The hippocampus has been associated with building context-specific memories. This latter form of memory can be contrasted with decontextualized memories about regularities across specific experiences. Using fMRI we investigated the neural correlates of both types of memories. Volunteers had to learn conjunctions between objects and positions. In an invariant learning condition, positions were held constant, enabling subjects to extract spatial regularities across trials. In a context-specific condition objects and positions were variable across trials. Performance increase in the invariant condition was paralleled by an increase of inferior middle frontal gyrus (iMFG) and a decrease of hippocampal activity. Conversely, in the context-specific condition the hippocampus was activated continously. These data suggest that the hippocampus is critically involved when information is represented in a context-specific way by binding variable objects to variable positions, whereas the iMFG mediates the extraction and maintenance of spatial regularities across episodes.
By increasing the number of object-position conjunctions and the duration of the learning phase in behavioral follow-up studies, we could show that performance increase within blocks was pronounced in an early phase of the experiment and diminished at the end. These data suggest that learning regularities might be based on two distinct mechanisms: A process operating within blocks and a process operating across blocks. A follow-up fMRI experiment will address two questions: (1) Are different brain regions involved in the proposed two learning mechanisms and (2) by contrasting a spatial and an object invariance condition, is the extraction of regularities a domain-general or a domain-specific neural mechanism?