Critical Period Plasticity

The time duration when the human brain is susceptible to change (structural or representational), where there is reorganization at the cellular level is called the critical period. The consensus during the early 1900s was the theory of a rigid brain, but the plasticity theory was accepted during the 1970s after the Nobel Prize-winning work of Hubel and Weisel. However, it was believed that the critical period is a rigid time window beyond which there was no scope for modification, there was much interest in studying critical periods and the malleability of the brain beyond that period. Although well-studied animals, it could not be done in humans primarily because it required participants who were congenitally blind and received the ability to see later in life.

India around the early 2000s had almost a quarter of the world’s blind population. It was believed that any congenital illness could not be treated beyond the critical period as there was no scope for recovery and as a result, no attempt of treatment was made to the millions of young congenitally blind Indians. Project Prakash by Dr Pawan Sinha and his research team at MIT aimed at providing ophthalmologic-surgical care for treatable congenital blindness in the Indian subcontinent. The treated patients necessarily were the perfect cohort to study critical period plasticity and the associated structural and functional changes in the brain.

The studies mostly showed that the critical period time window is not restricted to early postnatal life as thought earlier but varied extending up to many years. These findings were also replicated in few other studies from other parts of the world. After the restoration of vision following several years of blindness, the patients displayed visual perception of form, object and movement detection. Although perception was weak at first, they showed remarkable improvement with experience. Interestingly, they were relatively better with movement detection immediately after vision was restored. This was probably because motion perception develops the earliest in the developmental course, with little experience. The motion perception area of the brain may be preserved even in the absence of relevant stimuli which necessitates the question if the ability to perceive motion is innate?

The comprehensive findings of these studies suggested that the visual system is experience-dependent, i.e., the development of the visual system is dependent on the input received by the sensory organ, in this case, the eyes.