Still searching for the engram

Abstract

For nearly a century, neurobiologists have searched for the engram-the neural representation of a memory. Early studies showed that the engram is widely distributed both within and across brain areas and is supported by interactions among large networks of neurons. Subsequent research has identified engrams that support memory within dedicated functional systems for habit learning and emotional memory, but the engram for declarative memories has been elusive. Nevertheless, recent years have brought progress from molecular biological approaches that identify neurons and networks that are necessary and sufficient to support memory, and from recording approaches and population analyses that characterize the information coded by large neural networks. These new directions offer the promise of revealing the engrams for episodic and semantic memories.

Keywords: Comparative cognition; Episodic memory; Memory; Rat; Spatial learning.

Figure 1

Outline of brain systems that support different forms of memory.

Figure 2

A. T-maze alternation task. B. Left turn (light gray) and right turn (dark gray) paths through the maze and spiking patterns for left-turn and right-turn paths. C. Cartoon summary of locations of place fields on left-turn (yellow) and right-turn (blue) paths. Some cells fired equally on both paths, suggesting a mechanism of connecting the two types of episodes (from Wood et al., 2000).

Figure 3

Left. Context guided memory task. Right: Dendrogram illustrating the hierarchical organization of memories in the hippocampal memory space. X-axis indicates the 8 distinct rewarded events. Lines indicate mean correlation coefficients (r) between events and clusters of events. A & A′, etc., refer to odd and even numbered identical events. Pos = positions within each context (from Mackenzie et al., 2014).