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

Declarative Memory

Declarative memory is what most people think of as memory. Declarative memories can be consciously recalled and discussed. There are two kinds: Episodic, Semantic. Episodic memories are all the specific events you have experienced and remembered, and also your associations with people in which you can recall the specific occasion. Semantic memory is all the facts learned in school, ideas learned, mathematical procedures, general recollection of “grandma’s house,” etc.

Two types of declarative memory -- semantic and episodic. (Gazzaniga; Human, 303)

Brain areas    other than the hippocampus are also involved in declarative memory, e.g. the prefrontal cortex, which is known to be involved in memories after a delay. (Baddeley,; Memory, 273)


  Distinction between "episodic memory" events tied to specific time and place, as contrasted with "semantic memory" for knowledge that is time- and event-independent. (Eichenbaum; Neuroscience of Memory, 121)

 Link to — Episodic Memory

 Link to — Semantic Memory

Link to — Perception and Memory

Link to — Autobiographical Memory


 What we normally refer to as the memory of an object is the composite memory of the sensory and motor activities related to the interaction between the organism and the object. (Damasio; Self Comes to Mind, 133)

Memory is not a single entity but is composed of different systems.  Only one of these systems is accessible to awareness, the declarative memory system. (Squire & Kandel; Memory, 159)

We perceive what we remember as well as remember what we perceive. (Fuster; Cortex and Mind, 84)

Long-Term and Short-Term Memory

Functionally, cognitive psychologists make a demarcation between Long-Term Memory and Short-Term Memory.

One of the outstanding challenges to any general brain theory is to explain the structural basis of long- and short-term memory. (Edelman; Neural Darwinism, 204)

Memory processes in the hippocampus appear to be based on three different forms of plasticity within a serially organized anatomical circuit that comprises the corticohippocampal pathways    (entorhinal cortex -->    dentate -->    CA3 -->    Ca1).    (Guigon; Short-Term Memory, 869)

Synaptic potentiation can persist for hours in mossy fibers (between dentate and CA3),    for several days in cortical projections to the dentate gyrus,    for several weeks in CA1.    (Guigon; Short-Term Memory, 869)


Link to – Short-Term Memory functionality

Link to – Long-Term Memory functionality


Same Brain Areas for Short-Term and Long-Term Memory

The hippocampus is connected to other brain areas by two main bidirectional routes. (Eichenbaum; Neuroscience of Memory, 217)

Short-term and long-term memory share much of the same cortical substrate and simply reflect different activation states of that substrate. (Fuster; Memory in Cerebral Cortex, 4)

Same brain areas appear to be used for long-term memory as are used for visual perception and intermediate memory. (Squire & Kandel; Memory, 88)

Memory is a normal consequence of perception. (Squire & Kandel; Memory, 91)

Memory impaired patients perform well on tests that make large demands on their perceptual abilities. (Squire & Kandel; Memory, 92)

Link to — Long-Term MemoryLong-Term memories are (encrusted/etched) in the molecular structures of synapses.

Link to — Short-Term Memorymany local short-term memory circuits in the brain. Rehearsal and biochemical modification of synapses mediates the short-term memory function.

Long-Term Memory

Extended consciousness requires working memory and explicit long-term memory (including both semantic and episodic memories). (Damasio & Meyer; Consciousness Overview, 9)

Distinguish non-consolidated long-term memory and consolidated long-term memory.

Long-Term Potentiation

Long-term potentiation (LTP) is a mechanism for long-lasting facilitation of synaptic transmission;  a permanent increase in synaptic efficacy,  resulting from repeated activation of a presynaptic neuron and its firing the postsynaptic neuron. (Eichenbaum; Neuroscience of Memory, 346)

Long-term potentiation (LTP), the mechanism by which memories are formed in synapses. (Andreasen; Creating Brain, 60)

Long-term potentiation (LTP) can last for many hours or even days. (Kandel; Principles of Neural Science, 275)

LTP has phases.  Early LTP lasts 1-3 hours; does not require protein synthesis. (Kandel; Principles of Neural Science, 1262)

A more persistent phase of the LTP (called late LTP) that lasts for at least 24 hours and requires new protein and RNA synthesis. (Kandel; Principles of Neural Science, 1262)


Hippocampus-dependent Long-Term Memory

Forming new long-term memories requires the participation of the medial temporal hippocampal region, which operates in conjunction with cortex-wide assemblies of neurons that represent stored information. (Squire; Memory and Brain, 149)

Structures important for declarative memory include association areas of the neocortex, the cortical regions surrounding the hippocampus, and the hippocampus. (Squire; Fundamental Neuroscience, 1311)

Formation of a long-term memory requires the making of new protein. (Squire & Kandel; Memory, 132)

Long-term memory requires the activation of genes, new protein synthesis, and the growth of new synaptic connections. (Squire & Kandel; Memory, 155)

Information in long-term memory continues to change for many years. Some or all of it can eventually become independent of the medial temporal region (hippocampus). (Squire; Memory and Brain, 145)

Hippocampus does not store memories.  It has been likened to an intelligent collating machine, which filters new associations, decides what is important and what to ignore or compress, sorts the results, and sends various packets of information to other parts of the brain.  It is a way station that hands out the pieces. (Ratey; User's Guide to Brain, 188)


Research study — Overlapping Memory Trace Linking, but Not Recalling, Individual Memories

Research study — Memory Retrieval Via Prefrontal Cortex Projection to the Hippocampus

Research study — Hippocampal Neurogenesis Regulates Forgetting


For further discussion of declarative memory involving the hippocampus:

Link to — Declarative Memory Classifications


Link to — Memory Consolidatation

Link to — Declarative Memory Classifications

Link to — Memory Encoding

Hippocampus-Independent Long-term Memory

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

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.

Ultimate storage site for long-term memory is the various areas of the cerebral cortex that initially process information about people, places, and objects. (Squire & Kandel; Memory, 110)

The primate temporal cortex implements visual long-term memory. The primate inferotemporal cortex locates at the final stage of the ventral visual pathway and serves as a storehouse for visual long-term memory. (Sensory and Memory Processing in Temporal Cortex, Science 18 March 2011)

Formation of a long-term memory requires the making of new protein. (Squire & Kandel; Memory, 132)

Long-term memory requires the activation of genes, new protein synthesis, and the growth of new synaptic connections. (Squire & Kandel; Memory, 155)

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

The posterior brain regions, especially the posterior cingulate area, are involved in memory retrieval. (Alkire; General Anasthesia, 127)


Research study — Long-Term Memory Formation


Memory Traces (or Engrams)

Memory traces are thought to vary in their familiarity, depending on how much attention the item received at encoding or how many times it was repeated. (Baddeley,; Memory, 183)

Traces of memory are believed to be linked up to one another by connections that are usually called associations or links. (Baddeley,; Memory, 165)

Associations are synaptic efficacy linkages between traces that vary in strength. (Baddeley,; Memory, 165).

Signaled detection theory proposes that memory traces have strength values that reflect their activation in memory, which dictate how familiar they seem. (Baddeley,; Memory, 183)

 Memory Recall involves a Reconstruction of an Active Neural Network Pattern

Declarative memory is a reconstruction of a prior arrangement of synaptic connections over widely distributed areas of the brain.

Link to — Declarative Memory as Reconstructiondeclarative memories must undergo a complex process of reconstruction during retrieval.

A memory is a tiny bit different each time we remember it. (Ratey; User's Guide to Brain, 186)

Memories are retrieved by associative access through their component representations, by reconstruction from fragments. (Fuster; Memory in Cerebral Cortex, 199)

Memory is a replay of neural response patterns adequate to the performance, not some sequence or specific detail. (Edelman; Universe of Consciousness, 98)

Retrieval is a progression from one or more cues to the target memory, via associative connections linking them together, through a process of spreading activation. (Baddeley,; Memory, 166)


Episodic Memory

Episodic memory, the long term recall of sequences of events or narratives, depends on interactions between the hippocampus and the cerebral cortex. (Edelman; Wider than the Sky, 51)

Episodic memory is probably uniquely human. (Andreasen, Creating Brain, 71)

Episodic memory is uniquely human. (Gazzaniga; Human, 303)

Episodic memory is claimed to be uniquely human, a mental travel back in time that endows the individual with the capacity to reference personal experiences in the context of both time and space. (Buzsáki; Rhythms of the Brain, 292)

Episodic memory, enabled by the hippocampal system, is not essential for consciousness. (Carter; Mapping the Mind, 205)

Research work in humans suggests that the hippocampal -- entorhinal system is involved in both episodic and semantic memories. (Buzsáki; Rhythms of the Brain, 327)


Episodic memory is autobiographical memory, the recollection of information that is linked to an individual's personal experiences. (Andreasen; Creating Brain, 71)

Episodic memory is claimed to be uniquely human, a mental travel back in time that endows the individual with the capacity to reference personal experiences in the context of both time and space. (Buzsáki; Rhythms of the Brain, 292)


Research study — Episodic Memory — Medial Temporal Lobe


Semantic Memory

Semantic memory refers to knowledge of the world.  This system represents organized information such as facts, concepts, and vocabulary. (Squire; Memory and Brain, 169)


Link to — Semantic Memory

Link to — Semantic Working Memory System


Short-term Memory

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

Short-term memory is achieved by modifying pre-existing proteins and strengthening pre-existing connections, through the activity of one or another protein kinase.  These short-term forms of memory do not require new protein synthesis. (Squire & Kandel; Memory, 155)

Short-Term Memory Systems

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

Because principal neurons frequently discharge in bursts of action potentials, the degree of postsynaptic facilitation or depression during such bursts may contain much of the information transmitted through the network. (Andersen; Hippocampus Book, 211)

Iconic Memory

The shortest of short-term memories is iconic memory, which is the capacity to retain a sensory image or up to one second after presentation. (Fuster; Memory in Cerebral Cortex, 13)

Iconic memory, a high-capacity, rapidly-decaying visual form of storage, is quickly established and persists for at least a few hundred milliseconds. (Koch; Quest for Consciousness, 201)

The existence of iconic memory is well-established experimentally. (Crick & Koch; Consciousness and Neuroscience, 38)

One of the functions of iconic memory is to provide sufficient time to allow the brain to process brief signals. (Koch; Quest for Consciousness, 202)

Koch believes that iconic memory is necessary for visual perception. (Koch; Quest for Consciousness, 202)

Iconic memory is probably instantiated throughout the visual brain, starting as early as the retina and including the various cortical areas and their associated thalamic nuclei. (Koch; Quest for Consciousness, 202)

Iconic memory's neuronal substrate is the afterglow left by the waves of spikes sweeping up the visual hierarchy, amplified by local and more global feedback loops. (Koch; Quest for Consciousness, 204)

 Immediate Memory

Immediate memory refers to what can be held actively in mind that forms the focus of current attention and that occupies the current stream of thought. (Squire & Kandel; Memory, 84)

Immediate memory would last a few seconds longer than iconic memory.  It coincides of what is commonly understood as short-term memory. (Fuster; Memory in Cerebral Cortex, 13)

Capacity of immediate memory is quite limited (it can hold approximately 7 items), and unless its contents are rehearsed, it ordinarily persists for less than 30 seconds. (Squire & Kandel; Memory, 84)

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)

Amnesic patients, who have profound and impaired long-term memory, were not impaired in the immediate recall of prose.  These data suggest LTM is not necessary for immediate recall. (Martin; Cognitive Neuropsychology, Working Memory, 184)


 Working Memory

Baddeley’s working memory model assumes a four component system, comprising    (1) an attentional controller, the central executive,    and three temporary storage systems:   (2) the visuospatial's sketch pad,    (3) the phonological loop,    and (4) a more general integrated storage system, the episodic buffer. (Baddeley; Working Memory, 13)

Attention and working memory are closely intertwined, making it difficult to cleanly separate them. The more working memory is taxed, the less effective attention is at disregarding distractors. (Koch; Quest for Consciousness, 197)

Working memory appears to  go hand-in-hand with consciousness. (Koch; Quest for Consciousness, 199

Working memory, a concept of short-term memory, is essentially a temporary storage used in performance of cognitive behavioral tasks. (Fuster; Memory in Cerebral Cortex, 14)

Short-term retention of information in working memory is supported by sustained activity in cortical regions whose primary function is not working memory. (Postle; Activated Long-Term Memory, 344)

 Working memory is an extension of immediate memory by rehearsing. (Squire & Kandel; Memory, 84)

Ordinarily, information will slip from your conscious mind within a few seconds, but immediate memory can be extended in time and its contents retained for many minutes if you rehearse actively. (Squire & Kandel; Memory, 84)

Short-term or "working" memory is supported by ongoing activity that re-enters a neuronal loop. (analogous to continuously repeating a phone number while dialing), ongoing electrical activity produced by synaptic feedback. (Llinás; I of the Vortex, 188)

Working memory (used to be called short-term memory) implies not just a short term storage but an active processing mechanism used in thinking and reasoning. Working memory has a capacity limit of about seven pieces of information. Executive functions take care of overall coordination of the activities of working memory. (LeDoux; Emotional Brain, 270-271)


Memories are Stored by Semantic Relatedness

Memories are stored by semantic relatedness via trace overlaps forming neuronal associations in parietal and temporal through to frontal cortex. Each memory is reconstructed as a sparse, active neural network pattern (memory trace). Individual neurons are reused in different memories, each dendritic tree having a different pattern of active synapses for each individual memory.

According to spreading activation theory, semantic memory is organized on the basis of semantic relatedness or semantic distance. (Baddeley,; Memory, 119)

According to spreading activation theory, whenever a person sees, hears, or thinks about a concept, the appropriate neural network pattern in semantic memory is activated. (Baddeley,; Memory, 120)

The spreading activation model has generally proved more successful than the hierarchical network model. (Baddeley,; Memory, 121)

Given that most memories are essentially hierarchical, made up of cognitive contents of different hierarchical levels, and given that the memory contents at one level are better consolidated than those at another, not all contents of a memory are equally retrievable. (Fuster; Cortex and Mind, 133)


 Declarative Memory is not simply a readout -- it involves a Mental Reconstruction of a Prior Event

Declarative memory is a reconstruction of a prior arrangement of synaptic connections over widely distributed areas of the brain. Tens or hundreds of millions of synaptic connections are involved. The specific arrangement of synaptic connections is not precise. Each time the memory is regenerated, a somewhat different set of neurons and synapses could be active, but enough of the original synaptic connections will be involved to result in approximately the same thought. Memories will tend to change and fade over time. Newly stored memories will typically utilize some of the same neurons and synapses used by older memories. The newly stored memories could modify the synapses used by older memories. This multiuse of the synapses forming memories will eventually lead to fading of memories as newer memories are formed with reused and modified combinations of synaptic connections.


 Recency Effect

Ability to discriminate among memories decreases as the memories become more remote. (Baddeley; Working Memory, 106)

Recency effect is one of the most stable and reliable phenomena within the study of human memory. (Baddeley; Working Memory, 115)

Recency effect is what allows us to orient ourselves in time and space. (Baddeley; Working Memory, 115)


Link to article on Memory Consolidation


Phonological Loop

Visuospatial Sketch Pad



Semantic memory


Spatial Memory


Procedural Memory

Link to —  Procedural Memory


Link to — Declarative memory as Reconstruction

Return to — Memory