Critical Period

The critical period is a maturational stage in the lifespan of an organism during which the nervous system is especially sensitive to certain environmental stimuli. If, for some reason, the organism does not receive the appropriate stimulus during this crucial period, it may be difficult, or even impossible, to develop some functions later in life.

The concept of a critical period is particularly salient in the domain of language acquisition. Children exposed to a language or languages during the early years of life (roughly up to the age of 7, with decreasing plasticity until puberty) can acquire those languages effortlessly and without an accent. However, after this critical window, the ability to acquire language with native-like proficiency declines.

The concept of a critical period is not tied to a strict age range that applies universally across all domains or all individuals. However, in the context of language acquisition, the critical period often refers to the time from birth until around puberty. Some researchers argue the window starts closing around age 7, with diminishing plasticity until puberty. After puberty, the ability to acquire a new language with native-like proficiency—including accent, grammar, and syntax—becomes increasingly challenging.

Throughout history, there have been several accounts of "feral children" who were isolated or lived in the wild during their early years. In almost all these cases, the children faced profound challenges in acquiring language if they were discovered after the critical period.

Genie

One of the most cited cases in the study of the critical period for language is the case of Genie, a girl who was discovered at age 13 having been isolated and abused for much of her life. She had no exposure to language during her confinement. Despite intensive rehabilitation and language training after her discovery, Genie never fully acquired the grammatical intricacies of English.

Victor of Aveyron

Victor was a feral child found in France in the late 18th century. Having spent the majority of his formative years in the wild, he missed the critical period for language acquisition. Despite efforts to educate and socialize him, Victor never fully acquired language.

When babies are exposed to a language or languages different from their family's native tongue, their linguistic development can showcase some unique features. Here are a few examples of how babies can navigate differences in phonemes and other linguistic elements when exposed to non-native sounds:

1. Japanese 'l' and 'r' Sounds: The Japanese language does not differentiate between the 'l' and 'r' sounds in the way that English does. Instead, it has a sound that's somewhere in between, represented by the character ら (ra), り (ri), る (ru), れ (re), and ろ (ro). However, Japanese babies exposed to English from an early age can learn to differentiate between the 'l' and 'r' sounds just like native English speakers. Without this exposure, by around 10-12 months, Japanese babies, like adults, often struggle to distinguish between these two sounds, as shown in studies using the head-turn preference procedure.

2. Hindi Dental and Retroflex Sounds: Hindi has dental ('t' pronounced with the tongue against the upper teeth) and retroflex ('t' pronounced with the tongue curled back in the mouth) sounds that are distinct, but such distinction doesn't exist in English. English-speaking babies exposed to Hindi can learn to differentiate between these sounds, while monolingual English babies typically lose this ability as they age without exposure.

3. Swedish Pitch Accent: Swedish has a pitch accent system where the tone of a word can change its meaning. For instance, 'anden' can mean either 'the duck' or 'the spirit' depending on the tone. Babies exposed to Swedish learn to differentiate words based on pitch accent, while those not exposed to tonal or pitch accent languages would typically not make such distinctions.

4. Mandarin Tones: Mandarin Chinese is a tonal language, where the meaning of a word can change depending on its tone. For instance, the syllable "ma" can mean "mother," "hemp," "horse," or "scold," depending on the tone. Babies exposed to Mandarin learn to differentiate words based on these tones. Without exposure, this ability can diminish, and later acquisition of tonal languages becomes more challenging.

Synaptic pruning is a neural mechanism where excess or rarely used synaptic connections are eliminated, while frequently used synapses are strengthened. It’s a process of refinement, ensuring the brain is efficient in processing information and not overloaded with redundant connections. This process is pivotal in shaping neural networks based on experiences and environmental input.

This "use-it-or-lose-it" principle is most active during early childhood and adolescence. In the first few years of life, the brain experiences an overproduction of synapses. Post this period of synaptogenesis, pruning begins, and it becomes especially prominent during the adolescent years.

If a child is exposed to particular linguistic sounds during the early years, the synaptic connections associated with recognizing and producing those sounds are strengthened. Conversely, the lack of exposure can lead to pruning of those specific pathways. This can explain why children who aren’t exposed to certain phonemes during early years find it hard to recognize or produce them later in life.

GABA (Gamma-Aminobutyric Acid) is the primary inhibitory neurotransmitter in the brain. It plays a crucial role in regulating neuronal excitability. GABAergic neurons and their maturation are central to the timing of critical periods, essentially serving as a molecular “brake” to control when these periods start and finish.

In the early stages of development, the brain is highly plastic, meaning it can change and adapt rapidly. As GABAergic circuits mature, they increase inhibitory signaling, reducing this high plasticity. For a critical period to open, a balance between excitatory and inhibitory signaling is necessary. Once the critical balance shifts towards inhibition, the plasticity decreases, signaling the end of the critical period.

The relationship between GABAergic inhibition and critical periods has been most extensively studied in the visual system. For example, kittens deprived of visual stimuli during a critical window will have lifelong visual deficits. This is tied to the maturation of GABAergic circuits. By manipulating these circuits, researchers can artificially extend or shorten the critical period in experimental settings.

While the visual system provides a clear example, the principles can be extrapolated to language. The balance between excitatory and inhibitory signaling in language-associated regions of the brain can influence the ease with which new languages or phonemes are acquired. Once the balance shifts towards inhibition, acquiring a new language or achieving native-like pronunciation becomes more challenging.