The selective loss of unused connections among brain cells is called

Pruning of brain cells.

Explanation:

Basically, pruning is the removing of unwanted branches from a tree or in this case unwanted or unused connection removal among brain cells to increase their activity and speed.

Pruning of brain cells usually occurs in childhood and adolescence by removing of the unnecessary signals to increase the brain activity.

Pruning

Explanation:

Pruning or synaptic pruning is brain's way of clearing unnecessary signals that are no longer needed. It mostly occur in childhood and adolescence because brain want to get faster by avoiding unused impulses in Synapses. Synapse is the space between neurons through which signals are being transferred.