The important principle of language development states that if children are not exposed to any human language before a certain period, their language abilities will never fully develop. This is known as .

sensitivity period

Explanation:

One of the important factor in the language learning is sensitivity period. It the time period during which brain  readily and easily and most efficiently  grasp any language learning. Brain can accept and retain vocabulary and grammar rules required for the learning. It has been found that ability to learn language decreases with the age . It is high at the infant stage of child and hence sensitivity period gained prominence in the study of language learning.

In the problem it says that there is a certain period during which if children are not exposed to human language, then their language ability will not develop fully. Hence , it is the sensitivity period about which it is referred in the question based on above definition of sensitivity period.

permafrost layer, it exists beneath a layer of soil, rock or sediment.