Intrinsic motivation is a crucial mechanism for open-ended cognitive development since it is the driver of spontaneous exploration and curiosity. Yet, it has so far only been conceptualized in ad hoc manners in the epigenetic robotics com- munity. After reviewing different approaches to intrinsic motivation in psychology, this pa- per presents a unified definition of intrinsic mo- tivation, based on the theory of Daniel Berlyne. Based on this definition, we propose a landscape of types of computational approaches, making it possible to position existing and future models rel- ative to each other, and we show that important approaches are still to be explored.