How children learn is very different from how adults learn. Who has not envied the speed and the seemingly ease at which a child acquires a new language? The ability of our brain to create an auditory map of the outside world develops largely during childhood.
For example, children raised in an English-speaking environment will easily distinguish between the phonemes /la/ and /ra/. By contrast, children growing up in Japan will find it increasingly difficult to distinguish these two phonemes as they are not present in the Japanese language. Therefore, their brain has not been sculpted accordingly. The shaping of the neuronal circuits responsible for the comprehension of such perceptual differences is taking place during time windows of enhanced plasticity; they are known as critical periods for plasticity.
Critical periods are feature specific
Previous studies have shown that the brain is very much shaped by the sounds that it is exposed to just after its starts to have the ability to hear — but only if these sounds are the simplest ones, containing only one single frequency component (Barkat et al., 2011). What remained unclear is whether exposure to more complex sounds could equally change the brain and potentially even at a later stage. Complex sounds are for example frequency sweeps, where the frequency of the sound is not constant as in the simplest sounds, but is increasing (upward sweeps) or decreasing (downward sweeps) over time. The ability to perceive such sweeps is important for the understanding of speech and music in general.
To address this question, my research group in the Brain & Sound Lab at the University of Basel used a whole set of methods, including multi-channel electrophysiology, optogenetics, immunohistochemistry and behavioral assays. What we learned is that exposure to frequency sweeps can indeed sculpt the brain, but only if the exposure happens much later in development and well into adolescence, corresponding to people as old as 16 years of age.
This demonstrates that the brain is malleable to different features in distinct time windows. Or, to put it differently, the brain has numerous critical periods that are feature specific and asynchronous (Bhumika et al., 2020). By identifying the mechanisms that control such critical periods, we can control brain plasticity in very specific ways, independently of the postnatal development program of the sensory cortices (Nakamura et al., 2020).
Reinstating plasticity in the adult brain
What could this knowledge be used for? Identifying the mechanisms allowing for such plasticity in a developing brain opens new avenues for translational research. Say, it could inspire new strategies for reinstating plasticity in adults suffering from hearing abnormalities. For instance, it is known from human studies that a poor judgment of frequency sweep direction leads to decreased linguistic and emotional judgment, such as what can be observed in autism. One could speculate whether activating the mechanisms responsible for the enhanced plasticity during the critical period for frequency sweeps could help autistic adults to increase their perception of emotions, solely by improving their hearing capability.
But also in other ways this knowledge could be relevant. Understanding that our auditory environment continuously modifies brain function well into adolescence might allow us to raise more general questions. For instance, we may ask whether the very noisy auditory environments of school classrooms, in which our children spend so many hours, are appropriate for proper brain development? If taken seriously, these kinds of concerns could have an impact on educational policies and potentially even influence infrastructural developments.
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