A phase during which brain cell connections become more plastic and susceptible to the effects of a particular kind of experience. These connections, called synapses, can form or strengthen more easily during this period. Synaptic connections usually stabilize after this period of time and become more difficult to change. During a critical period of brain development, a new skill is usually formed with some experience. If the experience is not available during this time, then after it ends, it becomes much more difficult or even impossible to acquire the skill. This is proven in sensory systems such as vision and hearing.