Gibson believes in the direct theories of perception which he used the theory of bottom-up processing to explain visual illusions whereas Gregory believes in the indirect theories of perception and he used the theory of top-down processing to explain visual illusions. The bottom-up are based on the assumption that we work upwards in our analysis of the visual world form basic sensory inputs at the bottom level towards the higher, more cognitive levels of the brain. The top-down processing theories are based on the assumption that we can only perceive our visual world accurately if we use stored knowledge and problem-solving skills. Thus, there are differences between their theories of perception.
Gregory’s indirect theory of perception and Gibson’s direction theory of perception had led to the debate of nature-nurture. This is a big debate in Psychology whether perception is determined by genetics as proposed by Gibson’s theory or whether it is learnt or determined by upbringing and social context as proposed by Gregory’s theory. On the one hand, Nativists think that nature is more important factor which people are the products of their genetics and that we are born with certain behaviours. On the other hand, empiricists think that experience and nurture is the more important factor.