When neuronal connections that are used become stronger and faster, and the neuronal connections that are not used wither away, what is happening?

The answer is: synaptic pruning Synaptic pruning, which includes both axon and dendrite pruning, is the process of synapse elimination that occurs between early childhood and the onset of puberty in many mammals, including humans. Pruning starts near the time of birth and is completed by the time of sexual maturation in humans.