The "bottom-up" and "top-down" components of the hallucinatory phenomenon

This chapter provides and overview of the literature on bottom-up and top-down processes that have been suggested as mechanisms in the generation and/or perpetuation of hallucinations, both in the context of disorders, and in otherwise health individuals. We start from the bottom-up, and first consider the case of abnormal perceptual pathway to consider the impact of attentional mechanisms on perception, and discuss these in the context models which integrate sensory and attentional deficits to explain to occurrence of hallucinations, such as the recent Perception and Attention Deficit model. Finally, we turn to theoretical frameworks that are implicitly based on Bayesian inference theory or perception. Central to these models is the concept that top-down factors such as expectations and prior world-knowledge greatly influence our perception. We also consider experimental evidence for the hypothesis that hallucinations are caused by an imbalance between top-down influences and bottom-up processing.