A computational model of systems memory consolidation and reconsolidation

A coupled neural field model for the standard consolidation theory

The standard consolidation theory states that short-term memories located in the hippocampus enable the consolidation of long-term memories in the neocortex. In other words, the neocortex slowly learns long-term memories with a transient support of the hippocampus that quickly learns unstable memories. However, it is not clear yet what could be the neurobiological mechanisms underlying these differences in learning rates and memory time-scales. Here, we propose a novel modeling approach of the standard consolidation theory, that focuses on its potential neurobiological mechanisms. In addition to synaptic plasticity and spike frequency adaptation, our model incorporates adult neurogenesis in the dentate gyrus as well as the difference in size between the neocortex and the hippocampus, that we associate with distance-dependent synaptic plasticity. We also take into account the interconnected spatial structure of the involved brain areas, by incorporating the above neurobiological mechanisms in a coupled neural field framework, where each area is represented by a separate neural field with intra- and inter-area connections. To our knowledge, this is the first attempt to apply neural fields to this process. Using numerical simulations and mathematical analysis, we explore the short-term and long-term dynamics of the model upon alternance of phases of hippocampal replay and retrieval cue of an external input. This external input is encodable as a memory pattern in the form of a multiple bump attractor pattern in the individual neural fields. In the model, hippocampal memory patterns become encoded first, before neocortical ones, because of the smaller distances between the bumps of the hippocampal memory patterns. As a result, retrieval of the input pattern in the neocortex at short time-scales necessitates the additional input delivered by the memory pattern of the hippocampus. Neocortical memory patterns progressively consolidate at longer times, up to a point where their retrieval does not need the support of the hippocampus anymore. At longer times, perturbation of the hippocampal neural fields by neurogenesis erases the hippocampus pattern, leading to a final state where the memory pattern is exclusively evoked in the neocortex. Therefore, the dynamics of our model successfully reproduces the main features of the standard consolidation theory. This suggests that neurogenesis in the hippocampus and distance-dependent synaptic plasticity coupled to synaptic depression and spike frequency adaptation, are indeed critical neurobiological processes in memory consolidation.

The role of prelimbic and anterior cingulate cortices in fear memory reconsolidation and persistence depends on the memory age

Reconsolidation is a time-limited process under which reactivated memory content can be modified. Works focused on studying reconsolidation mainly restrict intervention to the moments immediately after reactivation and to recently acquired memories. However, the brain areas activated during memory retrieval depend on when it was acquired, and it is relatively unknown how different brain sites contribute to reconsolidation and persistence of reactivated recent and remote fear memories. Here, we sought to investigate the participation of prelimbic (PL) and anterior cingulate cortices (ACC) in recent (1 d old) and remote (21 d old) fear memory reconsolidation and persistence. Male Wistar rats were submitted to the contextual fear conditioning protocol. Tamoxifen (TMX), an estrogen receptor modulator known to inhibit protein kinase C activity was used to interfere with these processes. When infused into the PL cortex, but not into the ACC, TMX administration immediately or 6 h after recent fear memory reactivation impaired memory reconsolidation and persistence, respectively. TMX administered immediately after remote memory reactivation impaired memory reconsolidation when infused into the PL cortex and ACC. However, remote memory persistence was only affected when TMX was infused 6 h after memory reactivation into the ACC and no effect was observed when TMX was infused 6 h after memory reactivation into PL cortex. Together, the findings provide further evidence on the participation of PL cortex and ACC in reconsolidation of recent and remote fear memories and suggest that the persistence of a reactivated fear memory becomes independent on the PL cortex with memory age and dependent on the ACC.