If you moved from the United States to France as a child, you’d likely become fluent in French in a short period, but if you moved to France as an adult you might never become fluent. This difference in the capacity for learning language exists because there are sensitive periods in development when the brain is particularly plastic and able to receive and retain information with greater efficacy.
A well-established field of sensitive-period biology seeks to explain how people learn to speak, how birds learn to sing, and how our sensory systems wire up, among other things. The field has been particularly successful in explaining how the brain coordinates the information streaming in from the two eyes to allow binocular vision, useful for depth perception. In the last century, it was discovered that when a person was born with a “lazy” eye or had their vision clouded in one eye by a cataract, their binocular vision would be impaired for life. However, if a correction was made in early life, then the brain and binocular vision could recover to develop normally. This human phenomenon can be modeled in rodents by closing one eye in early life. Extensive study of this model provides the basis for our understanding of the cellular mechanisms regulating sensitive periods across the cortical regions of the brain.
You might conclude that early experiences are simply the most powerful; the juvenile brain is, in general, more plastic than the adult brain. However, the often glossed-over details show that the younger brain is not always more sensitive to experience than the older brain. When the biology can be studied in carefully controlled laboratory experiments, we find that periods of greater sensitivity are often delayed, perhaps even timed, until the incoming experience is appropriate to sculpt the brain. For example, the peak of sensitive-period plasticity for the development of binocular vision occurs about a month after birth in rodent brains, which is more than a week after eye opening. Scientists are still working on the why and how. Nonetheless, it’s clear that the brain can and does hold highly sensitive plasticity under wraps and then unveils it when appropriate. It’s thought that years of evolution have sculpted brain development to be not only experience-dependent but also carefully timed, such that it is experience-expectant—that is, dormant until needed.
What this means for the big picture is that human development probably involves a staggered sequence of undiscovered sensitive periods stretching late into the second or even third decade of life. Hence, we should be on the lookout for “sleeper” sensitive periods. For example, there may be teenage social-sensitive periods when we learn to interact with peers, or cognitive-sensitive periods when we sculpt our decision-making style. These sensitive periods might be timed to overlap with important transitions, as when we leave our parents’ protection to explore the world, or go through puberty, or become a parent. The boundaries may be sharp, triggered by events like puberty onset, or gradual slopes that rise and fall with age and experience. We don’t yet know when, where, and how these more subtle cognitive- and emotion-sensitive periods may work.
It may be easier to see evidence of complex sensitive periods in development in other species. Life-history ecologists have identified a wide array of nonhuman species that adapt their phenotype according to the sampled statistics of their particular environment. For example, if developing crickets are exposed to spiders in the environment, then the adult crickets are better at surviving where there are spiders. If food is scarce during development for a species of mite, then an alternate body type and foraging strategy may be used in adulthood. Less is known about the neurobiology of these phenomena in these non-mammalian species.
Sensitive-period biology may in future provide important insights into understanding and preventing mental illness. Sensitive-period plasticity enables adaptation to experience, but this adaptation doesn’t ensure an optimal or even favorable outcome. For example, negative experience during a sensitive period could generate a persistent negative bias in the processing of events, potentially leading to mental illness. It’s known that negative experiences do have different effects at different ages in humans and animal models, but we don’t know exactly when it’s better or worse to endure negative experience and why.
Sensitive-period biology may also influence behaviors commonly thought to make up a person’s personality. Experience at different times might alter someone’s appetite for risk or tolerance for delayed gratification, or kindle an interest in music. The experience of poverty, even during a brief window of development, could alter the brain and behavior for a lifetime. When money is available for educational or public-health intervention, knowledge of sensitive-period biology should become a central aspect of strategy. If sleeper sensitive periods exist in late childhood or teenage years, these periods may become more efficient target years.