Life experiences impact brain development and subsequent behavior. Sensitive and critical periods are developmental periods that are especially pertinent in shaping neural and behavioral outcomes. Sensitive periods refer to the developmental time windows during which experiences have an especially strong impact on brain organization. Of note, while similar experiences can still affect the brain outside of these sensitive periods, the consequences for brain reorganization will not be as strong. Critical periods refer to the limited time windows during which experiences, or lack thereof, have lasting effects on brain function and behavior (Knudsen, 2004). Indeed, disruptions during critical periods due to atypical experiences or adversity may lead to irreversible changes to brain structure. While sensitive and critical periods both share heightened neuroplasticity, sensitive periods are a broad time window during which experience may shape neural circuitry, whereas critical periods are a special class of sensitive periods that result in potentially irreversible changes in brain function (Knudsen, 2004).

Sensitive Periods of Development

Early childhood is a time that the brain is especially malleable and adaptive to environmental inputs. Early life experiences have a profound impact on how brain networks are organized and develop. For example, language acquisition occurs during early childhood. Research shows a close relationship between the age of exposure to a language and proficiency in that language—peak proficiency is far more likely for those who were exposed to that language in early childhood (Newport et al., 2001). This is especially pertinent for learning a second language. A seminal study examined second language acquisition in native Chinese or Korean speakers who moved to the United States and learned English at different ages (Johnson & Newport, 1989). Results indicated that children who began learning the second language (English) before age 7 were able to reach proficiency akin to native English speakers; children arriving between age 7 and puberty were less proficient; and after puberty an individual’s second language proficiency is likely to remain low (Figure 5.13). These findings support a brain maturation account, such that language-learning ability gradually declines and ultimately flattens as the brain matures. Importantly, this is not to say that learning a second language is impossible after brain maturation; but lower neuroplasticity after this sensitive period contributes to slower second language learning. The fact that second languages are still able to be learned throughout the lifespan, albeit at a slower rate, further exemplifies how age-related differences in second language learning reflect a sensitive, rather than critical, period in development. In summary, children may be better equipped to learn a second language during this sensitive period due to the heightened brain malleability.

Critical Periods of Development

Critical periods of development, during which exposure to environmental input causes irreversible changes to brain function and structure also occur during early childhood. Critical periods can be exemplified in sensory development and first language learning. Past experiments with animals have shown that sensory deprivation during infancy (e.g., an animal is deprived of sight or sound) can have lasting and irreversible consequences on their brain development (Hubel & Wiesel, 1970). For instance, in research with animals, visual deprivation to one eye during a critical period causes lasting vision loss due to decreased cortical neuron spiking responses to the deprived eye (Gordon & Stryker, 1996). In response to visual deprivation to one eye during a critical period, the brain reorganizes and prioritizes visual input from the non-deprived eye.

The brain’s adaptive nature can also be seen in individuals who are born blind or deaf and as a result may rely on other sensory systems. For example, in humans, the occipital cortex is typically involved in visual perception. In individuals with early blindness (who become blind during the first few years of life), the occipital cortex shifts from processing visual input to processing other sensory-related information, such as tactile and auditory sensations (Voss, 2013). This adaptive process is known as cross-modal plasticity. Recent research indicates that cross-modal plasticity may persist even after sensory functioning (i.e., vision) is restored, which may suggest that the manner in which the brain reorganizes itself during a critical period could persist throughout adulthood (Mowad et al., 2020).

Critical periods of development have also been discussed for first language acquisition. In the early 1970s, the tragic story of Genie, an adolescent girl who for most of her childhood experienced severe isolation and neglect, caught the world by storm. Upon encountering Genie, it was determined that she was unable to verbally communicate through language. A research team began working with Genie to study her linguistic development and ultimately concluded that, while Genie showed initial progress in learning speech production and grammatical structure, her language proficiency remained atypical and severely impaired (Curtiss, 1974). A more recent study assessed children who did not receive the required language input during the first year of life, due to either isolation or hearing difficulties, and found that those children later showed severe language syntax impairments (Friedmann & Rusou, 2015). In sum, the absence of key environmental inputs, especially during critical periods in early childhood, may be particularly detrimental to subsequent brain development.

Adolescence as a sensitive period of development

Adolescence, the phase of life between childhood and adulthood, which is generally considered ages 10-24 years, is marked by significant brain and behavioral changes. As a result of the ubiquitous social, cognitive, and emotional changes during adolescence, this stage of development is now widely considered to be a sensitive period of development.

How does brain development during adolescence shape behavior? Substantial neuroimaging research has shown that the frontal lobes, which include regions of the brain involved in executive function, such as the prefrontal cortex, are late-developing and undergo significant maturation that continues well into adolescence (Fuster et al., 2002; Casey et al., 1997; Giedd, 2004). Parallel to these brain development findings, prior work also suggests that adolescence is marked by increased sensation-seeking and risk-taking behaviors (Spear, 2000). Neuroscientists have since suggested that the increase in risk-taking during adolescence may emerge as a result of delayed development of self-regulatory capabilities, which may arise as the product of an interaction between heightened sensation-seeking and an immature executive function system that is not yet able to modulate reward-seeking impulses (Steinberg et al., 2004).