(2) Postnatal (After-Birth) Development
(i) At Birth
The evolutionarily old parts (reptilian brain) that regulate vital functions, such as respiration, cardiac rhythm, eating, sleeping, and eliminating metabolic waste products, are the most developed parts of the neonates. Interestingly, the evolutionarily new cerebral cortex (neo-cortex) that subserves higher-order functions, such cognition and behavior, is the least developed part of the brain.
(ii) First 18 Months of Life
(a) Synaptogenesis: Although most neurons are produced while we are still in the womb, the connections of neurons, known as synaptogenesis, occur postnatally. This happens at different rates in different parts of the brain. Synaptogenesis involves the growth of axons and dendrites that connect with each other creating new synapses. This process is mainly driven by the infant’s experiences; and occurs throughout the brain but notably in the previously underdeveloped cerebral cortex (Figure 4; Box -1) .
Figure 4. The Natural Selection of Brain Wiring: Synaptogenesis → Synaptic Pruning. During the ‘exuberant’ period of brain development, children generate about twice as many synapses (exhibited as small light pink/blue solid circles at the terminals of the axons) as they will eventually need, viz., synaptogenesis. Experience and/or electrical activity, will eventually determine which synapses will be preserved and which ones will be eliminated, viz., synaptic pruning. Thus, in general, no new neurons are added after birth, but new dendrites and synapses sprout furiously during a child’s earliest years, causing the cerebral cortex to thicken and its circuitry to grow massively more complex.
For example, on the one hand, the visual cortex gets wired together pretty soon and achieves its maximal synaptic density (i.e., the most connections it will ever have) around four months. On the other hand, the prefrontal cortex does not get to its maximal density until the child is about four years. Most of all, the other extraordinary thing that happens to the connections in the brain is that we produce far more of them than we need. As a result, by about the age of two years we have around 50% more connections in our brains that we typically have as an adult.
This means that, the proliferation of new nerve fibers (grey matter, body of neurons) is accompanied by increases in supportive and protective glial cells (white matter, myelinated axons), as a consequence of these combined growth processes the weight of the brain normally doubles between birth and 18 months.
(b) Synaptic Pruning: While there is a great deal of synaptogenesis during the first few years of life, thereafter, there is also a considerable amount of synaptic pruning. A process, in which the less-used axonal pathways are eliminated and solely the most-used pathways are retained (Figure 4; Box – 1).
Synaptic pruning has been likened to ‘moulding a sculpture from clay.’ We begin with excess clay, or neurons, and through a delicate process of refining and whittling down what is not needed, we are left with an object of beauty and efficiency. Synaptic pruning thus ensures the brain emerges into a smooth, well-oiled machine with connections that make sense.
The magnitude of this synaptic sorting is enormous. Children lose on the order of ‘20 billion synapses per day’ between early childhood and adolescence. While this may sound ruthless, it is generally a very beneficial thing. The elimination of stray synapses and the strengthening of survivors is what makes our mental processes more streamlined and coherent as we mature. Thus, it creates efficient channels for information transfer, and on the other hand it may also explain why our mental processes become less flexible and creative as we mature .
(c) Programmed Cell Death: Moreover, space for the proliferating axons and dendrites is made by the ‘programmed cell death or apoptosis,’ of some of the adjacent neurons. The prenatally formed surplus neuron ‘bank’ allows for this.
Ultimately, the combination of (a) synaptogenesis, accompanied by (b) synaptic pruning, and (c) programmed cell death is sometimes referred to as a process of ‘sculpting the brain’ into a functionally efficient form.
(d) Growth Spurts: Spurts in brain growth, with ensuing process of synaptic pruning and apoptosis, occur at approximately predictable ages from infancy through childhood through adolescence. The nature of these growth spurts is selective, building, and sculpting specific brain systems in turn (Figure 1, 3, & 4; Box – 1).
From middle age onwards, though, there is a gradual loss of grey matter in specific brain regions, associated with a decline in cognitive abilities. This deterioration may be exacerbated by age-related disease such as dementia.
(e) Myelination: The process by which the axons of neurons are covered with an insulating layer designed to increase the neuron’s efficiency. Myelin is a fatty substance that grows around the axons of neurons as they develop. It is generated by oligodendrocytes. This insulates the body of the neurons and so speeds up transmission of signals along the axon. It develops in different regions starting with the brain stem at about 29 weeks. It tends to happen from inferior to superior, and posterior to anterior; meaning that it happens from bottom to top and from the back areas to the front. Myelination occurs mostly during childhood but in some area continues into adolescence and even beyond .
As the connections in the head develop (by implication brain size and volume), so the size of the head itself needs to grow! It expands nearly 14 cm on average during the first two years of life outside the womb; and followed by a further 7 cm during early childhood and adolescence.
Like many other areas of development, brain growth is not smooth and continuous; rather it occurs in spurts as mentioned above. The brain undergoes several growth spurts over the course of its development. Recording of brain electrical activity as reflected in electroencephalograms (EEG) appears to show correlations between growth spurts and major periods of cognitive development. For example:
While it may be tempting to draw meaningful connections between developments within the brain itself (i.e., changes due to synaptogenesis, synaptic pruning, myelination, and development of the frontal lobe) and cognitive development, however, there exists a caveat in doing so, arguing the fact that just studying global brain changes in brain structure is less informative than examining how regional developments in the brain develop to support specific functions.
(f) Lateralization: An important aspect of brain development that begins prenatally and continues from infancy is ‘laterization.’ As we know, the cerebrum and cerebellum are divided into left and right divisions. Subcortical structures are also bilaterally paired. The left and right components of a pair of structures usually subserve different but complementary functions.
For example, the auditory cortex that is located in the left cerebral hemisphere receives sound coming predominantly from the right ear, while the auditory cortex situated in the right hemisphere receives sound coming predominantly from left ear. Likewise, speech production is typically carried out in the left hemisphere, however prosody (the emotion bearing patterns of pitch and intonation) is generally carried out in the right hemisphere.
Most importantly, and on the positive side:
In any case, the chemical processes involved in synaptogenesis are, however, mediated by gene expression products. Furthermore, the processes of pruning and programmed cells death (apoptosis) are also genetically controlled. Nevertheless, environmental factors also have an essential role in brain sculpting because experience determines which neuronal connections are well established (and consequently not pruned) as opposed to those that are poorly established (and consequently pruned).
This sort of interplay between genetic and environmental factors is typical of the process of brain development and change throughout the lifespan, ensuring that every neurotypical individual is unique.
Despite the above-mentioned facts, there is still some difference of opinion concerning the balance between the contributions of nature (as dictated by genes), and nurture (influenced by experience), particularly concerning the extent to which genes control precisely what circuits are built, and in which parts of the brain. This dialogue is absolutely relevant to theories of the immediate causes of the Socio-Emotional-Communicative (SEC) impairments and Restricted, Repetitive-Behaviors (RRBs) that are diagnostic of autism spectrum disorders (ASD); also to discussions of the extent to which ASD may or may not be ‘ameliorated’ or even for that matter ‘cured’ .
II. Neurotypical Brain Function
The whole brain functions as a hierarchy of nested systems referred to as neural circuits, systems, or networks. A network is a connection of many brain regions that interact with each other to give rise to a particular function. Every network consists of interconnected nuclei that operate together to subserve a particular function. Clusters of neurons, all of which are involved in transmitting and receiving the same information, are called nuclei (singular: nucleus).
Those neural circuits that involve relatively few nuclei and are situated close together in the brain are referred to as local networks. These networks carry out highly specific functions, for example, registering the color or shape of an object. However, if all salient features of an object simultaneously and as a whole or entirety, such as color, shape, size, and movement of an object or of a bird, in that scenario, activity in several local networks must be coordinated in a global network.
Certain global networks are very widely distributed within the brain, involving both left and right cerebral hemispheres. As an example, the hearing brain! Hearing is mediated by the auditory nerves that connect mechanisms in the left and right inner ears to nuclei in the brain stem; and these nuclei further connect to the primary hearing centers located in the left and right temporal lobes, and thereafter to adjacent auditory association areas in each temporal lobe.
The neural oscillations are named with letters of the Greek alphabet, such as alpha, beta, gamma, etcetera; as discussed below and corresponding brain waves are schematically illustrated in Figure 5 .
Alpha waves are neural oscillations in the brain with a frequency range of 7.5 – 12 Hz. This rhythm probably originating from the synchronous and coherent electrical activity of thalamic pacemaker cells in humans. Alpha waves are detected at different stages of the wake-sleep cycle. The alpha activity is centered in the occipital lobe, although it has been speculated that it has a thalamic origin.
Beta waves are neural oscillations in the brain with a frequency range of 12 – 30 Hz. Beta rhythm is associated with normal waking consciousness. Beta waves often conceived indicative of inhibitory cortical transmission medicated by gamma aminobutyric acid (GABA), which is the predominant inhibitory neurotransmitter of the mammalian nervous system. Consequently, beta waves are possibly biomarkers of GABAergic dysfunction, especially in neurodevelopmental disorders caused by genetic defects such as 15q deletions or duplications.
Figure 5. Neural Oscillations. ‘Brain or neural oscillations’ refers to the rhythmic and/or repetitive electrical activity generated spontaneously and in response to stimuli by neural tissue in the central nervous system. It can be recorded using various electrophysiological methods, such as electroencephalogram (EEG) or magnetoencephalogram (MEG); recorded either invasively or non-invasively, i.e., from inside the brain or from electrodes attached to the scalp. There are at least ten discrete brain rhythms, covering more than four orders of magnitude in frequency, from approximately 0.02 to 600 hertz (Hz). Most of the oscillations are short-lived, i.e., less than a second, while others can be sustained for longer periods. In humans: (i) Gamma wave is a pattern of neural oscillations with a frequency between 30 to 100 Hz; associated with motor function, problem solving, and concentration. (ii) Beta wave is a pattern of neural oscillations with a frequency between 12 to 30 Hz; linked to normal waking state, focus, concentration, and five physical senses. (iii) Alpha wave is a pattern of neural oscillations with a frequency between 7.5 to 12 Hz; correlated with relaxed, light meditation, creative, super learning, and consciousness. (iv) Theta wave is a pattern of neural oscillations with a frequency between 4 to 7.5 Hz; associated with light sleep, deep meditation, creative, recall, and fantasy. (v) Delta wave is a pattern of neural oscillations with a frequency between 0.1 to 4 Hz; associated with deep sleep, dreamless sleep, non-REM sleep, and unconscious.
Gamma waves are neural oscillations in the brain with a frequency range of 30 – 100 Hz. Gamma rhythms are correlated with large-scale brain network activity, including cognitive phenomena, for example, working memory, attention, and perceptual groupings. Altered gamma activity has been linked to certain mood and cognitive disorders, such as major depressive or bipolar disorders, schizophrenia, epilepsy, and Alzheimer’s disease. In addition, hypersensitivity and memory deficits due to fragile X syndrome (FXS) may be associated to gamma rhythm anomalies in the sensory cortex and hippocampus.
Delta waves are high amplitude neural oscillations in the brain with a frequency range of 0.1 – 4 Hz. Delta rhythms are typically associated with the deep stage 3 of non-REM sleep (slow-wave sleep, SWS). And it assists in characterizing the depth of sleep. As a result, suppression of delta waves drives to inability of body rejuvenation, brain revitalization, and poor sleep.
Theta waves are neural oscillations in the brain with a frequency range of 4 – 7.5 Hz. Theta rhythms underpin various aspects of cognition and behavior, including learning, memory, and spatial navigations in many animals. Broadly, two types of theta rhythm have been described.