Experimental study of apparent behaviour. Fritz Heider & Marianne Simmel. 1944
Movement is powerful way of establishing meaning and relationships between visual objects. In the example above, agency and a number of complex interactions are established through movement of simple shapes.
Motion perception is a complex theme, but some aspects are explained in simple terms in Gestalt psychology under “The Law of Common Fate”. This law stipulates that objects that move together, are perceived as single whole or group, something that is often utilised in GUI design.