The psychologists asserted that with respect to perception, the whole is greater than the sum of the parts.

The given blank can be filled with Gestalt.

Explanation:

A school of thought, which looks at the mind of a human and behavior as a whole is known as Gestalt psychology. In making an attempt to produce the sense of the world surrounding us, the Gestalt psychologists suggest that one does not basically focus on every small constituent. However, the minds seem to perceive objects as part of the greater whole and as the constituents of more composite systems.  

This branch of psychology plays a prime function in the contemporary development of the study of human perception and sensation.