Perceptual defence indicates to strategies that we use to avoid the cognitive dissonance. This strategy protects or defends our perception when we get challenged from experiences and information that are new to an individual.
Cognitive dissonances is a theory thats used to explain this process. An individual can feel discomfort when an experience or behaviour is in a disagreement with their belief. So a person says or does something and tend to advocate it instead of admitting that they were wrong or unwise.
We tend to change our perceptions little bit at a time instead of all at once. The easy perpetual change is accommodation to find a way to put the new perception in our old belief system.