Anatomical segregation of component processes in an inductive inference task

Inductive inference underlies much of human cognition. The essential compon ent of induction is hypothesis selection based on some criterion of relevan ce. The purpose of this study was to determine the neural substrate of indu ctive inference, particularly hypothesis selection, using fMRI. Ten volunte ers were shown stimuli consisting of novel animals under two task condition s, and asked to judge whether all the animals in the set were the same type of animal. In one condition, subjects were given a rule that specified the criteria for "same type of animal". In the other condition, subjects had t o infer the rule without instruction. The two conditions were further facto red into easy and difficult components. Rule inference was specifically ass ociated with bilateral hippocampal activation while the task by difficulty interaction was associated with activation in right lateral orbital prefron tal cortex. We interpret the former in terms of semantic encoding of novel stimuli, and the latter in terms of hypothesis selection. Thus, we show an anatomical dissociation between task implementation and task difficulty tha t may correspond to a critical psychological distinction in the processes n ecessary for inductive inference.