Hippocampus

The hippocampus is an elongated structure running along the lower edge of the temporal cortex, but it can be considered one of the subcortical structures connected to the cortex via a set of parallel, unidirectional pathways.  These pathways constitute one of the three topological networks of the brain.

 

Cellular Switchboard in Memory Circuits

Neurogliaform Cells Decouple Neuronal Synchrony Between Brain Areas

 

The hippocampus stores, probably for a few weeks or so, new long-term episodic memories before the information is established more permanently in the cortex. (Crick; Astonishing Hypothesis, 83)

We need a functioning hippocampus to reexperience unique moments in our past. (Corkin; Permanent Present Tense, 230)

Hippocampus is activated when people are asked to recall personal or 'episodic' memories. (Carter; Mapping the Mind, 166)

Hippocampus, which is phylogenetically old cortex, plays a crucial role in the acquisition and consolidation of memory and thus in the construction of neocortical representations. (Fuster; Cortex and Mind, 45)

The term hippocampus formation is applied to a group of distinct adjoining regions including the dentate gyrus,    hippocampus,    subiculum,    presubiculum,    parasubiculum,    and entorhinal cortex. (Andersen; Hippocampus Book, 42)

Diagram—Taxonomy of Mammalian Memory Systems

Diagram—Medial Temporal Lobe Memory System

Diagram—Systems Level Memory Consolidation

The six structures of the hippocampus formation are linked, one to the next, by a unique and largely unidirectional (functional) neuronal pathways. (Andersen; Hippocampus Book, 42)

The fornix   is a continuation of a bundle of hippocampal output fibers    to subcortical target structures. (Andersen; Hippocampus Book, 47)

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

Hippocampal anatomy; convergent-divergent  reentrant loop to the cortex; synaptic change by LTP; fornical connections related to value; temporal ordering of perceptual categorizations; short-term memory over seconds to minutes; initiation of long-term memory. (Edelman; Remembered Present, 132)

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

Hippocampuses, which are evolutionarily ancient structures deep inside the temporal lobes, are involved in the process whereby short-term memory gets transferred to long-term memory, and also spatial memory. (Gazzaniga; Human, 20)

It is convenient to think of the hippocampus having a binding function that binds together the storage sites that were established independently in several cortical regions, so that the storage sites are strongly connected with one another. (Squire & Kandel; Memory, 110)

The hippocampus grows as a single large multilayer space.  The evolutionary advantage of such an architectural solution is the creation of a giant random connections space, a requisite for combining arbitrary information. (Buzsáki; Rhythms of the Brain, 285)

Excitation in a particular part of the entorhinal area passes through the inner hippocampal loop and finds its way back roughly into the cortical region where it started 10-20 msec earlier. (Edelman; Remembered Present, 131)

 

Link to — Bayesian Inference in Brain Functionality

Research Study — Navigating Cognition

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

Research Study — Hippocampal Engram serves as an Index of Memory Content

Research Study — Hippocampal Diversity in Neural Firing Dynamics

Research Study — Neurogenesis Drops Sharply in Children to Adults

Research Study — Hippocampal Ripples Down-Regulate Synapses

Research Study — Hippocampal Cognitive Map Local Transformations

Research Study — Hippocampus Topographic Circuit Assembly

Research Study — Hippocampus Prospective Representation of Navigational Goals

Research Study — Hippocampus Spatial Representations of Self and Other

Research Study — Social Place-Cells in the Bat Hippocampus

Research Study — Entorhinal-Hippocampal Circuit Maturation  driven by Stellate Cells

Research Study — Strengthening Synaptic Connections by NMDA for Long-Term Potentiation (LTP)

Research Study — Memory Consolidation Engrams and Circuits

Research Study — Medial Entorhinal Cortex replays independently of the Hippocampus

Research Study — Hippocampal Gating by Entorhinal Cortex Long-Range Inhibition

Research study — Prefrontal–Thalamo–Hippocampal circuit for Goal-Directed Navigation

Research study — Hippocampus Logarithmic Sense of Space

Research study — NMDAR Receptor Structure

Research study — Hippocampal Neurogenesis Regulates Forgetting

Research Study — Neural Stem Cells can produce Neurogenesis in Adult Brain

Research Study — Movement-Related Theta Rhythm in Humans: Hippocampal Learning

Research Study — Hippocampal CA2 region is essential for Social Memory

Research Study — Hippocampus Dendritic Inhibition Supports Fear Learning

Research study — Entorhinal–Hippocampal Ensemble Activity during Associative Learning

 

Hippocampal Circuit

The entorhinal cortex can be considered the first step in the intrinsic hippocampal circuit. (Andersen; Hippocampus Book, 38)

The projections from the entorhinal cortex to the dentate gyrus form part of the major hippocampal input pathway called the perforant path. (Andersen; Hippocampus Book, 38)

The principal cells of the dentate gyrus, the granule cells, give rise to axons called mossy fibers that connect with pyramidal cells of the CA3 field of the hippocampus. (Andersen; Hippocampus Book, 38)

Pyramidal cells of CA3 are the source of the major input to the CA1 hippocampal field (the Schaffer collateral axons). (Andersen; Hippocampus Book, 38)

 

Link to — Hippocampus Diagram (Kandel) — pictorial diagram

Link to — Connections Within the Hippocampus (Diagram)

Link to — Hippocampus Connections with Neocortex — (diagram Edmund Rolls) hippocampus receives, via the adjacent parahippocampal gyrus and entorhinal cortex   inputs from virtually all association areas in the neocortex.

          

The hippocampus is the neocortex's librarian. (Buzsáki; Rhythms of the Brain, 285)

The computational properties of recursive organization, such as the extensive CA3 recurrent system, meet the requirements of an "autoassociator." (Buzsáki; Rhythms of the Brain, 289)

Marr (1971) proposed a theory for how the hippocampus could function as an associative memory. From this proposal have followed many extensions, usually focusing on the role of the CA3 recurrent collaterals. (Arbib, Handbook of Brain Theory; Burgess; Hippocampus Spatial Models, 469)

Hippocampus has myriad connections with many brain areas. (Llinas, Mind-Brain Continuum; Eichenbaum; Olfactory Perception and Memory, 192)

Hippocampal system contains afferents from all sensory modalities, which may be better conceived as functionally rather than perceptually defined inputs. Cortical areas that project to the hippocampal system contain multimodal information that defines the meaning of a stimulus according to functional designations of cortical modules. (Llinas, Mind-Brain Continuum; Eichenbaum; Olfactory Perception and Memory, 192)

Hippocampus is concerned with perception of the environment and therefore represents multisensory experience. (Kandel; Search of Memory, 308)

Hippocampus is densely connected to a multitude of different areas. (Revonsuo; Inner Presence, 312)

Major interconnections of the hippocampus with subcortical regions are mediated via the fornix. (Dudai; Memory from A to Z, 114)

Hippocampus, (Pinel; Anatomy of Human Brain, 175)

 Hippocampus Maturation

In both rodents and humans, the late development of the intrahippocampal associational connections may influence the maturation of the entire hippocampal network. (Andersen; Hippocampus Book, 128)

 

Hippocampus -- 3 major Afferent Pathways

Hippocampus -- 3 major afferent pathways (diagram) (Kandel; Principles of Neural Science, 1257)

Perforant fiber pathway from the entorhinal cortex forms excitatory connections with the granule cells of the dentate gyrus. (Kandel; Principles of Neural Science, 1257)

Granule cells of the dentate gyrus have axons that form the mossy fiber pathway, which connects with the pyramidal cells in area CA3 of the hippocampus. (Kandel; Principles of Neural Science, 1257)

Pyramidal cells in CA3 project to the pyramidal cells in CA1 by means of the Shaffer collateral pathway. (Kandel; Principles of Neural Science, 1257)

LTP is nonassociative in the mossy fiber pathway and is associative in the perforant fiber pathway and Shaffer collateral pathway. (Kandel; Principles of Neural Science, 1257)

Long term potentiation (LTP) of the mossy fiber pathway of the CA3 region of the hippocampus. (diagram) (Kandel; Principles of Neural Science, 1258)

Long-term potentiation (LTP) in the Shaffer collateral pathway to the CA1 region of the hippocampus. (diagram) (Kandel; Principles of Neural Science, 1259)

LTP in the Shaffer collateral pathway requires simultaneous firing in both the postsynaptic and presynaptic neurons. (Kandel; Principles of Neural Science, 1260)

 

Entorhinal Cortex Interconnects Hippocampus with Association Cortices

An extensive body of evidence suggests that the hippocampus is essential for fast encoding and storage of new episodic memories but has a more limited role in remote memory, which is thought to be stored primarily in the neocortex. Memory consolidation in the neocortex appears to be a slow and gradual process based on repeated interactions with the hippocampus. These interactions must be mediated largely through the entorhinal cortex, which interconnects the hippocampus with nearly all other association cortices. Understanding how information is processed in the entorhinal cortex is thus essential to resolving the interaction between the hippocampus and neocortex during encoding, consolidation, storage, and retrieval of memory. (Science 27 August 2004: Vol. 305. no. 5688, pp. 1258 – 1264)

The hippocampus and related structures in the medial temporal lobe are crucial components of a brain system that mediates spatial learning, context-dependent learning, and episodic memory. These forms of learning have both spatial and nonspatial components, and hippocampal neurons correspondingly have both spatial and nonspatial firing correlates. Characterizing the pathways by which spatial and nonspatial information reach the hippocampus is critical for a full understanding of the neural circuitry underlying these forms of memory. The major cortical input to the hippocampus originates in the entorhinal cortex, which is divided into two parts on the basis of distinctive cytoarchitecture and connectivity patterns. The medial entorhinal cortex (MEC) receives its predominant input from the postrhinal cortex (parahippocampal cortex in primates) and forms the medial perforant path into the hippocampus. The lateral entorhinal cortex (LEC) receives its major input from the perirhinal cortex and forms the lateral perforant path. (Science 17 June 2005: Vol. 308. no. 5729, pp. 1792 – 1794)

 

      Research study — Hippocampus — Synaptic Organization — excerpts from Shepherd text.

      Research study — Entorhinal Cortex for Temporal Association Memory

      Research study — Hippocampal Neurogenesis in Adult

 

Hippocampal Local Interactions between Neighboring Synapses

Long-term potentiation (LTP) of synaptic transmission underlies aspects of learning and memory. LTP is input-specific at the level of individual synapses, but neural network models predict interactions between plasticity at nearby synapses. A research study shows that in mouse hippocampal pyramidal cells, local interactions between neighboring synapses support clustered plasticity models of memory storage.

Hippocampal theta oscillations

Hippocampal theta oscillations are among the rare sustained rhythms in the brain. (Buzsáki; Rhythms of the Brain, 335)

Hippocampal theta oscillations are travelling waves

Theta oscillations clock hippocampal activity during awake behaviour and rapid eye movement (REM) sleep. Research results demonstrate that theta oscillations pattern hippocampal activity not only in time, but also across anatomical space.

Hippocampal Short- and Long-Term Plasticity Not Modulated by Astrocyte Signaling

The concept that astrocytes release neuroactive molecules (gliotransmitters) to affect synaptic transmission has been a paradigm in neuroscience research. A research study suggests that, at least in the hippocampus, the mechanisms of gliotransmission need to be reconsidered.

Hippocampal Learning in the Trisynaptic Pathway and Monosynaptic Pathway

The hippocampus is an area of the brain involved in learning and memory. It contains parallel excitatory pathways referred to as the trisynaptic pathway and the monosynaptic pathway. A research study found that synaptic output from CA3 in the trisynaptic pathway is dispensable and the short monosynaptic pathway is sufficient for incremental spatial learning.

Hippocampus Hypothesized to Mediate Pattern Separation of Mnemonic Representations

The dentate gyrus (DG) of the mammalian hippocampus is hypothesized to mediate pattern separation—the formation of distinct and orthogonal representations of mnemonic information—and also undergoes neurogenesis throughout life. The DG is one of two sites where neurogenesis is ongoing throughout life. Adult-born neurons integrate into DG circuitry and are thought to play a role in learning and memory. A research study provides experimental evidence of a role for newborn neurons in the adult DG in spatial discrimination, consistent with a role in spatial pattern separation.

 

Research study — Hippocampal Pattern Separation in CA3 and Dentate Gyrus — Pattern separation, the process of transforming similar representations or memories into highly dissimilar, nonoverlapping representations, is a key component of many functions ascribed to the hippocampus.

Research study — Hippocampal Neurons Respond During Free Recall

Research study — Gamma Oscillations in Hippocampus

Research study — Learning and Memory in Pyramidal Neuron Dendrites

Research study — Hippocampal Learning and CA3 Output

Research study — Interneurons Project Long-Range in Hippocampus and Entorhinal Cortex

Research study — Hippocampal Engram of Sparse Population for Specific Memory

 

Long-term memory  (Quest 189)  (Crick 83)  Long Term Potentiation (LTP)  (Neil’s 108)  (Delirium 115)

Procedural learning  (Quest 193)  (Delirium 116)

Declarative memory  (Quest 194)

Episodic

Semantic

 

 

HM  (Quest 194)

Patient HM had surgery to control epilepsy; both medial temporal lobes removed in 1953; extensively studied aftermath. (Llinás; I of the Vortex, 185)

 

Iconic memory  (Quest 201)

Temporary changes in synaptic strength such as synaptic depression and facilitation probably underlie our fading short-term memories. (Neil’s 108)

Habituation - loss of arousal as a result of repeated exposure. (Johnston; Why We Feel, 104)

Classical conditioning as Hebbian plasticity - (diagram) (LeDoux; Synaptic Self, 160)

Imprinting (Llinás; I of the Vortex, 199)

 

Long Term Potentiation (LTP)

(Llinás; I of the Vortex, 188)

At many synapses LTP and LTD are dependent on the activation of NMDA receptors. A requirement for the induction of LTP is that there must be a sufficient increase in the intracellular Ca²⁺ concentration near the stimulated synapses. This occurs by the influx of Ca²⁺ ions through NMDA receptors and/or voltage-gated Ca²⁺ channels. (Johnston; Hippocampus, 445, Shepherd; Synaptic Orgainzation of the Brain)

 

Emotional state has strong influence on memory

The hippocampus and amygdala likely interact in the formation of emotional memories. (Vogt; Cingulate Neurobiology, 455)

Novelty and unexpectedness merely arouse the nervous system, but this arousal is necessary and sufficient for learning to occur.  (Johnston; Why We Feel, 103)

Habituation - loss of arousal as a result of repeated exposure. (Johnston; Why We Feel, 103)

Events that elicit feeling continue to generate arousal and support creative learning throughout a lifetime. (Johnston; Why We Feel, 103)

Intensity of a feeling modulates the degree of arousal that is required for learning. (Johnston; Why We Feel, 104-106)

 

Hippocampus and memory

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

Dual role of the hippocampus in the formation and retrieval of memory although the mechanisms remained obscure. (Fuster; Cortex and Mind, 134)

Hippocampal formation and the adjacent entorhinal and perirhinal cortices are critical for declarative memory. Bilateral MTL lesions induce profound amnesia. The hippocampus, however, is not the ultimate storage site for explicit memories — the final repository is the neocortex, particularly the temporal and prefrontal lobes. The hippocampus combines the information coming from all sensory modalities for the to-be-remembered event and consolidates these in the relevant cortical areas over many weeks. (Koch; Quest for Consciousness, 194ff)

Hippocampus “Place Cells”  Firing Pattern Represents an Animal’s Location in Space

Mouse's whereabouts are signaled by the discharge of a unique population of hippocampal place cells. (Kandel; Principles of Neural Science, 1264)

Animal is thought to form a "place field," an internal representation of the space that it occupies. (Kandel; Principles of Neural Science, 1264)

When an animal enters a new environment, new place fields are formed within minutes and are stable for weeks to months. (Kandel; Principles of Neural Science, 1264)

Same pyramidal cells may signal different information in different environments and can be used in more than one spatial map. (Kandel; Principles of Neural Science, 1266)

 

Hippocampus and Amygdala involved with Anxiety States

Research study — Amygdalar and Hippocampal substrates of Anxious Temperament

 

LeDoux comments on Memory

Long term memory - memory that lasts for more than a few seconds. (LeDoux; Emotional Brain, 180)

Declarative or Explicit memory - conscious recall of some past experienced; memories brought to mind and described verbally. (LeDoux; Emotional Brain, 181)

Nondeclarative or Implicit memory - learning that does not depend on conscious awareness. . (LeDoux; Emotional Brain, 181)

Implicit, fear-conditioned memory, 'emotional memory'. (LeDoux; Emotional Brain, 182)

Explicit declarative memory, 'memory of an emotion'. (LeDoux; Emotional Brain, 182)

Memory is not mediated by any particular neural system but instead is diffusely distributed in the brain. (LeDoux; Emotional Brain, 182)

H. M., extreme epilepsy, 1953, Hartford, CT; - Explicit (conscious) memory, age 27, convulsive epileptic attacks since age 16. (LeDoux; Emotional Brain, 183)

Short-term memory lasts seconds. (LeDoux; Emotional Brain, 184)

Long-term memory lasts minutes to a lifetime. (LeDoux; Emotional Brain, 184)

What you are conscious of now, working memory, special kind of short-term memory. (LeDoux; Emotional Brain, 185)

What goes into your short-term memory is what can go into your long-term memory. (LeDoux; Emotional Brain, 185)

H. M. unable to form long-term memories. He could hold on to information for a few seconds, short-term memory. (LeDoux; Emotional Brain, 185)

Formation of long-term memories involves the temporal lobe; short-term memories involve other brain systems. (LeDoux; Emotional Brain, 185)

Brain system involved in forming new long-term memories is different from the one that stores old long-term memories. (LeDoux; Emotional Brain, 185)

H. M. had very severe anterograde amnesia but only a mild retrograde amnesia. (LeDoux; Emotional Brain, 185)

H. M.'s major deficit: depositing new learning into the long-term memory bank. (LeDoux; Emotional Brain, 185)

Temporal lobe is needed for forming long-term memories, but gradually over the years, memories become independent of this system. (LeDoux; Emotional Brain, 185)

Areas of the temporal lobe that were damaged in H. M. included major portions of the hippocampus and amygdala. (LeDoux; Emotional Brain, 186)

Hippocampus emerged as the brain region for laying down new memories. (LeDoux; Emotional Brain, 186)

Alzheimer's disease begins in the temporal lobe, particularly the hippocampus; forgetfulness is the first warning sign; eventually creeps into the neocortex; all aspects of memory along with a variety of other cortical cognitive functions. (LeDoux; Emotional Brain, 193)

 

Calvin comments

Temporary changes in synaptic strength such as synaptic depression and facilitation probably underlie our fading short-term memories. (Calvin; Neil's Brain, 108)

Following a burst of impulses, any pulse in the next minute or so may release more than the standard amount of neurotransmitter. When longer times and more impulses are involved, it can lead to Long Term Potentiation (LTP). (Calvin; Neil's Brain, 108)

Some short-term changes provide the scaffolding for making permanent changes. (Calvin; Neil's Brain, 108)

 

Edelman comments on the anatomical connections of the hippocampus

Extrinsic connections that link the entorhinal area and the cingulate to a large variety of secondary or tertiary areas in the temporal, frontal and parietal cortices are strongly reentrant. (Edelman; Remembered Present, 129)

Entorhinal area is small in comparison with the various input and output areas, suggesting extensive convergence and divergence in the circuitry. (Edelman; Remembered Present, 130)

Within the hippocampus, main input from the entorhinal area proceeds into the dentate fascia via granule cell mossy fibers to the pyramidal cells of subfield CA3. (Edelman; Remembered Present, 130)

CA3 cells send Schaffer collaterals to the pyramidal cells of subfield CA1, which in turn connect to the subiculum. (Edelman; Remembered Present, 130)

Subiculum connects back to the entorhinal area to close the loop. Edelman calls this the inner loop. (Edelman; Remembered Present, 130)

Laminar structure of the subfields is essentially made of a single layer of principal cells in CA1, CA3, and dentate fascia. (Edelman; Remembered Present, 130)

Fornical-thalamocortical path. (Edelman; Remembered Present, 130)

Fornical connections terminating in the cingulate cortex originate exclusively in the subiculum. Reentry from the subiculum to the entorhinal area. Edelman calls this the outer loop. (Edelman; Remembered Present, 130)

Brain stem and hypothalamic inputs enter the septum, which sends reentrant connections to the entorhinal area. (Edelman; Remembered Present, 130)

Fornical outputs connect to the mammillary bodies, then to the thalamus, and then to the cingulate gyrus. (Edelman; Remembered Present, 130)

Edelman's outer loop including the fornix connecting the hippocampus and cingulate gyrus is important for primary consciousness. (Edelman; Remembered Present, 130)

Cortical areas involving different modalities terminate on a given portion of the entorhinal area. (Edelman; Remembered Present, 131)

Excitation in a particular part of the entorhinal area passes through the inner hippocampal loop and finds its way back roughly into the cortical region where it started 10-20 msec earlier. (Edelman; Remembered Present, 131)

Through reentry, a number of the hippocampal groups would be repeatedly activated, allowing synaptic change involving long-term potentiation and an integrated response to perceptual input. (Edelman; Remembered Present, 131)

Hippocampal anatomy; convergent-divergent  reentrant loop to the cortex; synaptic change by LTP; fornical connections related to value; temporal ordering of perceptual categorizations; short-term memory over seconds to minutes; initiation of long-term memory. (Edelman; Remembered Present, 132)

 

(paraphrase of Yadin Dudai from Memory from A to Z, p. 117)

So what is the role of hippocampus in learning and memory? Despite thousands of man-years, smart para­digms, fascinating data, stimulating models, and the apparent neuroanatomical simplicity that has always attracted anatomists and physiologists, the truth is that we do not yet know for sure what the hippocampus does.

Judged by its connectivity to other brain areas, the hippocampus does seem fit to execute global operations. These could be of two types, which possibly overlap. First, computations necessary for the generation and processing of internal representations that are stored elsewhere. Examples are temporary binding of multi­ple representations to promote the formation of new ones, or ad hoc evaluation of the contextual impor­tance of on-line input or of retrieved information. Second, operations that involve activation of high-order representations that are themselves stored, at least partially, in the hippocampus; these could involve rep­resentations of specific times and settings of episodes, or of sets of other representations catalogued by yet unknown attributes. This latter view considers the hippocampus as some sort of a mental index that is used in the storage and retrieval of other representations.

With all this wealth of data, interpretations, and hypotheses, an attempt to encapsulate the proposed functions of the hippocampus in a catchy phrase, even if speculative, partial, and metaphoric, is worth a try. So let's try this one. In that same moment in a mental time travel in which we recall the specific contents, time, and setting of an episode all together, and sud­denly experience that familiar sense of 'I remember', it is the hippocampal system that is ticking in the background.

(end of paraphrase)

 

 

Link to — Memory

Link to — Consciousness Subject Outline

Further discussion -- Covington Theory of Consciousness