Multiple brain-memory systems: the whole does not equal the sum of its parts.

Most contemporary theories of memory are based on the assumption that memory can be divided into multiple psychological systems that are subserved by different neural substrates and that contribute to performance in a relatively independent manner. Although the study of individual memory systems has proved to be enormously useful, recent data increasingly point towards complex interactions between memory systems during performance of any given memory task. Three basic classes of interactions between different memory systems (competition, synergism and independence) are presented that appear to be congruent with the findings of many behavioral studies. Consideration of interactions among multiple memory systems will enhance our current understanding of memory by encouraging the view that memory systems are dynamic interactive units, rather than independent modules that act in isolation.