, learned with relative ease, and, in adulthood, is more difficult. We have seen how immature animals and children are unable to develop normal vision if they’re not exposed to a vivid, visual world during the period of brain development. It has, in fact, been repeatedly observed that, although many adults can rapidly learn a language, young children have an advantage in the sounds of language.
At age twelve, children begin to lose the ability to differentiate contrasting sounds they are unaccustomed to. Whereas normal infants can distinguish between two related but distinct sounds represented by the letter T in Hindi, those who hear, through their childhood, only English, lose this ability although Hindi speaking children retain it. Recent work has provided important human behavioral evidence consistent with the view that normal brain development involves the loss of synaptic connections, which, inevitably, result in the loss of certain skills as the brain approaches maturity.
These findings paint a picture of a developing brain that contrasts sharply from ‘genetic providentialism.’ Instead of the brain unfolding according to a specified blueprint, we see, instead, a process of selection by which abundant neuronal connections are eliminated, leaving only the connections that permit the animal to interact with its environment. Taken together, these findings paint a picture of the developing brain that contrasts sharply from the genetic providentialism favored by Sperry. Instead of the brain unfolding according to a genetically specified blueprint, we see instead a process of selection by which overly abundant neuronal connections are eliminated through a weeding-out process, leaving only those connections that permit the animal to interact successfully with its environment.