Space and Time in the Brain

Memory Consolidation

Memory consolidation, believed to be enhanced by sleep and dreaming, converts long-term memories involving the hippocampus, into long-term memories independent of the hippocampus. These long term memories can be invoked by working memory accessing synapse networks throughout the cortex, utilizing thalamocortical loops, without involving the hippocampus. Epilepsy patient HM could recall his childhood experiences, which had been consolidated prior to his surgery.

Consolidation of Short-Term memory into Long-Term memory

The hippocampus rehearses fresh memory traces with the cortex, especially those with emotional tags and even the unconscious traces, continuing repeatedly during sleep and dreaming, eventually consolidating memory patterns in the cortex via gene expression into the protein-modified synapses of long-term memory. The consolidation process could take days or even weeks for deeply etched long-term memories.

Animal experiments have showed the gradual consolidation of memory as a single store along a temporal continuum. There is no need for two distinct kinds of memory stores (short- and long-term) with a sharp boundary or temporal transitions between them. (Fuster; Cortex and Mind, 119)

Evidence for the consolidation of memory in one store implicates the entire cerebral cortex and synaptic change in cortical networks as the essence of consolidation. (Fuster; Cortex and Mind, 121)

Diagram — Systems Level Memory Consolidation

Research Study — Learning-Enhanced Coupling between Association Cortices and Hippocampus

Research Study — Memory Consolidation Engrams and Circuits

Research Study — Engram cells retain Memory under retrograde amnesia

Research Study — Sleep promotes formation of Dendritic Spines

Research Study — Long Term Memory Consolidation via DNA Methylation

Research Study — Memory Consolidation and Associative Changes in Neocortical Code

Research Study — Plasticity of Basket-Cell Network Regulates Adult Learning

The results of a research study support the hypothesis that plasticity changes in synapses during waketime lead to a net increase in synaptic strength in many brain circuits and that sleep is required for synaptic renormalization.

Research study — Thalamus in Fear and Memory — Through systems consolidation, in which memories are transferred from the hippocampus to the cortex, memories become generalized. A study defines a neural circuit—composed of the medial prefrontal cortex, the nucleus reuniens (NR), and the hippocampus—that controls fear memory generalization. Highly specific memories are proposed to be maintained through "pattern separation," but can be generalized during retrieval through "pattern completion".

Research study — Sleep and Synaptic Homeostasis

Widespread Short-Term Memories Become Consolidated into Long-Term Memory

During the lengthy process of consolidation, the critical neural system within the medial temporal region may maintain the organization of distant memory storage sites, until such time as the coherence of these sites increases and they can be activated as an ensemble without the participation of the medial temporal region. (Squire; Memory and Brain, 209)

Widespread areas of the neocortex contain the details of the information that is to be remembered, and medial temporal area support the capacity to retrieve the memory during this period shortly after learning. (Squire; Fundamental Neuroscience, 1310)

cAMP-dependent protein kinase, MAP kinase, and CREB, to convert labile short-term memory into long-term memory. (Kandel; Principles of Neural Science, 1274)

Hippocampus slowly feeds new memories to the cortex during sleep. (LeDoux; Synaptic Self, 113)

Medial temporal areas reactivate the cortical representations repeatedly, inducing plasticity and intra-cortical connections that provide the permanent linkages and organization of the cortical memory. (Squire; Fundamental Neuroscience, 1311)

How the hippocampus contributes to the long-term consolidation of memories. (Andersen; Hippocampus Book, 738)

The same representational function the hippocampus provides in encoding and linking episodic memories at the time of learning continues for a period to support the establishment of intracortical linkages of memories within a large-scale memory network. (Squire; Fundamental Neuroscience, 1311)

Hippocampus affects recent memories, but not old ones that have been consolidated in the cortex. (LeDoux; Synaptic Self, 107)

Memory representations over 4 weeks old presumably have already been consolidated in the neocortex. (Fuster; Cortex and Mind, 46)

Old memories are the result of accumulations of synaptic changes in the cortex. (LeDoux; Synaptic Self, 107)

One of the reasons we need sleep is to permanently encode memories. (Hobson; Dreaming as Delirium, 115)

Memories are ultimately encoded as proteins in the synapses. (Hobson; Consciousness, 74)

Memory consolidation during sleep. During sleep, neural patterns repeated in the hippocampus, dreaming about the event. Hippocampal playback during sleep is read and used by the cortex? (LeDoux; Synaptic Self, 107)

Hippocampus - major role in consolidating short-term memory into long-term memory in the cerebral cortex. (Edelman; Universe of Consciousness, 45)

Immediate Consolidation Period

Some agents have been proposed as potential memory enhancers, but their beneficial effect seems to be limited to the encoding and immediate consolidation period. (Research Study — Enhancement of Consolidated Long-Term Memory)

Long-term memories can experience significant memory fading. Over time, synapses undergo “synaptic homeostasis” like processes, i.e. a regression to the mean.

Research study — Synaptic Remodeling, and Network Activity

Short-term to long-term memory via consolidation

Repetition and consolidation establish, through synaptic strengthening, preferred feedforward/feedback pathways that use subsets of neurons within the network. Each new memory ensemble overlays on the composite ensemble of prior memories, reuses neurons involved in many other memories, and modifies the synapses to form a revised composite pattern of synaptic strengths. A new input signal pattern will distribute itself in the neural network so that the signaling pattern coincides with the optimal pathways ensemble of synaptic strengths.

Initial basis of long-lasting forms of synaptic plasticity in the mammalian CNS, such as LTP and LTD, entail post-translational changes that lead to altered distributions or density of postsynaptic AMPA receptors. (Purves; Neuroscience, 597)

Research study — Long-Term Memory Formation

Research study — Dendritic Spines and Memories — in the mouse cortex, learning and novel sensory experience lead to spine formation and elimination by a protracted process.

Research Study — Hippocampal Place Cells and Long-Term Memories

Research study — Sensory and Memory Processing in Temporal Cortex

Memory Reinforcment by Repeated Re-creation

Memories are reinforced by repeatedly being asked to relive the events being remembered. (Mlodinow; Subliminal, 66)

When we repeatedly re-create a memory, we reinforce it each time, so that in a way we are remembering the memory, not the event. (Mlodinow; Subliminal, 66)

An important trend in the evolution of a memory -- there isn't just memory loss, there are also memory additions. (Mlodinow; Subliminal, 69)

As the original reading of a story fades into the past, new memory data is fabricated. (Mlodinow; Subliminal, 69)

Research study — Disruption of Reconsolidation Erases Fear Memory Trace — After a conditioned fear memory was formed, reactivation and reconsolidation left a memory trace in the basolateral amyglada. Reactivation followed by disrupted reconsolidation suppressed fear, abolished the memory trace, and attenuated fear-circuit connectivity.

Degeneracy, Stochasticity, Robustness

The pattern of synaptic strengths in the ensemble of cortical and subcortical neurons has the property of degeneracy as pointed out by Edelman. An input signal pattern will distribute itself in the cortical network with variability in the ensemble of pathways it invokes each time it appears as an input. Many thousands of synapses in the active pattern are likely to be on the verge of connectivity, "twinkling" in-and-out of connectivity, while most of the synapses in the pattern are firmly in the active connected state. The stochastic properties of neuron functionality and its dendritic tree of about 10,000 synapses will require that populations dynamics determine network functionality rather than the stochastic properties of individual neurons and small neuronal assemblies.

The degeneracy characteristic of the network provides robustness in network functionality, not depending on the stochastic characteristics of individual neurons, but rather population dynamics of large ensembles. Individual neurons can die or otherwise become nonfunctional, leaving the network patterns of memories largely intact.

Memory Consolidation Potentiated by Slow Oscillations During Sleep

There is compelling evidence that sleep contributes to the long-term consolidation of new memories. A research study shows that inducing slow oscillation-like potential fields by transcranial application of oscillating potentials (0.75 Hz) during early nocturnal non-rapid-eye-movement sleep, that is, a period of emerging slow wave sleep, enhances the retention of hippocampus-dependent declarative memories in healthy humans.

(paraphrase from Eichenbaum, Cognitive Neuroscience of Memory, 308)

There are two aspects of memory consolidation: (1) a short-term one that involves molecular and cellular processes that support the fixation of memory within synapses over a period of minutes or hours, and (2) another that involves interactions within the declarative memory system to support a reorganization of memories that occurs over weeks to years.

There is a cascade of molecular and microstructural events by which short-term synaptic modifications lead to permanent changes in connectivity between neurons. There are also modulatory mechanisms that can facilitate fixation of memory in the synapses. An important modulatory system involves the release of glucocorticoids and adrenergic mechanisms via the amygdala that can influence memory fixation in the declarative and habit systems in both animals and humans.

In addition, the declarative memory system mediates the prolonged reorganization of memories via synaptic strengths throughout the brain. The precise nature of mechanisms is poorly understood, but current models propose that the role of the hippocampus is to rapidly store indices of cortical representations, and to slowly facilitate the interconnection of cortical representations by repeated two-way interactions between the cortex and hippocampus. Over a protracted period, these interactions ultimately result in an asymptotic level of reorganization and connections among network synapse patterns to represent the new knowledge. Notice that any of the synapses can participate at different times in any synaptic pattern forming a memory. Of course, some synapses will participate frequently, while other will be used rarely. Those synapses that are used extremely rarely will tend to atrophy.

(end of paraphrase)