New Evidence for Early Formation of Long-Term Memory

In the study abstract below, Kitamura and others show that long-term memory traces in the outer cerebral cortex of the mouse are laid down early in the experience to be remembered. Prior to this study the prevailing opinion was that neuronal traces for short-term memories are initially created only in the hippocampus and transferred later to long term cortex. The study below shows that initial weak traces of long term memory are created simultaneously with the strong short term memory traces in the hippocampal short term memory region, and that, over a couple weeks, the cortical area traces become stronger and more active and the corresponding hippocampal areas fade in their activity.

This suggests that prior theories were too simplistic, perhaps because they tended to model memory after human information storage, such as writing or memory disk, where information is generally laid down once and remains the same strength thereafter unless erased. Modeling the brain after our technology metaphors can be misleading.

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ABSTRACT

Engrams and circuits crucial for systems consolidation of a memory

Takashi Kitamura1,*, Sachie K. Ogawa1,*, Dheeraj S. Roy1,*, Teruhiro Okuyama1, Mark D. Morrissey1, Lillian M. Smith1, Roger L. Redondo1,2,†, Susumu Tonegawa1,2,‡

Science 07 Apr 2017

Vol. 356, Issue 6333, pp. 73-78

DOI: 10.1126/science.aam6808

The network of memory consolidation

Memories are thought to be formed in the hippocampus and later moved to the neocortex for long-term storage. However, little is known about the mechanisms that underlie the formation and maturation of neocortical memories and their interaction with the hippocampal network. Kitamura et al. discovered that at the onset of learning, neurons for contextual fear memory are quickly produced in the prefrontal cortex. This process depends on the activity of afferents from both the hippocampus and the amygdala. Over time, the prefrontal neurons consolidate their role in memory expression. In contrast, the hippocampal neurons slowly lose this function.

Episodic memories initially require rapid synaptic plasticity within the hippocampus for their formation and are gradually consolidated in neocortical networks for permanent storage. However, the engrams and circuits that support neocortical memory consolidation have thus far been unknown. We found that neocortical prefrontal memory engram cells, which are critical for remote contextual fear memory, were rapidly generated during initial learning through inputs from both the hippocampal–entorhinal cortex network and the basolateral amygdala. After their generation, the prefrontal engram cells, with support from hippocampal memory engram cells, became functionally mature with time. Whereas hippocampal engram cells gradually became silent with time, engram cells in the basolateral amygdala, which were necessary for fear memory, were maintained. Our data provide new insights into the functional reorganization of engrams and circuits underlying systems consolidation of memory.