Neuroplasticity plays a major part. The brain changes or rewires itself, neuron want to fire and they get healthier from firing and so change will occur in that direction when a person is born without sight or hearing as the cells that are normally use for the missing sense find a way to get some action(electrically speaking and stay healthy).
https://en.wikipedia.org/wiki/Neuroplasticity
Neuroplasticity, also known as brain plasticity or neural plasticity, is an umbrella term that describes lasting change to the brain throughout an individual's life course. The term gained prominence in the latter half of the 20th century, when new research[1][2] showed that many aspects of the brain can be altered (or are "plastic") even into adulthood.[3] This notion is in contrast with the previous scientific consensus that the brain develops during a critical period in early childhood and then remains relatively unchanged (or "static").[4]
Neuroplasticity can be observed at multiple scales, from microscopic changes in individual neurons to larger-scale changes such as cortical remapping in response to injury. However, cortical remapping is more extensive early in development.[5] Behavior, environmental stimuli, thought, and emotions may also cause neuroplastic change through activity-dependent plasticity, which has significant implications for healthy development, learning, memory, and recovery from brain damage.[5][6][7]
At the single cell level, synaptic plasticity refers to changes in the connections between neurons, whereas non-synaptic plasticity refers to changes in their intrinsic excitability.