The human brain goes through major changes as we grow from babies to adults.
Scientists are interested in mapping out these changes to understand when and where the brain is most flexible to new experiences during development.
Knowing when the brain is most flexible, called neuroplasticity, is important because experiences during these times have the biggest impact on shaping the brain.
Researchers have proposed that there are “critical periods” of development when the brain is extra flexible to learn from experiences.
Just like there are key periods for learning language as a kid, there may be critical times for developing skills like self-control, emotion regulation, and social skills during adolescence and young adulthood.
Understanding when and where critical periods happen in the brain will allow us to better understand healthy development.
It can also help explain when and how things can go wrong, like in mental illnesses that often begin during adolescence and young adulthood like depression, bipolar disorder, and schizophrenia.
Key Facts:
- The brain develops from back to front, with senses like vision and hearing developing before high-level thinking areas.
- There are “critical periods” when brain regions are extra flexible to learn from experiences.
- Critical periods likely progress from sensory to higher-order thinking regions of the cortex as we age.
- Scientists are developing new imaging techniques in humans and animal models to study critical period mechanisms.
- Understanding critical period timing will help optimize interventions and explain vulnerabilities in mental illness.
Source: Trends in Neurosciences (Aug 28, 2023)
The Brain Develops Back to Front
Studies using brain imaging technology like MRI in kids, teens, and adults have shown that the brain develops from the back to the front.
Areas at the back of the brain involved in vision, hearing, and movement tend to develop first.
Areas at the front involved in complex thought, decision-making, and social skills are the last to fully develop.
For example, the cortex processes information from senses and controls movement.
Within the cortex, there is a hierarchy from sensory cortex at the back to association cortex at the front:
- Sensory cortex: Processes sensory information, like visual cortex for sight and auditory cortex for sound. Develops earliest.
- Motor cortex: Controls movement. Also develops early.
- Association cortex: Integrates information to support complex abilities like attention, decision-making, emotion regulation, self-control, and social interaction. Develops last.
Studies tracking brain structure over development have shown that sensory and motor cortex areas mature earlier than association cortex.
For example, areas involved in movement and sensation finish developing gray matter, the outer brain tissue, in late childhood.
But prefrontal cortex gray matter is still changing into the 20s.
Similarly, the connections between brain areas strengthen from back to front over time.
Kids have already developed networks between sensory and motor areas, while networks between higher-order thinking areas continue maturing through adolescence and into adulthood.
Overall, these imaging studies suggest that brain development goes through back-to-front, sensory-to-association progression as kids grow up.
But what drives these developmental changes?
Critical Periods May Progress From Sensory to Association Areas
Scientists think time periods called “critical periods” may help explain the back-to-front brain development timeline.
Critical periods are windows of time when brain regions are extra flexible and can be strongly shaped by experiences.
For example, when young animals have one eye sewn shut, the part of visual cortex wired to the open eye takes over the closed eye’s territory.
This only happens during a critical period in early development, not before or after.
Critical periods have been studied extensively in animal models.
Research suggests they are driven by a specific process involving excitation and inhibition balance:
- Excitation: Brain cell activity sends signals to other cells
- Inhibition: Some cells reduce or stop activity in other cells
- Excitation/inhibition balance: The ratio between cell excitation and inhibition
Here is what happens during a critical period:
- Inhibitory cells that use GABA as their chemical messenger strengthen their connections. This tips the excitation/inhibition balance toward more inhibition.
- With this new balance, cells can change their connections more easily in response to experiences. This is the height of neuroplasticity and the critical period window.
- Inhibitory cells form structural changes like myelin and perineuronal nets that stabilize their connections. Excitation is further reduced through pruning unused connections. These changes close the critical period by reducing neuroplasticity.
Research in animals shows critical periods first open in sensory areas like vision and hearing cortex.
As sensory circuits mature, critical periods progress up the hierarchy to association areas like prefrontal cortex.
The timing matches the back-to-front timeline of brain development.
Scientists think critical periods progressing from sensory to association cortices may mechanistically drive human brain development.
But new techniques are needed to study if this happens in the same way in the human brain.
Studying Critical Periods in the Human Brain
Unlike animal studies, scientists can’t directly look at human brain cells and molecules involved in critical periods.
But new imaging methods allow them to investigate critical period mechanisms like the excitation/inhibition balance.
Here are three promising techniques:
Pharmacological MRI (phMRI): Participants get a drug that changes excitation/inhibition balance while getting a brain scan. Comparing scans on-drug versus off-drug reveals the brain patterns linked to shifting the balance. Researchers can then look for those patterns in brain development.
Chemogenetic imaging: Scientists use techniques in animal models to manipulate specific cells involved in the excitation/inhibition balance. Then they relate the cell changes to patterns in brain imaging scans. This helps identify signals of shifting balance in standard human brain imaging.
Biophysical modeling: Researchers build computer models of how neural excitation and inhibition generate patterns in brain imaging data. By testing different ratios in the models and comparing to real brain scans, they can infer the underlying balance during development.
Early studies using these techniques provide some support for the idea that excitation/inhibition balance shifts toward more inhibition during adolescence, especially in higher-order thinking regions.
But more work is still needed to confirm the critical period model of human brain development.
Why Do Critical Periods Matter?
If critical periods organize brain development from back to front, it would have big implications for understanding the brain and mental health.
First, it would help explain the optimal timing of interventions to shape brain development.
Treatments like behavioral therapy may have the biggest impact if timed to critical periods underlying relevant abilities.
For example, anxiety therapy in pre-teens may benefit developing emotion circuitry more than later in adolescence after it has matured.
Second, it may reveal windows of vulnerability where problems can permanently alter brain development trajectories.
Many mental illnesses like depression and schizophrenia arise during times linked to association cortex development in adolescence and young adulthood.
If these disorders involve disrupted critical periods in higher-order brain regions, it explains why interventions later in adulthood can’t fully reverse symptoms – the critical window has closed.
Finally, identifying the specific cellular mechanisms that drive critical period shifts could inform new treatment targets, like medications or non-invasive brain stimulation methods.
If excitation/inhibition balance proves central to critical periods, treatments targeting GABA or other involved molecules could rescue disrupted development.
Much more research is still needed to confirm if critical periods organize human brain development and understand how genetics and environment interact with these mechanisms.
But this framework provides an exciting starting point to elucidate why adolescence and young adulthood are such important times for brain development – for better or worse.
Understanding critical periods in human brain development will lead to new insights into optimizing health and treating illness.
References
- Study: A critical period plasticity framework for the sensorimotor-association axis of cortical neurodevelopment
- Authors: Bart Larsen et al. (2023)
