Critical Period in Brain development

Imagine trying to mould wet clay—each touch, impression, and carve you make is critical because the clay is soft and malleable. You can shape it into a tumbler, pot, or plate. You choose how you wish to sculpt. However, there is a limited time within which you must work to give it your desired form. Once the clay is dry and hard, the opportunity to shape it according to your wish vanishes, and the structure becomes permanent.

The brain has similar characteristics. During a critical period, certain factors can influence the brain. If someone fails to seize this window, the brain loses its flexibility and becomes unyielding, like a hard, dried piece of clay. Let’s understand what this critical period in brain development is all about and how the brain is most receptive to learning during this time.  

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What Does Critical Period Mean?

The Critical Period (CP) is a specific time window during early development when the brain is highly plastic, meaning it is more malleable. This window varies for different skills. Essentially, CP refers to the timeframe during which environmental influences can assist the brain in acquiring specific skills. It’s like the brain being as pliable as wet clay during this time.

As the name suggests, a critical period is crucial. Receiving exposure to a particular stimulus during this time is essential for the individual to acquire the skill associated with that stimulus. A critical period is limited and does not repeat itself. Once that timeframe elapses, there’s no way for it to occur again. The inability to experience a certain stimulus within its corresponding critical period could lead to the inability to learn that particular skill, leading to functional deficits, suggesting that the individual may not develop that skill in the future.

To get a clearer understanding, we need to explore brain plasticity and how it’s influenced by synaptic connections and the pruning of neural networks. As stated before, the brain’s plasticity is at its optimum during infancy and early childhood. This is because, at birth, an infant has more than 100 billion neurons. Due to the brain’s malleability, the neural networks that are relevant or used more frequently get strengthened, while those that are not used or are irrelevant fade away.

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Jeff Lichtman, a neuroscientist at Harvard, said that we are “wired up for every contingency possible,” but we are left with a smaller portion of that wiring because ultimately, it is “tuned exactly to the world you found yourself in.” This means that the experiences that impact our early development influence the neural connections that are left with us. Those early experiences determine the kind of networks that are strengthened or weakened.

Thus, brain plasticity is greater in children than in adults. In children, neural circuits are still developing, while in adults, they are already solidified, making it easier for children to learn new skills such as language and music compared to adults. This solidification of skills is not necessarily a bad thing, as it enables the consolidation of one set of skills upon which the next set develops.

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However, if a child is not exposed to certain sensory experiences, or if there are abnormalities in certain sensory functions and the critical period for those skills elapses, then the child may not be able to attain those skills. While abnormalities may be fixed later, a functional deficit may occur as the brain may not be as plastic as it was during childhood. The solidification of neural circuits may prevent the brain from fully processing those sensory inputs as it would have earlier in the critical period.

This leads us to our next important point:

What influences Brain Development?

Genetics and sensory input are two factors associated with brain development. Sensory experiences are crucial for growth in children. For cortical development, sensory input becomes imperative. Simply having sensory experiences is enough to alter the development of the brain. For example, just seeing and hearing can cause the brain to mature and strengthen these particular pathways.

Sensations thus become the foundation of experience during critical periods. The sensory input received is imperative for shaping how the brain processes information. For example, when the critical period for vision appears, the child needs to receive visual stimuli for the brain’s visual cortex to develop. In instances of sensory deprivation, such as cataracts, where a child lacks visual stimuli during the critical period, their visual cortex may not fully develop. Even with correction, vision may never fully recover. ‘

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Critical Period and Language Acquisition

With an understanding of critical periods and the role of sensory inputs, we must now delve into language acquisition. The concept of critical periods is highly relevant in the context of language acquisition. Eric Lenneberg (1967) proposed that the brain’s ability to acquire certain language skills, especially those associated with a first language, is effective during a critical period that ends at puberty. This is due to cerebral lateralization, where certain functions, including language, become more specialized in one hemisphere of the brain.

This implies that language development that occurs after the critical period is significantly different from language development during the critical period. The brain processes involved later are just not as natural or efficient as those during the critical period. While Lenneberg’s critical period stretched from 2 years of age to puberty, researchers today do not define a specific starting point for the critical period. However, they do identify the end of the critical period at the age of 12, 15, 16, or, at the most, 18 years. The case of an early 18th-century boy called Victor became central to understanding language acquisition during the critical period.

A feral child of 12 or 13, found in the forest of Aveyron, Victor had little to no human contact. According to the concept of the critical period, Victor was isolated from human language for a span of time when he could have learned it easily and naturally. Jean-Marc Gaspard Itard, a physician, cared for Victor after he was found in the forest. He tried to socialize him and teach him a few words, but these efforts met with limited success.

Since Victor was secluded from human interaction during the critical period of language development, his brain’s plasticity to learn a language was lost. Victor’s case shows us how language acquisition in later periods is qualitatively different than during the critical period, as it requires conscious effort. For Victor, despite the intensive efforts put in, it was difficult for him to develop the fluency and skills required for language acquisition. This suggests that brain lateralization for language development needs to be stimulated at a particular time (critical period) to develop properly.

Despite these findings, research conducted by Snow and Hoefnagel-Höhle on language learning abilities across different ages found that acquiring Dutch as a second language was easier for those 15 years and above, as compared to 3- to 5-year-olds who were acquiring Dutch as their second language.

This finding suggests that older learners, including those beyond the critical period, can be more efficient in learning certain aspects of a new language. It challenges the strict interpretation of the CP, which traditionally posits that language learning ability significantly declines after puberty. Instead, it indicates that there may be advantages to learning a second language later in life, such as more developed cognitive skills and learning strategies in older individuals.