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Archicortex

The archicortex, also known as the archipallium, is the phylogenetically oldest region of the cerebral cortex, distinguished by its simplified three-layered structure consisting of molecular, pyramidal, and polymorphic layers, and primarily encompassing the hippocampal formation.^[1]^[2] Located in the medial temporal lobe as a key component of the limbic system, it forms a C-shaped continuum of gray matter that includes the hippocampus proper, dentate gyrus, subiculum, and related structures such as the fasciolar gyrus and supracallosal gyrus.^[3]^[4] This ancient cortical tissue, which develops in association with the olfactory system, contrasts with the more complex six-layered neocortex and represents about 3.5% of the total cerebral cortical surface area in humans.^[5]^[3] Phylogenetically, it emerges from primordial zones linked to early vertebrate brain evolution, predating the expansion of neocortical areas in mammals.^[1] In terms of connectivity, the archicortex integrates with regions like the entorhinal cortex via the perforant path and participates in the Papez circuit, facilitating bidirectional communication with the thalamus and other limbic components.^[2]^[1] Functionally, the archicortex is essential for episodic memory consolidation, spatial navigation, and emotional regulation, with the hippocampus serving as its core for encoding declarative memories and contextual learning.^[2]^[1] It also contributes to olfactory processing in its anterior extensions, though its primary role in modern neuroscience emphasizes cognitive and affective integration.^[4] Dysfunctions in the archicortex are implicated in disorders such as Alzheimer's disease, where hippocampal atrophy leads to profound memory deficits, and temporal lobe epilepsy, highlighting its vulnerability to neurodegenerative and excitotoxic processes.^[2]

Overview

Definition and Classification

The archicortex, also known as the archipallium, represents the phylogenetically oldest division of the cerebral cortex, characterized by a simplified three-layered cytoarchitecture that distinguishes it from more recent cortical formations.^[6] This structure consists of a molecular layer (layer I), a pyramidal cell layer (layer II), and a polymorphous layer (layer III), in contrast to the six-layered neocortex (isocortex) that dominates in higher mammals.^[7] The reduced layering reflects its primitive evolutionary origins, with layers oriented parallel to the cortical surface and featuring a high density of pyramidal neurons in the middle layer, contributing to its compact organization.^[8] Within the broader classification of cerebral cortex, the archicortex forms a key subtype of the allocortex, which encompasses phylogenetically older cortical regions comprising approximately 10% of the total cortical surface and lacking direct thalamic input.^[7] Alongside the paleocortex, which exhibits three to five layers and is primarily olfactory in function, the archicortex serves as a transitional form between the allocortex and the more differentiated neocortex, bridging primitive and advanced cortical processing.^[6] This taxonomic position underscores its role in early vertebrate brain evolution, where it emerged as a foundational element before the expansion of the six-layered neocortex.^[8] The primary components of the archicortex are located within the hippocampal formation, including the hippocampus proper (comprising fields CA1 through CA3), the dentate gyrus, and the subiculum.^[8] The hippocampus proper and dentate gyrus each display the characteristic three-layered arrangement, while the subiculum acts as an output gateway, facilitating connections to adjacent cortical areas.^[7] These elements collectively embody the archicortex's unique anatomical features, such as its agranular nature and elevated neuronal packing in specific laminae, which support its specialized integrative functions.^[6]

Historical Development

The understanding of the archicortex emerged from early anatomical explorations of the cerebral cortex in the late 18th and 19th centuries, building on detailed dissections and initial histological analyses. Samuel Thomas von Sömmering's seminal 1788 treatise Vom Hirn und Rückenmark offered one of the first comprehensive illustrations and descriptions of the human brain's gross structure, including the hippocampal formation and associated limbic components later classified as archicortical.^[9] Theodor Meynert further refined these observations in the mid-19th century through his studies on cortical lamination and connectivity, using early staining techniques to delineate primitive cortical districts and association pathways that distinguished allocortical regions from neocortex.^[10] Key advancements in the early 20th century integrated comparative anatomy and cytoarchitectonics to formalize the archicortex as a distinct entity. Ludwig Edinger, in his evolutionary framework around 1908, introduced the terms "archipallium" and "archicortex" to describe phylogenetically ancient cortical layers homologous to reptilian medial pallium, contrasting them with the newer neopallium proposed by Grafton Elliot Smith.^[11] Concurrently, Korbinian Brodmann's 1909 cytoarchitectonic parcellation of the human cerebral cortex identified specific archicortical zones, such as areas 27, 28, and 48 in the hippocampal and entorhinal regions, based on laminar differences revealed by Nissl staining.^[12] Ariëns Kappers' comparative neuroanatomy, detailed in his 1909–1911 volumes, reinforced these findings by tracing archicortical homologues in reptilian brains, emphasizing their conserved three-layered organization across vertebrates.^[13] Nomenclature evolved significantly in the 1920s, shifting from the 19th-century "rhinencephalon" concept—which broadly encompassed olfactory-related limbic structures—to more precise allocortical subdivisions. Max Rose's 1926 studies on comparative cytoarchitectonics proposed a tripartite model of the cortex (archicortex, paleocortex, neocortex), highlighting the archicortex's primitive histology and distinguishing transitional periallocortex zones.^[14] These refinements were enabled by improved histological methods, particularly Nissl staining developed in the 1890s, which illuminated the sparse lamination and granule cell scarcity characteristic of archicortical tissue.^[15]

Anatomy

Location and Gross Morphology

The archicortex, comprising the hippocampal formation, is situated primarily in the medial temporal lobe, forming a critical component of the limbic system on the medial surface of each cerebral hemisphere. It includes three main structures: the hippocampus proper (also known as cornu ammonis), the dentate gyrus, and the subicular complex, as well as the indusium griseum and gyrus fasciolaris (fasciola cinerea), which together constitute a paired organ extending from the region near the amygdala anteriorly to the splenium of the corpus callosum posteriorly.^[16] This formation lies buried within the parahippocampal gyrus and occupies the floor and medial wall of the temporal (inferior) horn of the lateral ventricle, positioned medial to the inferior horn and along the brain's longitudinal axis. The indusium griseum forms a thin layer of gray matter adherent to the superior surface of the corpus callosum, while the gyrus fasciolaris represents a folded extension of the dentate gyrus near the splenium.^[16] Grossly, the archicortex presents as a prominent, curved, C-shaped structure resembling a seahorse or ram's horn, with an elongated and convex profile approximately 5 cm in length in humans. The hippocampus folds into the temporal horn via the hippocampal sulcus, creating an infolded sheet of cortex divided into three segments: the bulbous head (pes hippocampi, featuring 2-3 shallow grooves), the elongated body, and the tapering tail. The dentate gyrus appears as a narrow, crenated strip of gray matter adjacent to the hippocampus, while the subicular complex forms a transitional zone. Key macroscopic connections include the fornix, a bundle of efferent white matter fibers arising from the alveus (a thin sheet covering the ventricular surface) and fimbria, which wraps around the thalamus to project to subcortical targets; afferent inputs arrive primarily from the entorhinal cortex via the perforant pathway.^[17]^[16]^[18] The archicortex relates closely to surrounding structures, bordering the six-layered neocortex laterally through transitional zones and the three-layered paleocortex anteriorly, particularly the olfactory-related regions. It is embedded inferiorly in the parahippocampal gyrus, separated from adjacent areas like the collateral sulcus by the hippocampal sulcus, and lies in proximity to the amygdala anteriorly. In terms of mammalian variations, the gross morphology in primates, including humans and non-human species, is characterized by greater elongation and pronounced C-shaped folding compared to the more linear form in rodents; within primates, human examples show a particularly developed anterior bulb and overall convexity, with species-specific differences in size and sulcal depth.^[17]^[16]^[19]

Cellular and Microscopic Structure

The archicortex, particularly the hippocampus, exhibits a distinctive trilaminar organization at the microscopic level, consisting of three primary layers: the molecular layer, the pyramidal (or granular) cell layer, and the polymorphous layer. The molecular layer, the outermost, is primarily composed of unmyelinated axons, dendrites, and glial processes, forming a dense neuropil that receives major extrinsic inputs such as the perforant pathway from the entorhinal cortex. The middle pyramidal cell layer contains the somata of principal excitatory neurons, densely packed in a single row, while the innermost polymorphous layer includes a heterogeneous mix of interneurons, glial cells, and axonal arborizations, providing local inhibitory and modulatory connections. This simplified laminar structure contrasts with the six-layered neocortex and is a hallmark of allocortical regions like the archicortex.^[20] Key neuronal cell types define the functional architecture of the archicortex. Pyramidal neurons, the predominant excitatory cells in the cornu ammonis (CA) regions (CA1–CA3), feature large triangular somata in the pyramidal layer and extend prominent apical dendrites into the molecular layer, as well as basal dendrites into the polymorphous (oriens) layer; these neurons use glutamate as their primary neurotransmitter and form the backbone of intra-hippocampal projections. In the dentate gyrus, granule cells occupy the granular layer, possessing spiny apical dendrites in the molecular layer and unmyelinated axons that form mossy fibers—large, filopodial boutons that synapse onto CA3 pyramidal neurons and hilar mossy cells for robust, low-fidelity transmission. Mossy cells, multipolar glutamatergic interneurons in the polymorphous layer of the dentate hilus, provide associative feedback to granule cells, enhancing pattern separation. Interneurons, such as basket and chandelier cells, are scattered across layers and mediate GABAergic inhibition to regulate principal cell activity.^[20]^[21]^[22] The synaptic architecture of the archicortex is dominated by excitatory glutamatergic synapses, with approximately 80–90% of connections involving ionotropic receptors on principal neurons. Mossy fiber synapses in the CA3 stratum lucidum are notable for their large, multivesicular boutons contacting thorny excrescences on pyramidal dendrites, enabling high-release probability transmission. The canonical trisynaptic circuit exemplifies this organization: layer II entorhinal cortex neurons project via the perforant path to granule cell dendrites in the dentate molecular layer; granule cell mossy fibers then excite CA3 pyramidal cells; and CA3 Schaffer collaterals target CA1 pyramidal dendrites in the stratum radiatum, forming a feedforward excitatory chain with embedded inhibitory loops from interneurons. Inhibitory synapses, primarily GABAergic, target somata and dendrites to control excitability, while gap junctions may link certain interneurons for synchronized activity.^[23]^[24] Neurochemical markers highlight regional specializations in the archicortex. NMDA receptors (NMDARs), critical for synaptic plasticity, show high expression on pyramidal and granule cell dendrites, particularly in the CA1 and CA3 molecular and radiatum layers, where GluN2A and GluN2B subunits predominate to facilitate calcium influx during coincident pre- and postsynaptic activity. Calcium-binding proteins like calbindin-D28k are prominently expressed in subsets of interneurons and select principal cells, such as CA1 pyramidal neurons and some granule cells, buffering intracellular calcium to modulate excitability and protect against excitotoxicity; expression is weaker in the mossy fiber zone and dentate granule cells overall. These markers underscore the archicortex's role in calcium-dependent signaling without delving into broader functional implications.^[21]^[25]^[26]

Functions

Olfactory Processing

The archicortex plays a key role in the olfactory pathway by receiving inputs from the olfactory bulb via the entorhinal cortex, which serves as a primary interface for integrating olfactory sensory information with higher cognitive processes. The lateral entorhinal cortex (LEC), in particular, encodes odor representations via layer 2 principal neurons, such as fan and pyramidal cells, enabling rapid processing of olfactory stimuli.^[27] The subiculum functions as a critical relay station within this pathway, channeling processed olfactory signals from the hippocampus to other brain regions, facilitating the tagging of odors for memory consolidation.^[28] This relay mechanism allows the archicortex to link raw olfactory data with contextual elements, distinguishing it from primary olfactory cortices like the piriform cortex.^[29] Olfactory processing in the archicortex involves specialized mechanisms for refining sensory input, including pattern separation in the dentate gyrus, which helps distinguish similar scents by transforming overlapping olfactory inputs into distinct neural representations. This process relies on the sparse firing properties of dentate granule cells, reducing interference between closely related odor patterns and supporting fine-grained odor discrimination.^[30] Additionally, interactions with the amygdala modulate these representations by assigning emotional valence to odors, enhancing the salience of aversive or appetitive scents through strengthened connectivity between the amygdala and entorhinal-hippocampal circuits during encoding.^[31] Such modulation ensures that emotionally charged odors are prioritized in archicortical processing, influencing behavioral responses without overlapping into broader memory functions.^[32] A prominent circuit in this processing is the perforant path, originating from layer II of the entorhinal cortex and projecting to the dentate gyrus and CA fields of the hippocampus, which enables the integration of olfactory cues with contextual information for associative olfaction. This pathway conveys odor-specific inputs to the hippocampus, allowing for the binding of scents to environmental contexts, as seen in tasks requiring odor-place associations.^[33] Activation along the perforant path is particularly evident in response to olfactory tract stimulation, underscoring its role in relaying sensory details for contextual refinement.^[34] Experimental evidence from lesion studies highlights the archicortex's necessity for intact olfactory discrimination. Bilateral lesions of the entorhinal cortex impair learning and retention in successive-cue olfactory discrimination tasks, where animals fail to differentiate rewarded from unrewarded odors, indicating a disruption in pathway integration.^[35] Similarly, selective damage to the lateral entorhinal cortex disrupts odor-context associative memory while sparing single-item odor recognition, emphasizing its specific contribution to combined sensory-contextual processing.^[36] These findings, observed in rodent models, demonstrate that archicortical integrity is essential for precise odor discrimination, with deficits manifesting as reduced accuracy in fine odor differentiation tasks following perforant path interruption.^[37]

Memory and Learning

The archicortex, particularly the hippocampus, plays a pivotal role in synaptic plasticity underlying memory formation through mechanisms such as long-term potentiation (LTP). LTP, first demonstrated in the dentate gyrus and later in the CA3 and CA1 regions, involves a persistent strengthening of synapses following high-frequency stimulation, serving as a cellular basis for learning and memory storage. In the CA1 region, LTP enhances excitatory transmission at Schaffer collateral synapses from CA3, while in CA3, it strengthens recurrent connections among pyramidal cells, facilitating associative memory networks. These processes enable the hippocampus to support episodic memory, which involves recalling personal events with contextual details, and spatial memory, mediated by place cells in CA1 and CA3 that fire selectively in specific locations within an environment.^[38] Key mechanisms for memory encoding and consolidation in the archicortex include theta rhythm synchronization and neural replay. The hippocampal theta rhythm (4-8 Hz), prominent during exploratory behavior, coordinates the timing of neuronal firing to separate encoding from retrieval phases, with afferent inputs arriving during the theta peak to promote LTP induction.^[39] During sleep, particularly slow-wave sleep, hippocampal neurons replay sequences of activity observed during wakefulness in forward or reverse order, strengthening engrams and transferring memories to neocortical storage for long-term consolidation.^[40] The dentate gyrus contributes specifically to pattern separation, orthogonalizing similar input representations via sparse granule cell activation to prevent memory interference, as evidenced by differential neural responses to overlapping spatial contexts.^[41] Interactions between the archicortex and prefrontal cortex further integrate hippocampal outputs into working memory processes. Bidirectional connections allow hippocampal place and time cells to inform prefrontal representations of spatial and temporal contexts, enabling flexible decision-making and maintenance of goal-relevant information over short delays. This interplay supports the updating of working memory buffers with episodic details, such as linking current stimuli to past experiences. The critical role of the hippocampus in declarative memory was starkly illustrated by the case of patient H.M., who underwent bilateral medial temporal lobe resection including the hippocampus in 1953, resulting in profound anterograde amnesia while sparing working memory and procedural learning. This deficit highlighted the archicortex's necessity for forming new explicit memories, influencing decades of research on memory systems.

Development and Evolution

Embryonic Origins

The archicortex, encompassing structures such as the hippocampus and dentate gyrus, originates from the medial wall of the telencephalic vesicle during early human embryonic development. The prosencephalon, or forebrain, divides into telencephalon and diencephalon around the fifth gestational week, establishing the foundational vesicle from which the dorsal telencephalon patterns into cortical regions, including the archicortical primordium.^[42] This medial positioning distinguishes the archicortex from lateral neocortical areas, reflecting its conserved role in early neural organization.^[43] Key developmental stages commence with the invagination of the neural plate, which forms the neural tube by the fourth week, followed by the emergence of the hippocampal primordium in the dorsal medial telencephalon around the eighth to ninth gestational week. Neuroblasts migrate from the ventricular zone along radial glia, contributing to the establishment of the trilaminar structure characteristic of archicortex—comprising a molecular layer, pyramidal cell layer, and polymorphic layer—by the end of the eighth week. This migration and layering process is guided by gradients of signaling molecules, ensuring proper areal specification within the expanding telencephalon.^[44]^[45] Genetic regulation plays a pivotal role in regional specification and patterning of the archicortex. Transcription factors such as Emx2 and Lhx5 are expressed in the medial telencephalon, where Emx2 promotes hippocampal growth and field delineation, while Lhx5 ensures proper dorsoventral organization and prevents ectopic differentiation. Wnt signaling, particularly through Wnt3a secreted from the cortical hem—a transient organizer at the pallial-subpallial boundary—induces hippocampal fate and regulates progenitor proliferation in a dose-dependent manner, with high levels favoring archicortical identity over neocortical expansion.^[46]^[47]^[48] The timeline of archicortical maturation extends through gestation, with the dentate gyrus beginning to differentiate around the third month (12-14 weeks), as proliferating precursors form the primary germinal matrix and initiate granule cell neurogenesis. Lamination of the hippocampal formation approaches completion by birth, though the dentate gyrus remains immature, with ongoing addition of granule cells postnatally via the subgranular zone, continuing into infancy and adulthood at reduced rates. This protracted development underscores the archicortex's adaptability in response to environmental cues during early life.^[45]^[49]

Evolutionary Role

The archicortex, encompassing the hippocampal formation in mammals, traces its phylogenetic origins to the medial pallium of early tetrapods, where it emerged as a specialized structure for spatial navigation. This medial pallium is homologous to the reptilian medial cortex, which processes allocentric spatial information and supports memory formation for environmental landmarks. In vertebrates, the medial pallium likely specialized for map-like representations of space early in evolution, predating the divergence of major lineages around 360 million years ago during the Devonian period. The transition to terrestrial life in tetrapods amplified its role, integrating olfactory cues with spatial mapping to aid navigation in complex habitats.^[50]^[51]^[52] Comparatively, the archicortex exhibits a simpler organization in non-mammalian vertebrates, featuring a two- to three-layered structure in fish and amphibians, which contrasts with the more elaborated six-layered neocortex. In fish, the pallial homolog—often termed a rudimentary archicortex—consists of basic neuronal layers without distinct subdivisions, primarily handling olfactory-spatial integration. Amphibians show a slightly more defined medial pallium with two layers, supporting basic spatial learning but lacking the laminar complexity seen in higher forms. Mammalian innovations, such as the dentate gyrus, introduced ongoing neurogenesis and enhanced pattern separation, enabling finer-grained memory encoding that built upon these ancestral foundations.^[13]^[53]^[54] These evolutionary developments conferred adaptive advantages, particularly in early mammals, by facilitating odor-based foraging and detailed environmental mapping, which were crucial for survival in nocturnal, scent-reliant niches following the Permian-Triassic transition. The archicortex's expansion allowed integration of olfactory inputs with hippocampal place cells, supporting efficient resource location and predator avoidance in diverse ecosystems. Fossil and genetic evidence underscores this conservation: hippocampal circuitry, including spatial processing networks, has remained remarkably stable over approximately 300 million years since the divergence of reptilian and mammalian lineages. Genes like Foxg1, which regulate telencephalic patterning and hippocampal development, show deep evolutionary conservation across amniotes, with orthologs present in reptiles and birds. In non-mammalian analogs, such as the avian hippocampus, relative size correlates with ecological demands like food caching in species such as chickadees, where larger structures enhance spatial memory for cache retrieval.^[55]^[52]^[56]^[57]^[58]

Clinical and Research Aspects

Associated Disorders

The archicortex, particularly the hippocampus, is implicated in several neurological and psychiatric disorders characterized by structural and functional abnormalities. Hippocampal sclerosis, a key pathological feature in mesial temporal lobe epilepsy (mTLE), involves neuronal atrophy and gliosis primarily in the CA1 and CA3 regions of the hippocampus, leading to recurrent seizures originating from the temporal lobe. This condition is prevalent in approximately 60-70% of mTLE cases, often resulting from early-life insults such as febrile seizures or hypoxia, and contributes to drug-resistant epilepsy by disrupting normal excitatory-inhibitory balance in archicortical circuits.^[59]^[60] In Alzheimer's disease (AD), archicortical dysfunction manifests early with tau pathology in the entorhinal cortex and amyloid plaques accumulating in the subiculum, preceding widespread neocortical involvement and directly correlating with initial episodic memory deficits. These pathological changes impair the perforant path connections between the entorhinal cortex and dentate gyrus, leading to synaptic loss and circuit hyperexcitability that exacerbate cognitive decline. Neuroimaging studies reveal that such alterations in the archicortex are among the earliest detectable signs of AD, with tau tangles in layer II of the entorhinal cortex serving as a predictor of progression from mild cognitive impairment.^[61]^[62] Other conditions linked to archicortical pathology include schizophrenia, where reduced hippocampal volume—particularly in the CA1 subfield—reflects chronic neurodevelopmental disruptions and is associated with deficits in working memory and reality testing. In post-traumatic stress disorder (PTSD), hyperactivity within hippocampal-amygdala fear circuits impairs contextual processing of traumatic memories, resulting in persistent fear responses and extinction deficits. Major depressive disorder involves impaired neurogenesis in the dentate gyrus, diminishing the archicortex's role in mood regulation and pattern separation, which contributes to anhedonia and cognitive biases toward negative stimuli.^[63]^[64]^[65] Diagnostic markers for these archicortical disorders often rely on MRI volumetry, which quantifies hippocampal atrophy with 20-40% volume loss in affected regions compared to healthy controls, aiding in differential diagnosis for epilepsy and AD. For instance, unilateral hippocampal volume reductions of around 28% are typical in mTLE, while bilateral losses of 30-35% characterize AD progression. These metrics, combined with T2 signal hyperintensities indicating gliosis, provide objective evidence of archicortical involvement without invasive procedures.^[66]^[67]

Modern Neuroimaging and Studies

Modern neuroimaging techniques have significantly advanced the understanding of archicortex function, particularly in the hippocampus, by enabling non-invasive visualization of neural activity and connectivity. Functional magnetic resonance imaging (fMRI) has been widely employed to detect hippocampal activation during memory tasks, revealing robust engagement in declarative memory retrieval for words, objects, and episodic events.^[68] For instance, high-resolution fMRI studies demonstrate increased hippocampal signals during successful recollection in recognition paradigms, linking subfield-specific activity to memory performance.^[69] Diffusion tensor imaging (DTI) complements this by facilitating tractography of the fornix, the primary efferent pathway from the hippocampus, to quantify microstructural integrity and connectivity alterations.^[70] In healthy adults, DTI metrics of the fornix correlate with memory efficiency, showing reduced fractional anisotropy in aging populations indicative of disrupted archicortical outputs.^[71] Optogenetics in animal models provides causal insights into hippocampal circuit dynamics, allowing precise manipulation of neuronal populations to dissect memory-related processes. Studies using optogenetic stimulation of CA1 neurons in mice have shown that rhythmic activation modulates synaptic plasticity and theta oscillations, enhancing spatial memory encoding.^[72] Similarly, targeting GABAergic inputs to the hippocampus reveals inhibitory stabilization of excitatory networks, challenging prior views of sparse connectivity in archicortical regions.^[73] Post-2000 research has confirmed ongoing adult neurogenesis in the dentate gyrus, a key archicortical structure, with approximately 700 new neurons integrated daily in each human hippocampus, contributing to an annual turnover of 1.75% of granule cells.^[74] This process supports adaptive plasticity, particularly in pattern separation for memory discrimination. High-resolution 7T MRI has further elucidated the archicortex's role in prospective memory, showing subfield-specific activation during future-oriented tasks like event simulation, where anterior hippocampus engagement predicts successful intention retrieval.^[75] In aging, 21st-century neuroimaging reveals preserved yet compensatory plasticity in the hippocampus, with structural adaptations in subfields like CA1 mitigating cognitive decline through enhanced connectivity rerouting.^[76] Recent advances in the 2020s include connectomics projects that map full hippocampal wiring at nanoscale resolution using electron microscopy. For example, large-scale 3D datasets of mouse CA1 and CA3 subfields have uncovered spatially graded mossy fiber inputs and selective inhibitory circuits, informing models of human archicortical organization.^[77] Human studies highlight unique wiring patterns that optimize memory storage via sparse, strong connections.^[78] Additionally, AI-assisted analysis of EEG theta patterns has improved detection of hippocampal oscillations, with machine learning tools identifying rhythms linked to memory consolidation and navigation in rodent models.^[79] These methods address gaps in earlier research by quantifying dynamic plasticity in aging, showing that interventions like cognitive training can restore theta coherence and neurogenesis rates.^[80]

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Sep 12, 2005 · This paper reviews work on network models in which theta rhythm contributes to the following functions: (1) separating the dynamics of encoding and retrieval.
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Reactivation of Hippocampal Ensemble Memories During Sleep
Information acquired during active behavior is thus re-expressed in hippocampal circuits during sleep, as postulated by some theories of memory consolidation.
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The Basics of Brain Development - PMC - PubMed Central
Human brain development is a protracted process that begins in the third gestational week (GW) with the differentiation of the neural progenitor cells and ...
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Development of the mammalian cortical hem and its derivatives
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Development of the human fetal hippocampal formation during early ...
From Kier's study, the development of the fetal HF was complex during the time from 13 GW to 24 GW (Kier et al., 1997). The rotation process began at about 14– ...Missing: timeline | Show results with:timeline
[44]
Perinatal compromise affects development, form, and function of the ...
Hippocampal development commences within weeks of conception and continues through the first years of life in human infants (Fig. 2). By week 8−9 of human ...
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Hippocampus development and generation of dentate gyrus granule ...
Feb 1, 2000 · Mutations of the LIM homeodomain genes Lhx2 (Porter et al., 1997) and Lhx5 (Zhao et al., 1999) eliminate the entire hippocampus. Emx2 mutants ...Results · Lef1-Lacz/lacz Mice Lack The... · Role Of Wnt And Bmp...
[46]
Emx2 Is Required for Growth of the Hippocampus But Not for ...
Because Emx2 mutant mice die at birth, any disruption of the normal schedule of hippocampal development would have a particular impact on the dentate gyrus.Missing: Lhx5 | Show results with:Lhx5
[47]
A local Wnt-3a signal is required for development of the mammalian ...
We show that Wnt-3a acts locally to regulate the expansion of the caudomedial cortex, from which the hippocampus develops.
[48]
Developmental profile of neurogenesis in prenatal human ...
Proliferating cells peaked at 14 weeks in dentate gyrus, 9 weeks in cornu ammonis. •. Mature neurons appeared at 9 weeks and peaked at 22 weeks in cornu ammonis ...Missing: timeline | Show results with:timeline
[49]
Spatial memory and hippocampal pallium through vertebrate evolution
These data suggest that early in vertebrate evolution, the medial pallium of an ancestral fish group that gave rise to the extant vertebrates became specialized ...
[50]
Circuits Regulating Pleasure and Happiness: The Evolution of the ...
The medial pallium is considered to evolve into the hippocampal complex in all tetrapods, and perhaps all jawed vertebrates (Medina and Abellán, 2009).
[51]
How the evolution of air breathing shaped hippocampal function
Dec 27, 2021 · Once on land, the hippocampus of terrestrial mammals continued to scaffold memory on olfactory gradients, mediated as the bearing map by the ...
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Amphibian spatial cognition, medial pallium and other supporting ...
As such, hippocampal evolution in amniotes, especially mammals, can be seen as progressing toward a cytoarchitecture with well-defined subdivisions, regional ...<|separator|>
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Evolution of the Mammalian Dentate Gyrus - PMC - PubMed Central
In the classification of mammalian cortical regions, DG is designated as an area of archicortex, on the basis of its resemblance to reptilian medial cortex ( ...
[54]
Olfaction, navigation, and the origin of isocortex - Frontiers
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[55]
On the Value of Reptilian Brains to Map the Evolution of the ...
Sep 4, 2017 · The first model holds that M and DM constitute the entire medial pallium and thus the equivalent of the entire mammalian hippocampal formation.
[56]
Evolutionary conservation and conversion of Foxg1 function in brain ...
Jun 5, 2017 · Conversion in Foxg1 downstream regulatory network and function during evolution. Foxg1 target genes and direct binding sites by Foxg1.
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[PDF] 34-46 - The ecology of the avian brain: food-storing memory and the ...
Food-storing birds have an enlarged hippocampus, which is important for retrieving stored food, and this has occurred independently in multiple lineages.
[58]
Review: Hippocampal sclerosis in epilepsy: a neuropathology review
Hippocampal sclerosis (HS) is a common pathology encountered in mesial temporal lobe epilepsy (MTLE) as well as other epilepsy syndromes and in both surgical ...
[59]
Mesial temporal sclerosis | Radiology Reference Article
Jun 18, 2025 · Mesial temporal sclerosis, also commonly referred to as hippocampal sclerosis, is the most common association with intractable temporal lobe epilepsy.
[60]
Entorhinal cortex dysfunction in Alzheimer's disease - PMC - NIH
Feb 1, 2023 · This review will focus on recent findings on EC dysfunction in AD, and discuss the potential pathways for mitigating AD progression by protecting the EC.
[61]
Neuronal Vulnerability of the Entorhinal Cortex to Tau Pathology in ...
Oct 6, 2024 · Accumulation of abnormal tau in the EC is one of the first events in AD and precedes any memory deficits and cognitive decline. Given that ...
[62]
Hippocampal Volume Is Reduced in Schizophrenia and ... - PMC - NIH
Feb 20, 2014 · These findings from a large psychosis sample support decreased hippocampal volume as a putative biomarker for schizophrenia and schizoaffective ...
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https://pmc.ncbi.nlm.nih.gov/articles/PMC4266285/
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Depression, Antidepressants, and Neurogenesis: A Critical ... - Nature
Sep 21, 2011 · The neurogenesis hypothesis of depression posits (1) that neurogenesis in the subgranular zone of the dentate gyrus is regulated negatively by stressful ...
[65]
Identifying Morphological Patterns of Hippocampal Atrophy in ...
Jan 22, 2020 · MTLE and AD have distinctive morphologic patterns of hippocampal atrophy, which provide new insight into the radiology-pathology correlation in these diseases.
[66]
Mapping local hippocampal changes in Alzheimer's disease and ...
Hippocampal volumes of Alzheimer's disease patients were consistent with expectations, exhibiting 30% and 35% tissue loss to the left and right relative to ...
[67]
Functional Magnetic Resonance Imaging (fMRI) Activity in the ... - NIH
The finding of robust hippocampal activation during retrieval of words or objects shows that such activation occurs in traditional recognition memory tasks and ...
[68]
Recollection-related hippocampal fMRI effects predict longitudinal ...
Mar 20, 2020 · Prior fMRI studies have reported relationships between memory-related activity in the hippocampus and in-scanner memory performance, ...
[69]
The role of hippocampal subfield volume and fornix microstructure in ...
Parahippocampal gyrus volume was estimated using Freesurfer and DTI scans were used to obtain diffusion metrics from tractography of the fornix. Item and ...
[70]
Diffusion tensor tractography of the fornix in cerebral amyloid ...
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[71]
Article Hippocampal Neural Circuits Respond to Optogenetic Pacing ...
Apr 23, 2018 · We found that rhythmic optogenetic stimulation of medial septal GABAergic neurons controlled the hippocampal LFP frequency outside of the endogenous theta ...
[72]
Optogenetics reveals paradoxical network stabilizations in ...
Apr 5, 2023 · Contrary to long-standing dogma, both CA1 and CA3 hippocampal regions display strongly recurrent excitation, which is stabilized through inhibition.
[73]
Dynamics of Hippocampal Neurogenesis in Adult Humans
Jun 6, 2013 · In adult humans, 700 new neurons are added in each hippocampus per day, corresponding to an annual turnover of 1.75% of the neurons within the renewing ...
[74]
High-resolution 7T fMRI of Human Hippocampal Subfields during ...
In this study we investigated hippocampal subfield activity in healthy participants using an associative memory paradigm during high-resolution functional ...
[75]
Distinguishing adaptive plasticity from vulnerability in the aging ...
Hippocampal circuits are among the best described networks in the mammalian brain, particularly with regard to the alterations that arise during normal aging.Missing: neuroimaging 21st
[76]
Large-scale 3D EM connectomics dataset of mouse hippocampal ...
Apr 5, 2025 · Here, we present the first large-scale three dimensional (3D) electron microscopy dataset of mouse CA1 at nanometer-scale resolution.Missing: 2020s | Show results with:2020s
[77]
The unique neural wiring of the human hippocampus may maximize ...
Jan 6, 2025 · An abundance of nerve cells combined with sparse but strong connections is a design that maximizes memory retrieval and storage.
[78]
[PDF] A Software for Identification and Characterization of Theta Rhythms ...
Mar 26, 2025 · The software is a MATLAB-based tool with a user interface for identifying and analyzing theta rhythms in the hippocampus, which are related to ...
[79]
Neural ageing and synaptic plasticity: prioritizing brain health in ...
In this review, we explore the multifaceted aspects of healthy brain ageing with emphasis on synaptic plasticity, its adaptive mechanisms and the various ...Missing: century | Show results with:century