APA Dictionary of Psychology

the theory that the strength of memory depends on the degree of cognitive processing the material receives. Depth has been defined variously as elaboration, the amount of cognitive effort expended, and the distinctiveness of the memory trace formed. This theory evolved an expanded empirical investigation of the levels-of-processing model of memory. [expounded in 1972 by Canadian psychologist Fergus I. M. Craik (1935–  ) and Robert S. Lockhart and in 1975 by Craik and Endel Tulving]