Dimensions and mechanisms of memory organization

Abstract

Memory formation is dynamic in nature, and acquisition of new information is often influenced by previous experiences. Memories sharing certain attributes are known to interact so that retrieval of one increases the likelihood of retrieving the other, raising the possibility that related memories are organized into associative mnemonic structures of interconnected representations. Although the formation and retrieval of single memories have been studied extensively, very little is known about the brain mechanisms that organize and link related memories. Here we review studies that suggest the existence of mnemonic structures in humans and animal models. These studies suggest three main dimensions of experience that can serve to organize related memories: time, space, and perceptual/conceptual similarities. We propose potential molecular, cellular, and systems mechanisms that might support organization of memories according to these dimensions.

Keywords: engram overlap; inferential reasoning; memory allocation; memory linking; memory organization; mnemonic structures.

Figure 1.. Dimensions of mnemonic structures.

A) Relative organization of Single memories, Mnemonic Structures and Memory Schemas. Memory Schemas are high-level cognitive structures that represent the abstract relation between different memories and can serve as a scaffold for the formation of other structures. Mnemonic structures represent specific relations between specific memories. Schemas and Mnemonic structures may interact. B) Example of a mnemonic structure composed of 3 elements. Common element “A” serves as a link to element “B and C”. The linking element can belong to different dimensions of experience. C) Theoretical representational space where temporal, spatial and perceptual/conceptual dimensions interact to determine linking and integration of memories into the same mnemonic structure. Note that Time and Space are not necessarily orthogonal.

Figure 2.. Systems and cellular mechanisms of mnemonic structures.

A) Schematic representation of the potential circuits involved in the formation of mnemonic structures. Different pathways convey information about what, where, and when to the hippocampus. These different pathways are involved in the formation of item-, space-, and time-bases mnemonic structures, respectively. The PFC, together with additional factors, determine how memories are co-allocated in the HPC and integrated into mnemonic structures. B) Cellular mechanisms of HPC mnemonic structures. Cells encoding a common element A (▴) restrict which cells are available for memory allocation (▵) and are co-encoded with different elements B (▴) and C (▴). This leads to the overlap between some cells representing B and C (). Upon retrieval of B, overlapping cells can indirectly reactivate memory C. The same effect is not observed when the two memories B and C do not share a common element (bottom panel). Grey triangles (▴) represent cells that are not available for encoding.