A window of opportunity for learning

February 14th, 2022

Written by: Sophie Liebergall

Have you ever tried to learn a foreign language? Was it difficult? My guess would be yes, even in the age of language learning apps, podcasts, and online tutors. Mastering a new language as an adult requires thousands of hours of memorizing grammar rules, vocabulary, and usually some degree of language immersion. And even when adults achieve full mastery of the grammar of a new language and speak that language fluently for many years, they are almost always perceived as having a foreign accent to native speakers.

In contrast to the challenge of learning a new language, we don’t generally send babies to classes to learn their native language (or languages, for those with the fortune of being raised as bi- or multilingual). For most people, learning their native language is a seamless process that requires very little real studying. In fact, you likely didn’t explicitly learn the grammar rules of your own native language until you tried to learn a foreign language!

So why is it so much more difficult to learn a language as an adult than as a child? A long-standing hypothesis in the fields of linguistics and developmental psychology proposes that we have a critical period for learning language. According to this theory, there is a narrow window during our childhood development during which our brains are much more open to learning a language and more easily achieve full fluency.¹

But critical periods have a role far more significant than making your high school German class a real drag… In fact, you also have critical periods for gaining the ability to use the different senses like vision and hearing, as well as for learning other, more complex skills. “Missing” the critical period for a certain skill can have meaningful and sometimes permanent consequences on your ability to learn that skill in the future.

What is a critical period?

A critical period is a window of time during development when the brain is especially sensitive to external experience.² Critical periods are associated with high levels of plasticity, which is the brain’s ability to flexibly reorganize the connections between different brain cells.³ This plasticity is necessary for the brain to be able to flexibly respond to incoming information as it learns to perform new functions during critical periods. (Learn more about the ability for the brain to flexibly remap itself in this PennNeuroKnow post!) Critical periods have been observed in animals ranging from birds to apes, which has allowed us to investigate this developmental phenomenon in animals that are much easier than humans to study in the laboratory.

It is important to note that there is no single critical period for learning all of the brain’s functions. Instead, each of the brain’s functions has its own critical period during which experience is necessary to learn that function.⁴ Though all of the critical periods occur sometime between birth and puberty, the exact timing of critical periods for different brain functions in humans remains controversial. Nevertheless, scientists have been able to gain a more detailed understanding of critical period timing in animals with less complex brains like mice (Figure 1).

The most well studied and understood critical period is the critical period for vision. Scientists have learned a great deal about the critical period for vision by studying a condition called amblyopia. Amblyopia occurs when the visual input to one eye is impaired during early childhood.⁵ This can be a result of the eye having trouble focusing, aligning with the other eye, or having something that clouds the eye’s vision like a cataract. If the child doesn’t get treatment before the age of five, their brain will always have trouble processing visual input to the previously impaired eye, even if the eye itself is working totally fine later in life.

Scientists have been able to simulate amblyopia in research animals by blocking vision to one of the animal’s eyes during the critical period for vision (usually by closing the lid or using an eye patch). This has allowed us to gain a better understanding of how exactly the brain responds to sensory experience during critical periods. Work done all the way back in the 1960s by Hubel and Wiesel, which went on to win the Nobel Prize in Physiology or Medicine, showed that the part of the brain that is primarily responsible for processing visual information is actually organized in a striped pattern, with alternating stripes devoted to the left and right eyes (Figure 2). Hubel and Wiesel observed that if you block vision in one of the animal’s eyes during the critical period for vision, even after you unblock the eye, the organization of the stripes in the visual processing center in the brain is permanently altered.⁶ Essentially, the unblocked eye completely takes over the visual real estate in the brain during the critical period.

What controls the timing of critical periods?

Studies of the critical period for vision have shown that the brain increases plasticity during the critical period, and that this increased plasticity makes the brain especially sensitive to the external experience. But scientists are still in the process of trying to understand exactly how the brain increases plasticity during the critical period and how the brain decides when the critical period starts and ends.

One factor that scientists have found to be very important for the regulation of critical periods is the action of inhibitory neurons. The brain has two major types of brain cells, or neurons: excitatory neurons, whose primary function is to activate other cells, and inhibitory neurons, whose primary function is to silence other cells. Interestingly, the maturation of inhibitory neurons lags behind the maturation of excitatory neurons.⁷ And scientists noticed that the timing of the maturation of inhibitory neurons in a certain brain region occurs at the beginning of the critical period for development of that brain region. For example, the inhibitory neurons in the vision control center of the brain mature at the beginning of the critical period for vision.⁴

In support of the importance of inhibitory neurons in regulating critical periods, scientists have found that if you apply drugs to the brain that block the action of inhibitory neurons, you can delay the beginning of the critical period.⁸ On the other hand, if you apply drugs to the brain that enhance the activity of inhibitory neurons, you can jumpstart the beginning of the critical period. Perhaps most strikingly, if you harvest immature inhibitory neurons from a mouse embryo and transplant these neurons into the brain of an adult mouse, you can actually induce a new critical period in the adult mouse.⁹ In summary, we still have a lot to learn about the factors that control critical periods, but there seems to be a special role for inhibitory neurons, making them a promising target for future study.

 Why is it important for us to learn more about critical periods?

The critical period is an important concept that is crucial to our understanding of brain development and how we acquire the ability to execute complex brain functions such as visual perception and language. But studying critical periods may serve a purpose beyond strengthening our grasp of how the healthy brain works. There is mounting evidence that many brain diseases may be the result of errors in regulation of critical periods. For example, some researchers suspect that autism may actually be a disease of problems with critical periods. This theory is supported by evidence that many patients with autism have abnormal activity of inhibitory neurons. Additionally, many models of autism in mice show changes in the timing and/or duration of their critical periods.¹⁰

Though this still remains the stuff of science fiction, one could imagine a future in which we can manipulate critical period timing to prevent conditions such as amblyopia, or to treat neurodevelopmental disorders like autism and psychiatric disorders like depression. Perhaps one day we could even harness the extreme plasticity of critical periods to rapidly learn a new language before a trip overseas!

References:

1.         Lenneberg, E. H. Biological foundations of language. (R.E. Krieger, 1984).

2.         Hensch, T. K. Critical period plasticity in local cortical circuits. Nat Rev Neurosci 6, 877–888 (2005).

3.         Levelt, C. N. & Hübener, M. Critical-period plasticity in the visual cortex. Annu Rev Neurosci 35, 309–330 (2012).

4.         Reh, R. K. et al. Critical period regulation across multiple timescales. Proceedings of the National Academy of Sciences 117, 23242–23251 (2020).

5.         Jefferis, J. M., Connor, A. J. & Clarke, M. P. Amblyopia. BMJ 351, h5811 (2015).

6.         Hubel, D. H. & Wiesel, T. N. Binocular interaction in striate cortex of kittens reared with artificial squint. Journal of Neurophysiology 28, 1041–1059 (1965).

7.         Larimer, P. & Hasenstaub, A. R. Chapter 19 – Functional maturation of neocortical inhibitory interneurons. in Synapse Development and Maturation (eds. Rubenstein, J. et al.) 423–442 (Academic Press, 2020). doi:10.1016/B978-0-12-823672-7.00019-3.

8.         Hensch, T. K. & Stryker, M. P. Columnar architecture sculpted by GABA circuits in developing cat visual cortex. Science 303, 1678–1681 (2004).

9.         Davis, M. F. et al. Inhibitory Neuron Transplantation into Adult Visual Cortex Creates a New Critical Period that Rescues Impaired Vision. Neuron 86, 1055–1066 (2015).

10.      LeBlanc, J. J. & Fagiolini, M. Autism: a ‘critical period’ disorder? Neural Plast 2011, 921680 (2011).

Cover Photo from PxHere

Figure 1: Made with Adobe Illustrator. Adapted from Reh et al., “Critical period regulation across different time scales,” PNAS, 2020.

Figure 2:  Made with Adobe Illustrator. Adapted from Hubel DH and Wiesel TN (1977) Ferrier lecture: Functional architecture of macaque monkey visual cortex. Proceedings of the Royal Society of London, Series B: Containing Papers of a Biological Character 198: 1–59.