Scientific Understanding of Consciousness
Consciousness as an Emergent Property of Thalamocortical Activity

Short-Term Memory

 

Cognitive psychologists subdivide short-term memory into two major components: immediate memory and working memory. (Squire & Kandel; Memory, 84)

The only plausible way to create a neural pathway in a short time is to strengthen an existing synapse in some way. (Francis Crick; Searchlight Hypothesis, 267)

Local Short-Term Memory Circuits in the Brain

There are many local short-term memory circuits in the brain. Rehearsal and biochemical modification of synapses mediates the short-term memory function. Consolidation to long-term memory continues persistently, especially for emotionally tagged memories that are frequently and continually rehearsed.

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.

Thus, synapses in the cortex that originally functioned in the circuits of short-term memory, through rehearsal and consolidation, become molecularly engraved as long-term memory.

An individual neuron can be a part of many memory patterns. The ~10,000 synapses of its dendritic tree would have vastly different activity configurations for each of the many memory patterns in which it participates.

Two central hypotheses of dynamic link architecture: (1) binding by signal correlations, and (2) short-term synaptic modification. (von der Malsburg; Binding Problem, 141)

Forceful activity events can modify synapses in a small fraction of a second, perhaps a few milliseconds. When there is no further activity in the two cells connected by a synapse, J slowly returns to its resting value, with the time constant of short-term memory. (von der Malsburg; Binding Problem, 140)

 

Immediate memory

Immediate memory refers to the information that occupies our current stream of thought. Immediate memory can be extended to last minutes or more by rehearsal. (Squire & Kandel; Memory, 131)

Immediate memory (fractions of a second) (Purves; Neuroscience, 734)

After a few seconds, immediate memory or working memory can no longer support recollection unless the information is maintained by rehearsal. (Squire & Kandel; Memory, 92)

Extension of immediate memory is called working memory. (Squire & Kandel; Memory, 84)

Immediate memory and working memory are best thought of as a collection of temporary memory capacities that operate in parallel. (Squire & Kandel; Memory, 85)

Bilateral damage to the medial temporal lobe; patient perceives the material normally and holds it satisfactorily in immediate memory; material cannot persist in long-term memory. (Squire & Kandel; Memory, 91)

Damage to the medial temporal lobe spares immediate memory and working memory, because these early-stage forms of memory depend on areas of cortex outside the medial temporal lobe. (Squire & Kandel; Memory, 92)

Memory buffers in prefrontal cortex?

Two concepts have been discussed in the scientific literature for the cortex areas facilitating short-term memory that comprises working memory.  The traditional concept has been that working memory is supported by a cognitive system of prefrontal cortex (PFC) memory buffers that store the information, making it available for manipulation, interaction with other systems, and/or for guidance of behavior.  An alternative concept is that working memory is served by a facility of sustained attention to information represented in non-PFC cortical systems that have evolved to perform perception, representation, or action-related functions.

My hypothesis is that working memory is supported by the second concept with the short-term memory facilitated by sustained activity in the same non-PFC brain regions that process this information.

(Postle; Activated Long-Term Memory, 333)

 

Unitary view of memory with a common cortical substrate -- working memory is a temporary activation of updated long-term memory networks for orchestrating actions in the near term. (Fuster; Prefrontal Cortex, 4)

Currently activated memory elements are represented as a subset of long-term memory, and the focus of attention is represented as a subset of the currently activated memory elements. (Cowan; Attention and Memory, 139)

 

One short-term memory system, the human auditory-verbal short-term memory, appears to be implemented in the left hemisphere at the junction between the temporal, parietal, and occipital lobes. (Rolls & Treves; Neural Networks, 247)

 

 Short-Term Memory Mechanisms

Short-term memory can be mediated in at least two ways.

· Rehearsal of neural activity

· Synaptic facilitation by a biochemical mechanisms.

 

Research Study — Working Memory Fronto-Parietal Synchronization — Results indicate that short-term memories are represented by large-scale patterns of synchronized activity across the fronto-parietal network.

 

Remembering something in the short term uses proteins that are already present in synapses. (Ratey; User's Guide to Brain, 195)

Storage of biochemical information in the nerve cell after a brief period of activity, leads to a strengthening of the presynaptic connection that persist for many minutes. (Kandel; Principles of Neural Science, 275)

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)

 

Neural Network — Short-Term Memory

There are a number of different short-term memory systems, each implemented in a different cortical area. (Rolls & Deco; Noisy Brain, 91)

The short-term memory is implemented by subpopulations of neurons that show maintained activity in a delay., while a stimulus or event is being remembered. (Rolls & Deco; Noisy Brain, 91)

The autoassociation could be implemented by associatively modifiable synapses between connected pyramidal cells within an area, or by the forward and backward connections between adjacent cortical areas in a hierarchy. (Rolls & Deco; Noisy Brain, 91)

In the inferior temporal cortex the firing of neurons for short-term memory may be maintained for a few hundred milliseconds. In more ventral temporal cortical areas such as the entorhinal cortex the firing may be maintained for longer periods.  In the prefrontal cortex the firing may be maintained even for tens of seconds. (Rolls & Deco; Noisy Brain, 94)

The firing of neurons may be maintained by the operation of associatively modified recurrent collateral connections between nearby pyramidal cells producing attractor states in autoassociative networks. (Rolls & Deco; Noisy Brain, 94)

The ability of the dorsolateral prefrontal cortex to provide multiple separate short-term attractor memories could provide the basis for its function in planning. (Rolls & Deco; Noisy Brain, 95)

Neuronal activity in the inferior temporal visual cortex (IT) is driven by each new incoming visual stimulus, whereas in the prefrontal cortex, neurons start to fire when the sample stimulus is shown, and continue the firing that represents the sample stimulus, even when potential match stimuli are being shown. (Rolls & Deco; Noisy Brain, 95)

 

 

    Link to — Short-Term Memory — Synaptic Facilitation Diagram

 

Return to — Memory

Return to — Declarative Memory Classifications