A basic tenet of the guidelines set forth by the Council of Europe with respect to housing is that the animal must be able to engage in its natural activities to the fullest possible extent. The rat must be able to move about and rear up so it can explore (see movie 1) and engage in social behaviour vis a vis its cage mates (see movie 2 on the page Health and activity).
Rats are capable of recognizing individual humans. Consequently, it is important that they are familiar with the people who handle and feed them.
Cage enrichment materials, such as tissues, paper, tubes or boxes are beneficial, particularly if the animals have grown up with them (see movie 2).