Human brain development timeline
||The lead section of this article may need to be rewritten. (January 2013)|
In the 1950s, animal research showed development in the sensory regions after birth. During sensitive periods, the environment plays a major role in normal development.
This research indicated that from early postnatal time through the next several months or years, the brain went through synaptogenesis followed by synaptic pruning which represent the creation and elimination of synapses during growth.
In the 1960-70s, studies were done on human brains to reveal development past the early childhood years, especially in the prefrontal cortex. This was identified by the process of myelination where the developed regions axons' were myelinated first while the association areas were still able to develop through adolescence.
Synaptic reorganization takes place most predominantly during childhood and adolescence. During these periods the brain becomes sensitive to change which allows it to develop in unique ways dependent upon the individuals age, gender, and environment along with many other variables.
The concept of "self-organization" indicates that the brain actually organizes itself based on the individuals experiences.
In 2012, a team of scientists created a statistical model that could predict the age of an individual under the age of 20 from an MRI scan with 92% accuracy. The model measures 231 biomarkers of brain anatomy and was constructed with data from 885 people. This work provides a uniquely holistic view of adolescent brain development and suggests that the responsible processes are more strongly genetically pre-programmed than is typically thought.
- 1 Descriptors
- 2 Neuroimaging
- 3 Current Research
- 4 Further Research
- 5 See also
- 6 References
- 7 External links
The human brain development timeline is easier to understand when the data is organized into charts and images however there are discrepancies between researchers about the exact time frame of each developmental stage. The images and charts provided on this page are approximate. Multiple charts and images are provided for disambiguation.
|33||posterior commissure appears||Ashwell et al. (1996)|
|33||medial forebrain bundle appears||Ashwell et al. (1996)|
|44||mammillothalamic tract appears||Ashwell et al. (1996)|
|44||stria medullaris thalami appears||Ashwell et al. (1996)|
|51||axons in optic stalk||Dunlop et al. (1997)|
|56||external capsule appears||Ashwell et al. (1996)|
|56||stria terminalis appears||Ashwell et al. (1996)|
|60||optic axons invade visual centers||Dunlop et al. (1997)|
|63||internal capsule appears||Ashwell et al. (1996)|
|63||fornix appears||Ashwell et al. (1996)|
|70||anterior commisure appears||Ashwell et al. (1996)|
|77||hippocampal commissure appears||Ashwell et al. (1996)|
|87.5||corpus callosum appears||Ashwell et al. (1996)|
|157.5||eye opening||Clancy et al. (2007)|
|175||ipsi/contra segregation in LGN and SC||Robinson and Dreher (1990)|
Neuroimaging is responsible for great advancements in understand how the brain develops. EEG and ERP are effective imaging processes used mainly on babies and young children since they are more gentle. Infants are generally tested with fNIRS. The MRI and fMRI are widely used for research on the brain due to the quality of images and analysis possible from them.
Magnetic Resonance Imaging
MRI's are helpful in analyzing many aspects of the brain. The magnetization-transfer ratio (MTR) measures integrity using magnetization. Fractional anisotropy (FA) measures organization using the diffusion of water molecules. Additionally, mean diffusivity (MD) measures the strength of white matter tracts.
Functional Magnetic Resonance Imaging
fMRI's test mentalising which is the theory of the mind by activating a network. The posterior superior temporal sulcus (pSTS) and temporo-parietal junction (TPJ) are helpful in predicting movement. In adults, the right pSTS showed greater response than the same region in adolescents when tested on intentional causality. These regions were also activated during the "mind in the eyes" exercise where emotion must be judged based on different images of eyes. Another key region is the anterior temporal cortex (ATC) in the posterior region. In adults, the left ATC showed greater response than the same region in adolescents when tested on emotional tests of mentalising. Finally, the medial prefrontal cortex (MPFC) and the anterior dorsal MPFC (dMPFC) are activated when the mind is stimulated by psychology.
Higher resolution photography has allowed three-dimensional sonography to aid in identifying the human brain development during the embryonic stages. Studies report that three primary structures are formed in the sixth gestational week. These are the forebrain, the midbrain, and the hindbrain, also known as the prosencephalon, mesencephalon, and the rhombencephalon respectively. Five secondary structures from these in the seventh gestational week. These are the telencephalon, diencephalon, mesencephalon, metencephalon, and myelencephalon which later become the lateral ventricles, third ventricles, aqueduct, and upper and lower parts of the fourth ventricle from the telencephalon to the myelencephalon, during adulthood. 3-D sonographic imaging allows in-vivo depictions of ideal brain development which can aid in recognizing irregularities during gestation.
White Matter Development
Using MRI, studies showed that while white matter increases from childhood (~9 years) to adolescence (~14 years), grey matter decreases. This was observed primarily in the frontal and parietal cortices. Theories as to why this occurs vary. One thought is that the intracortical myelination paired with increased axonal calibre increases the volume of white matter tissue. Another is that synaptic reorganization occurs from proliferation and then pruning.
Grey matter development
The rise and fall of the volume of grey matter in the frontal and parietal lobes peaked at ~12 years of age. The peak for the temporal lobes was ~17 years with the superior temporal cortex being last to mature. The sensory and motor regions matured first after which the rest of the cortex developed. This was characterized by loss of grey matter and it occurred from the posterior to the anterior region. This loss of grey matter and increase of white matter may occur throughout a lifetime though the more robust changes occur from childhood to adolescence.
Current research has been able to make new discoveries for various parts of the brain thanks to the noninvasive imaging available.
- Medial Prefrontal Cortex (MPFC)
In this region, more activity is noted in adolescents than in adults when faced with tests on mentalising tasks as well as communicative and personal intent. Decreased activity from adolescence to adulthood. In a mentalising task employing animation, the dMPFC was more stimulated in adults while the ventral MPFC was more stimulated in children. The can be attributed to the use of objective strategy associated with the dMPFC. Theories for decrease in activity from adolescence to adulthood vary. One theory is that cognitive strategy becomes more automatic with age and another is that functional change occurs parallel to neuroanatomical change which is characterized by synaptogenesis and pruning.
The MPFC is an example of one specific region that has become better understood using current imaging techniques. Current research provides many more findings like this.
The development of the brain will vary between males and females since each gender matures at different times and in different ways however the details are yet to be understood. An important bodily change like puberty was tested to show drastic effects on the peaks in cortical development. In preliminary studies, gray matter increased in the amygdala, a region linked to emotional response. In females, there was increased oestradiol and increased limbic gray matter. In males, there was increased testosterone and parietal cortex gray matter. A decrease in the volume of the hippocampus was also noted. These results need further support.
Differences in environment can affect how the brain develops and at what pace. The environment can include factors like location and surroundings as well as circumstances in those environments. Environment can also be identified as an individuals emotions or response to certain stimuli. In this case, the concept of "self-organization" which postulates that the brain organizes itself based on each individual, must be explored further.
Different regions of the brain depend on each other for specific functions. They communicate by their connectivity. Much is still unknown about these networks and how exactly they are connected to carry out the necessary functions. Most current research is taken from animal studies; testing in humans is vital to gain more information of this topic. Task-dependent connectivity can be analyzed functionally or effectively. A trend from some preliminary research implied there was a change in organization occurring with increasing age. Due to the complexity of the brain and the range of its capabilities, extensive research will have to be done to understand this phenomenon. The Resting-state functional MRI can be used to explore further into connectivity.
The effects of drugs on brain development has yet to be thoroughly understood. Potential research should be targeted to find the long-term effects of administering stimulants and the variables should include type of drug, dosage, and age of patient. Animal studies must be conducted first to get a thorough understanding of potential consequences and mechanisms.
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- Translating Time — a website providing translation of brain developmental times among different species