Perception- it is a way of conceiving things. Different people will differently understand and respond to what they see or feel via different senses.
Perception really matters and sometimes causes serious problems in HCI as its designers-(people that create GUI) can perceive things in a different way than their clients and actual users.
Visual Phenomena & Optical Illusions
Visual Phenomena & Optical Illusions
Visual Phenomena & Optical Illusions
Examples of optical illusions: