In this book the authors relate and discuss the idea that perceptual processes can be considered at many levels. A phenomenon that appears at one level may not be the same as a superficially similar phenomenon that appears at a different level. For example "induced motion" can be analyzed in terms of eye movements or at the retinal level or at a much higher cognitive level: how do these analyses fit together? The concept of levels also makes us think of the flow of information between levels, which leads to a consideration of the roles of top-down and bottom-up (or feed-forward, feed-back)...