One shift in activation that occurs as children develop is from diffuse, larger areas to more focal, smaller areas. this shift is characterized by synaptic in which, areas of the brain not being used lose synaptic connections and those being used show an increase in connections.

pruning

Explanation:

Synapses are brain structures that permits a  nerve cell (neuron) to transmit an electrical or a chemical signal from one neuron to another neuron.

Synaptic pruning can be defined as a natural process that occurs in the brain during early childhood.  During synaptic pruning, the brain eliminates extra synapses, in which, areas of the brain not being used lose synaptic connections and those being used show an increase in connections.

Synaptic pruning is thought to be the brain’s way of removing connections in the brain that are no longer needed. Synaptic pruning is the body’s way of maintaining more efficient brain function as we grow older and learn new complex information.

i think the answer is

mass