Many topics have inspired significant amounts of neuroimaging research in the recent years, and the study of mental imagery was one of the earliest that elicited a thorough empirical investigation. Twenty years later, the goal of understanding this pervasive but elusive phenomenon continues to motivate a number of sustained research programs on the part of cognitive psychologists and neuroscientists. The issues at stake are easy to formulate, even if the answers sometimes may be difficult to obtain:
* Which parts of the human brain are active when a person generates a memory image of an...