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Human Infant Brain

Infant brain development occurs mainly in the first two years of life and is for the most part pre-programmed, meaning that it follows a standard set of steps that most infants go through. This time period is critical for human development, as neurodevelopmental disorders (disorders that arise due to a specific factor of development) are typically at risk of developing at this point of life (Morgan, White, Bullmore, & Vértes, 2018). The infant brain is structurally relatively similar to the brain of a human adult, as the majority of development occurs prenatally, meaning most major structures are already in place by the time the infant is born (Linderkamp & Linderkamp-Skoruppa, 2021). However, infants are born with very few neural connections, or connections between brain cells, meaning these are typically formed in the first weeks of life. These connections are typically experience dependent (formed based on specific experiences of an infant, such as learning new information) and are important for the development of human cognitive abilities such as memory, learning and adaptation (Linderkamp & Linderkamp-Skoruppa).

Infant brain development can be studied using a number of techniques and using a number of indicators. For example, structural magnetic resonance imaging (sMRI) has been used to study the volume of grey and white matter in an infant’s brain which can be examined to test whether development in an infant is typical (Morgan, White, Bullmore, & Vértes, 2018). Another use for this technology is to examine the cortical thickness of a developing brain, which can similarly be used to test for atypical development (Morgan et al.). When testing for typical or atypical development, longitudinal studies are typically the most beneficial type of research as they allow for a participant or participants to be studied over a longer period of time (Schaie, 2005). The results can then be compared to those of other participants at the same point in development to identify potential abnormalities.

Structure

The brain of a human child is made up of a number of important structures, including the cerebrum, the brainstem and the cerebellum. The cerebrum consists of the right and left hemisphere (divided into four lobes; the frontal lobe, temporal lobe, parietal lobe and occipital lobe). It is thought that some of the lobes are not fully developed in the early years of childhood, for example the frontal lobe has a number of important functions, such as emotional regulation, that do not fully develop until later in life (Romine & Reynolds, 2005). The brain also consists of the brainstem, which is the part of the brain that extends downwards towards the spinal cord, and the cerebellum, a cauliflower shaped structure that protrudes from the back of the brainstem. Research has found that the shape of the cerebrum and the brainstem may be altered in premature births, and these structural changes may be reflected in growth rates of various brain structures. For example, Wu et al. (2020) highlight that in premature babies, regions of the Midbrain, Pons, and Medulla (the three major brainstem structures) are impaired in their development, meaning they do not grow at the rate of typical infants. Additionally, the same study found that premature infants typically had smaller right and left hemispheres compared to age matched infants that were not premature.

The human skull consists of several bone structures which protect the brain from damage, the two main structures being the neurocranium and facial skeleton (also called the viscerocranium). These structures are similar in adult humans and infants, with a major difference being the size of the skull. The skull bones of human infants are not always fully fused when they exit the womb (Esteve-Altava, Vallès-Català, Guimerà, Sales-Pardo, & Rasskin-Gutman, 2017), meaning that their heads are generally soft to the touch. This can be exacerbated in pre-mature infants, whose skulls take longer to fuse due to leaving the womb earlier than during typical births, meaning the new-born has reached fewer developmental milestones. However, premature skull fusion (Craniosynostosis) can also be an issue for developing infants, as early fusing of the skull can result in morphological (structural) changes to the cerebral cortex (Esteve-Altava et al., 2017).

Development

The brain of a human infant begins its development during the foetal stages of development (meaning before exiting the womb), with simple neural connections, also known as synapses, beginning to form. These connections between neurons are a method of communication within the brain. Post-birth, simple neural connections which support more basic brain functions begin to form throughout childhood, alongside more complex neural circuits that are responsible for higher level functioning and behaviour (Linderkamp & Linderkamp-Skoruppa, 2021). During the early years of life over a million new neural connections are formed each second (via a process known as synaptic proliferation or synaptogenesis), however this process soon results in pruning, where unused or unnecessary connections are cut back to increase the efficiency of the brain and reduce energy consumption (Paolicelli et al., 2011). This process of pruning is vital for typical brain development, as research has found that deficits in the pruning process may contribute to synapse abnormalities associated with neurological dysfunction (Paolicelli et al., 2011). The pruning process relies on a phagocytic cell (a type of cell which specialises in removing harmful material from the body) known as microglia to find and clear out unnecessary or unused synapses. Although new neural connections continue to be formed throughout the lifespan, the early years of life are the most active in terms of making new connections and for pruning them (Linderkamp & Linderkamp-Skoruppa, 2021; Paolicelli et al., 2011). However, this is a very dynamic process, meaning that it is impossible to say for certain at what point in the lifespan the brain is developed to a specific percentage.

Structurally, most of the infant brain has been fully formed by the time the infant comes to term, meaning that little structural development occurs once the infant has exited the womb. However, the development of important primary circuits typically occurs post-birth in order to provide structure to later developing regions. During the first twelve months post-term, two major structures that are still developing are the hippocampus and the striatum, which are important structures for information consolidation, memory and spatial navigation, and motor and reward systems, respectively. These structures are also important for myelinating axons (Ábrahám et al., 2012), which increases processing speed by boosting transmission via the axons, meaning that these structures allow for faster communication between cells. Structures such as the hippocampus and the striatum are also important for later development of the prefrontal cortex, a region that plays a major role in executive function.

Influences

A number of factors can influence infant brain development, including environmental factors such as socialisation and nutrition (Georgieff, Ramel, & Cusick, 2018), and genetic factors. Specifics of birth can also affect infant development, for example, pre-mature babies typically take longer to develop a fully formed skull and are therefore at higher risk for some syndromes such as Plagiocephaly (colloquially known as Flat Head Syndrome), where infants may suffer from an altered skull shape, either flattened on one side or on the back. Flat Head Syndrome generally does not affect an infants’ brain development, and typically resolves itself over time, however it may be resolved by introducing the infant to additional stimuli, to avoid them laying in one position for too long.

Nutrition in infancy can have a major impact on brain development. Evidence suggests that nutrition in the neonatal period of life has a significant impact of neurodevelopment throughout life, not just in infancy. Georgieff, Ramel, and Cusick (2018) suggest that deficits of certain nutrients, such as protein and iron can result in long-term risks, including alterations to growth, neurotransmitter function and myelination. Georgieff et al. also found that malnutrition can also affect non-neuronal cells such as oligodendrocytes, astrocytes and microglia, as well as signal pathways.

Neuropsychological conditions in infancy can also have an influence of further neurodevelopment. For example, a number of conditions (such as hydrocephalus, hypoxia, and meningitis) can have a major effect on object recognition in infants and can result in changes to the visual pathways (Chen, Weinberg, Catalano, Simon, & Wagle, 1992). These changes were detected using computed tomographic (CT) scans or magnetic resonance imaging (MRI) scans in infants with abnormal object recognition and perception skills (for their age and development milestones), which enabled researchers to identify neuropsychological conditions such as hydrocephalus, a condition wherein a build up of cerebral-spinal fluid CSF) in the brain has a negative impact of neurological function due to expansion of the ventricles (Chen et al., 1992; Khale, Kulkarni, Limbrick, & Wharf, 2016).

References

Ábrahám, H., Vincze, A., Veszprémi, B., Kravják, A., Gömöri, E., Kovács, G. G., & Seress, L. (2012). Impaired myelination of the human hippocampal formation in Down syndrome. International Journal of Developmental Neuroscience, 30(2), 147-158. doi: j.ijdevneu.2011.11.005

Chen, T. C., Weinberg, M. H., Catalano, R. A., Simon, J. W., & Wagle, W. A. (1992). Development of object vision in infants with permanent cortical visual impairment. American Journal of Ophthalmology, 114(5), 575-578. doi: 10.1016/S0002-9394(14)74485-X

Esteve-Altava, B., Vallès-Català, T., Guimerà, R., Sales-Pardo, M., & Rasskin-Gutman, D. (2017). Bone fusion in normal and pathological development is constrained by the network architecture of the human skull. Scientific Reports, 7(3376). doi: 10.1038/s41598-017-03196-9

Georgieff, M. K., Ramel, S. E., & Cusick, S. E. (2018). Nutritional influences on brain development. Acta Paediatricia, 107(8), 1310-1321. doi: 10.1111/apa.14287

Khale, K. T., Kulkarni, A. V., Limbrick Jr, D. D., & Wharf, B. C. (2016). Hydrocephalus in children. The Lancet, 387(10020), 788-799. doi: 10.1016/S0140(15)60694-8

Linderkamp, O., & Linderkamp-Skoruppa, D. B. (2021). Prenatal structural brain development: Genetic and environmental determinants. In: Evertz, K., Janus, L., & Linder, R. (Eds). Handbook of prenatal and perinatal psychology. Springer, Cham. doi: 10.1007/978-3-030-41716-1_3

Morgan, S. E., White, S. R., Bullmore, E. T., & Vértes, P. E. (2018). A network neuroscience approach to typical and atypical brain development. Neuroimaging, 3(9), 754-766. doi: 10.1016/j.bpsc.2018.03.003

Paolicelli, R. C., Bolasco, G., Pagani, F., Maggi, L., Scianni, M., Panzanelli, P., … & Gross, C. T. (2011). Synaptic pruning by microglia in necessary for normal brain development. Science, 333(6048), 1456-1458. doi: 10.1126/science.1202529

Romine, C. B., & Reynolds, C. R. (2005). A model of the development of frontal lobe functioning: Findings from a meta-analysis. Applied Neuropsychology, 12(4), 190-201. doi: 10.1207/s15324826an1204_2

Schaie, K. W. (2005). What can we learn from longitudinal studies of adult development? Research in Human Development, 2(3), 133-158. doi: 10.1207/s15427617rhd0203_4

Wu, Y., Stoodley, C., Brossard-Racine, M., Kapse, K., Vezina, G., Murnick, J., … & Limperopoulos, C. (2020). Altered local cerebellar and brainstem development in preterm infants. Neuroimage, 213. doi: 10.1016/j.neuroimage.2020.116702