Book excerpt: Perception is the first step in the whole of the cognitive processes (attention, learning, memory, categorization, imagery, intuition, inference, comprehension, thought, judgement, expression) which culminate in the reasoning activity and to which emotions make a contribution. The production of perception representations is correlated with the perception events. Such perception representations occur by means of the contribution of two kinds of factors: sensory signals which reproduce the spatio-temporal characteristics of the receptor modifications, and interpretation of the intrinsic ambiguity of such signals by means of unconscious inferences. Various interactions intervene between bottom-up signals from peripheral receptors and top-down signals from higher centres.