We know that it’s the plasticity of our brain which allows us to learn n continuously adapt to ever-changing environments. But how sensitive is our brain to the plasticity of neurons and does the brain always remain plastic throughout its lifetime?
“Ut est rerum omnium magister usus” (Experience is the teacher of all things) — Julius Caesar
Experience exercise a powerful influence on the brain and, in turn, on behavior. We know that children, in particular, are very sensitive compared to adults to their surroundings and impact essential aspects of their personalities as they grow. To study these dynamics of brain plasticity we must understand the periods during which the brain is most sensitive to experience driving internal changes. When the impact of experience on the brain is strong during a limited period in development, this period is known as a sensitive period. Such periods allow the experience to instruct neural circuits to process or encode information in a way that is adaptive for the individual. When the experience provides information that is crucial for normal development and alters performance permanently, such sensitive periods are referred to as critical periods.
To visualize the plasticity dynamics, we can use a physics-based metaphor, called the “energy stability landscapes”. These so-called energy landscapes represent the “neural state space” i.e. the combinatorics of all possible states of neural architecture and their signals that the brain might represent. Now many different neural circuits and their signal patterns may be able to perform a particular function of a brain. But not all of them might be efficient in terms of energy expended and the accuracy achieved. So how does our brain is able to tune into a suitable representation out of the seemingly infinite neural state space, for achieving optimal performance? Of course through sheer experience and plasticity!
Plasticity represents the flexibility of the energy landscape metaphor we talked about. And Experience helps to achieve the optimal or “least cost” state in the overall neural state space.
For example, say for a particular foodstuff there is a binary state space of whether you will ingest or expel the food item depending on its taste and so on. Now initially before eating the item, the state space is in a dilemma, in an unstable energy state of the undecided outcome. Once you eat an item, the experience changes the landscape in such a way that either expel or swallow becomes the least cost or optimal state and thus achieves stability. More experience of eating the same or different foodstuff reinforces this neural state-space thus helping the brain achieve optimal performance.
Now, these metaphors help us to explain and understand the concept of brain plasticities at various time periods. But these are, well, metaphors in the end and one must be very careful when such metaphors work and when they might fail.