By Nicole Lim, Senior Assistant Director, Communications & External Relations
From birth, a baby’s brain is primed for development and learning, much of which happens during ‘critical periods’. These periods or windows have become the focus of intense study in recent years in attempts to mitigate the long-term impact of sensory deprivation early in life and may one day even lead to new interventions that enhance learning.
Acting Director for NTU’s Centre for Research & Development on Learning and LKCMedicine Professor of Psychology Annabel Chen, part of whose work focuses on the neuroscience of learning and education, said that research into critical periods can largely be divided into two categories. “First is to understand what goes on in the brain during these periods that helps the animal or child to learn so well and fast. Second is the critical window itself, when does it occur and can it be prolonged or stimulated to occur again later in life or after injury to the brain for recovery,” said Prof Chen.
In a paper published recently in Scientific Reports, scientists at LKCMedicine have taken another step towards the latter goal by defining, for the first time, the critical period for the system in charge of touch.
Led by LKCMedicine Professor of Neuroscience & Mental Health George Augustine, the team studied mice, which have a particularly well developed sense of touch, teasing apart the function of individual circuits in the somatosensory cortex using a laser light, a method known as optogenetics.
Prof Augustine said, “Critical periods are critical because they are the times when our brains are optimised for learning. Thus, there has been an intense amount of interest in understanding what goes on during these critical periods, to better understand what permits our brains to learn so well during critical periods. Understanding what’s going on in the brain on a circuit level gives us important new insights into the mechanisms that underlie learning anything from what our mother’s face looks like to speaking new languages.”
During the critical period of somatosensory development, the brain is particularly receptive to tactile feedback from our surroundings. Neuronal networks extract and process the information coming from our senses, compare that to what’s stored in memory, and then translate this into a response. Responses are fine-tuned like an old-fashioned radio signal using two dials – excitatory neurons, which trigger a response, and inhibitory neurons, which suppress it. The inhibitory circuits, in particular, have been thought to play a key role, a hypothesis the team put to the test.
Optogenetic maps (colours) of circuits between interneurons and excitatory neurons in the somatosensory cortex. Circuits in the somatosensory cortex of the mice were much more developed on the control side compared with the side where the whiskers had been removed (Source: Scientific Reports)
Removing one set of whiskers of newborn mice at different ages (at birth, three, seven, 14 and 21 days) allowed the researchers to confirm that experience shaped inhibitory circuits as well as the extent to which they were influenced by comparing signals from the side with and the one without whiskers in the same mouse. They found that the critical period in mice closes around 14 days after birth.
“Our results are the first to pin down the precise time that inhibitory circuitry is sculpted by sensory experience during the critical period. This paves the way for deeper understanding of how our brain circuitry learns more efficiently during the critical period,” said Prof Augustine.
The team is now looking to determine the molecular mechanisms that underlie these experience-dependent changes. The aim is to identify ways in which the critical period can be extended or maybe even re-opened.
We already know that the critical period is an important time for learning in humans. For example, it is why children learn languages much more readily than adults. Similar phenomena are also important for regaining motor skills after the loss of limbs, for example,” said Prof Augustine. “And eventually, we hope to find ways to lengthen our brain’s critical period by manipulating gene expression.
Being able to reopen critical period windows could allow better treatment for people whose senses are either impaired from birth or who may have lost the use of a limb or eye. More generally, it could create opportunities for adults to improve their learning skills after their critical periods have closed, for example, to efficiently learn a new language long after they have passed the age for optimal language learning, said Prof Augustine.
Commenting on these findings, Prof Chen, who was not involved in this study, said, that it would be a real breakthrough if we could identify a mechanism that can extend this window and translate this to humans. She said, “For example, if we can somehow prolong the language learning window in children, we could have more time to ameliorate mechanisms of reading disabilities and these children could still catch up faster to their typical peers, or if we can stimulate such windows again after brain injury during rehabilitation to speed up the recovery process in adults. Although translation of this research may take some time, it is exciting that we have a clearer framework now with Prof Augustine’s work.”