Although Eric Lenneberg (1967) is considered as father of the idea of Critical Period Hypothesis and the one who popularized this phenomenon, Penfield and Roberts (1959) are the one who proposed the same. In the beginning of their studies, these two neurosurgeons were exploring the neuroscience of language, concluding that it was dominant in the left hemisphere of the brain. Penfield and Roberts’ research was focused on the linguistic performance of individuals with brain damage, rather than on linguistic performance of ‘normal’ individuals. Repeating Chomsky’s fact that all children are not born as tabula rasa or in other words no human being is born without any real innate language ability, Penfield and Roberts added the fact that children up to the age of nine can learn three languages thanks to the human’s ability to learn languages. The early children exposure to languages activates a kind of reflex in their brain which helps them to switch one language with another without the need to translate the same into their native language. Lenneberg supports the theory given by Penfield and Roberts, felling free to develop the hypothesis for the critical period, and claiming that the critical period for language acquisition ends around puberty and after this period the acquisition of the first language will be impossible. He accepted the Penfield and Roberts’s ideas that neurological mechanisms are responsible for the maturation change in the language learning abilities, connecting the same with the brain lateralization and left hemispherical specialization for language around thirteen.
According to Lenneberg up to the age of thirteen the language learning functions are present in both hemispheres. Later or more accurate at the beginning of the puberty the cerebral functions of the hemispheres separate, and make the language acquisition very difficult. With his studies Lenneberg is pointing out on the idea that there are certain ages that are appropriate for learning a language, somehow demonstrating the existence of the Critical Period Hypothesis and the difficulty to acquire a second language during and after it.