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Learning during development is regulated by unexpected brain region

During childhood, the brain goes through critical periods in which its learning ability for specific skills and functions is strongly increased. It is assumed that the beginning and ending of these critical periods are regulated in the cortex, the outermost layer of the brain. However, scientists from the Netherlands Institute for Neuroscience discovered that a structure deep in the brain also plays a crucial role in the regulation of these critical periods. These findings, published today in the leading journal Nature Neuroscience, have important implications for understanding developmental problems ranging from a lazy eye to intellectual disability.

Critical periods

We can only flawlessly learn skills and functions such as speaking a language or seeing in 3D through binocular vision during critical periods of development. When these developmental forms of learning fail, lifelong problems arise.

Scientists have been investigating the mechanisms by which critical periods are switched on and off in the hope to extend or reopen them for the treatment of developmental problems. Half a century of research on how the brain learns to integrate visual inputs from the two eyes has provided important insights in critical period regulation, leading to the conclusion that it occurs within the cortex. Neuroscientist Christiaan Levelt and his team now made the surprising discovery that a brain region that passes on input from the eyes to the cortex also plays a crucial role in opening the critical period of binocular vision.

Using electrophysiological recordings in genetically modified mice, they showed that this brain region, known as the thalamus, contains inhibitory neurons that regulate how efficiently the brain learns to integrate binocular inputs. Levelt: “To improve developmental problems resulting in learning problems during critical periods, reinstating flexibility in the visual cortex may not be sufficient. Scientists and clinicians should not limit themselves to studying cortical deficits alone. They should also focus on the thalamus and the way it preprocesses information before it enters the cortex.”

Further implications

The study may also provide some hope for people with albinism, who often have limited binocular vision due to misrouting of inputs from the eyes to the thalamus. Levelt’s team found that in contrast to what is generally assumed, plasticity of binocular vision also occurs in the thalamus itself, suggesting that this might be improved in children with albinism through training.

 

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Plasticity of the neocortex is crucial for us to learn and adapt to our environment. Once tasks or functions are learned, the brain can carry them out very efficiently, in a routine-like fashion. However, learning and carrying out routine functions do not go hand in hand. During development the brain is highly malleable, but processes information rather slowly and erratically. Vice versa, when we perform routine tasks, little learning occurs and we ignore many inputs. This situation can suddenly change when a routine procedure results in an unexpected outcome. We rapidly become aware of additional circumstances and learn what caused the unexpected result.

Recent evidence, also from our laboratory, suggests that these increases in plasticity levels during critical periods of development or in response to reinforcement signals are achieved by a temporary reduction in cortical inhibition. Possibly, high levels of inhibition increase performance of neuronal networks by suppressing inputs that are irrelevant for the execution of routine tasks. Reduced inhibition may support learning by allowing such inputs to be taken into consideration to solve a novel challenge.

Using the mouse visual cortex as a model, the Levelt lab studies how inhibition regulates cortical plasticity levels at the right time. To achieve this goal the lab employs a combination of state-of-the art two-photon microscopy, electrophysiology, optogenetics and gene manipulation.

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