Perceptual Illusion

Pareidolia, or the phenomenon of recognizing patterns where none exists. This is strong tied to the Rorschach test, where subjects' perceptions of relatively symmetric inkblots are recorded and analysed using psychological interpretation, complex algorithms, or both (as described by Wikipedia). It is more effective when patients are unwilling to speak directly on the topic that concerns them. 

The human mind is very adept at finding patterns in a given stimulus, so one can often imagine seeing certain common shapes in clouds, or faces in a coincidental arrangement of features. This is what pareidolia is about.

The Tritone paradox, is an illusion where two Shepard tones (a series of notes which seems to infinitely ascend or descend) are separated by tritone intervals.  It is heard by some people as ascending and descending by others.