Connections between the anterior inferotemporal cortex (area TE) and CA1 of the hippocampus in monkey

The Effective Connectivity of the Human Hippocampal Memory System

Effective connectivity measurements in the human hippocampal memory system based on the resting-state blood oxygenation-level dependent signal were made in 172 participants in the Human Connectome Project to reveal the directionality and strength of the connectivity. A ventral "what" hippocampal stream involves the temporal lobe cortex, perirhinal and parahippocampal TF cortex, and entorhinal cortex. A dorsal "where" hippocampal stream connects parietal cortex with posterior and retrosplenial cingulate cortex, and with parahippocampal TH cortex, which, in turn, project to the presubiculum, which connects to the hippocampus. A third stream involves the orbitofrontal and ventromedial-prefrontal cortex with effective connectivity with the hippocampal, entorhinal, and perirhinal cortex. There is generally stronger forward connectivity to the hippocampus than backward. Thus separate "what," "where," and "reward" streams can converge in the hippocampus, from which back projections return to the sources. However, unlike the simple dual stream hippocampal model, there is a third stream related to reward value; there is some cross-connectivity between these systems before the hippocampus is reached; and the hippocampus has some effective connectivity with earlier stages of processing than the entorhinal cortex and presubiculum. These findings complement diffusion tractography and provide a foundation for new concepts on the operation of the human hippocampal memory system.

Human hippocampal responses to network intracranial stimulation vary with theta phase

Hippocampal-dependent memory is thought to be supported by distinct connectivity states, with strong input to the hippocampus benefitting encoding and weak input benefitting retrieval. Previous research in rodents suggests that the hippocampal theta oscillation orchestrates the transition between these states, with opposite phase angles predicting minimal versus maximal input. We investigated whether this phase dependence exists in humans using network-targeted intracranial stimulation. Intracranial local field potentials were recorded from individuals with epilepsy undergoing medically necessary stereotactic electroencephalographic recording. In each subject, biphasic bipolar direct electrical stimulation was delivered to lateral temporal sites with demonstrated connectivity to hippocampus. Lateral temporal stimulation evoked ipsilateral hippocampal potentials with distinct early and late components. Using evoked component amplitude to measure functional connectivity, we assessed whether the phase of hippocampal theta predicted relatively high versus low connectivity. We observed an increase in the continuous phase-amplitude relationship selective to the early and late components of the response evoked by lateral temporal stimulation. The maximal difference in these evoked component amplitudes occurred across 180 degrees of separation in the hippocampal theta rhythm; that is, the greatest difference in component amplitude was observed when stimulation was delivered at theta peak versus trough. The pattern of theta-phase dependence observed for hippocampus was not identified for control locations. These findings demonstrate that hippocampal receptivity to input varies with theta phase, suggesting that theta phase reflects connectivity states of human hippocampal networks. These findings confirm a putative mechanism by which neural oscillations modulate human hippocampal function.

Extensive cortical functional connectivity of the human hippocampal memory system

The cortical connections of the human hippocampal memory system are fundamental to understanding its operation in health and disease, especially in the context of the great development of the human cortex. The functional connectivity of the human hippocampal system was analyzed in 172 participants imaged at 7T in the Human Connectome Project. The human hippocampus has high functional connectivity not only with the entorhinal cortex, but also with areas that are more distant in the ventral 'what' stream including the perirhinal cortex and temporal cortical visual areas. Parahippocampal gyrus TF in humans has connectivity with this ventral 'what' subsystem. Correspondingly for the dorsal stream, the hippocampus has high functional connectivity not only with the presubiculum, but also with areas more distant, the medial parahippocampal cortex TH which includes the parahippocampal place or scene area, the posterior cingulate including retrosplenial cortex, and the parietal cortex. Further, there is considerable cross connectivity between the ventral and dorsal streams with the hippocampus. The findings are supported by anatomical connections, which together provide an unprecedented and quantitative overview of the extensive cortical connectivity of the human hippocampal system that goes beyond hierarchically organised and segregated pathways connecting the hippocampus and neocortex, and leads to new concepts on the operation of the hippocampal memory system in humans.

Extensive Cortical Connectivity of the Human Hippocampal Memory System: Beyond the "What" and "Where" Dual Stream Model

The human hippocampus is involved in forming new memories: damage impairs memory. The dual stream model suggests that object "what" representations from ventral stream temporal cortex project to the hippocampus via the perirhinal and then lateral entorhinal cortex, and spatial "where" representations from the dorsal parietal stream via the parahippocampal gyrus and then medial entorhinal cortex. The hippocampus can then associate these inputs to form episodic memories of what happened where. Diffusion tractography was used to reveal the direct connections of hippocampal system areas in humans. This provides evidence that the human hippocampus has extensive direct cortical connections, with connections that bypass the entorhinal cortex to connect with the perirhinal and parahippocampal cortex, with the temporal pole, with the posterior and retrosplenial cingulate cortex, and even with early sensory cortical areas. The connections are less hierarchical and segregated than in the dual stream model. This provides a foundation for a conceptualization for how the hippocampal memory system connects with the cerebral cortex and operates in humans. One implication is that prehippocampal cortical areas such as the parahippocampal TF and TH subregions and perirhinal cortices may implement specialized computations that can benefit from inputs from the dorsal and ventral streams.

The pre/parasubiculum: a hippocampal hub for scene-based cognition?

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We consider internal representations of the world in the form of scenes.

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The anterior medial hippocampus is implicated in scene-based cognition.

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This region contains the pre/parasubiculum.

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The pre/parasubiculum is a primary target of a major visuospatial processing system.

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The pre/parasubiculum may be the hippocampal hub of the scene processing network.

Internal representations of the world in the form of spatially coherent scenes have been linked with cognitive functions including episodic memory, navigation and imagining the future. In human neuroimaging studies, a specific hippocampal subregion, the pre/parasubiculum, is consistently engaged during scene-based cognition. Here we review recent evidence to consider why this might be the case. We note that the pre/parasubiculum is a primary target of the parieto-medial temporal processing pathway, it receives integrated information from foveal and peripheral visual inputs and it is contiguous with the retrosplenial cortex. We discuss why these factors might indicate that the pre/parasubiculum has privileged access to holistic representations of the environment and could be neuroanatomically determined to preferentially process scenes.

The ventral visual pathway: an expanded neural framework for the processing of object quality

Since the original characterization of the ventral visual pathway, our knowledge of its neuroanatomy, functional properties, and extrinsic targets has grown considerably. Here we synthesize this recent evidence and propose that the ventral pathway is best understood as a recurrent occipitotemporal network containing neural representations of object quality both utilized and constrained by at least six distinct cortical and subcortical systems. Each system serves its own specialized behavioral, cognitive, or affective function, collectively providing the raison d'être for the ventral visual pathway. This expanded framework contrasts with the depiction of the ventral visual pathway as a largely serial staged hierarchy culminating in singular object representations and more parsimoniously incorporates attentional, contextual, and feedback effects.

The role of recollection and familiarity in the functional differentiation of the medial temporal lobes

The components of the medial temporal lobes (MTL) receive different kinds of input. The perirhinal cortex receives primarily object/item information, the parahippocampal cortex receives contextual information, and the hippocampus receives high-level inputs that include object/item, context, and other information. Critically, the perirhinal and parahippocampal cortices have similar cytoarchitectonics, which differ considerably from that of the hippocampus and suggest that these cortices process their inputs differently from the way that the hippocampus processes its inputs. Much evidence indicates that the hippocampus is designed to rapidly bind together pattern-separated representations that support recall/recollection well. In contrast, the newer MTL cortices rapidly create poorly pattern-separated memories that support familiarity well, but recall/recollection very poorly. For over a decade, there has been disagreement about whether recall/recollection is primarily mediated by the hippocampus and familiarity by the evolutionarily newer MTL cortices or whether the MTL mediates these kinds of memory in an integrated, homogeneous fashion. Common misconceptions about familiarity, recollection, item, and associative memory are discussed as are methodological problems with MTL lesion and functional imaging research. The possible confound of familiarity with weaker memory and recollection with stronger memory is discussed and the implications of the Montaldi et al. (2006) functional Magnetic Resonance Imaging (fMRI) study, which matched memory strength between strong familiarity and recollection, finding that only recollection activated the hippocampus, are discussed. A suggestion is made about how the long-running conflict of findings in the human hippocampal lesion literature may be resolved.

Network interactions explain effective encoding in the context of medial temporal damage in MCI

Selective dysfunction in the medial temporal lobe (MTL) in amnestic mild cognitive impairment (MCI) results in a relatively circumscribed impairment in episodic memory. Previously, we found that activation extent in MTL during encoding correlated with subsequent recognition (hit rate) in controls but not in MCI patients (Mandzia et al. [2009]: Neurobiol Aging 30:717-730). Here, we examined whether functional connectivity amongst MTL and cortical regions might better explain differences in subsequent recognition success. Participants underwent fMRI scanning during picture encoding, and multivariate analysis was used to characterize the relationship between network activations and recognition. Both patients and controls activated a canonical MTL encoding network. However, this network correlated with hit rate only for controls. In MCI patients, recognition variability was best explained by the engagement of an additional network including BA 20. We propose that this pattern represents functional reorganization caused by reduced efficiency in the MTL network. Our findings suggest that understanding brain-behavior relationships in neurological disorders requires examination of large-scale networks, even when dysfunction is relatively focal as in MCI.

Anomia for people's names after left anterior temporal lobe resection--case report

A 47-year-old man was admitted to our hospital with an intrinsic brain tumor in the left anterior temporal lobe. Preoperative sodium thiopental test demonstrated left hemispheric dominance. Awake craniotomy was performed for dominant-hemispheric tumor resection using language mapping to identify the stimulation-induced positive language area. The tasks of object naming and repetition were used, along with specific tests for famous people's names. The language area was detected on the superior temporal gyrus and preserved. Following surgery, this patient was unable to retrieve the names of famous individuals (i.e. anomia for people's name) despite preservation of semantic knowledge for those individuals. This anomia for people's names showed no improvement at all for a period of 15 months. This case report and other sporadic cases with this type of deficit reveal the left anterior temporal lobe is an important brain area for retrieving people's names.

Complementary circuits connecting the orbital and medial prefrontal networks with the temporal, insular, and opercular cortex in the macaque monkey

The origin and termination of axonal connections between the orbital and medial prefrontal cortex (OMPFC) and the temporal, insular, and opercular cortex have been analyzed with anterograde and retrograde axonal tracers, injected in the OMPFC or temporal cortex. The results show that there are two distinct, complementary, and reciprocal neural systems, related to the previously defined "orbital" and "medial" prefrontal networks. The orbital prefrontal network, which includes areas in the central and lateral part of the orbital cortex, is connected with vision-related areas in the inferior temporal cortex (especially area TEav) and the fundus and ventral bank of the superior temporal sulcus (STSf/v), and with somatic sensory-related areas in the frontal operculum (OPf) and dysgranular insular area (Id). No connections were found between the orbital network and auditory areas. The orbital network is also connected with taste and olfactory cortical areas and the perirhinal cortex and appears to be involved in assessment of sensory objects, especially food. The medial prefrontal network includes areas on the medial surface of the frontal lobe, medial orbital areas, and two caudolateral orbital areas. It is connected with the rostral superior temporal gyrus (STGr) and the dorsal bank of the superior temporal sulcus (STSd). This region is rostral to the auditory parabelt areas, and there are only relatively light connections between the auditory areas and the medial network. This system, which is also connected with the entorhinal, parahippocampal, and cingulate/retrosplenial cortex, may be involved in emotion and other self-referential processes.

Spatial organization of direct hippocampal field CA1 axonal projections to the rest of the cerebral cortex

The spatial distribution of axonal projections descending from rat field CA1 to thalamus and hypothalamus was analyzed previously with the PHAL method [Cenquizca, L.A., Swanson, L.W. 2006. An analysis of direct hippocampal cortical field CA1 axonal projections to diencephalon in the rat. J Comp Neurol 497:101-114.]. The same experimental material was used here to define the topography of field CA1 association projections to other cerebral cortical areas. First, the results confirm and extend known intrahippocampal formation inputs to dentate gyrus, subiculum, presubiculum, parasubiculum, and entorhinal area, which are arranged generally along the formation's transverse axis and dominated by the subicular projection-by far the densest established by field CA1 anywhere in the brain. And second, field CA1 innervates a virtually complete ring of extrahippocampal formation cortex via three routes. A dorsal pathway from the dorsal third of field CA1 innervates moderately the retrosplenial area; a moderately strong ventral pathway from the ventral two thirds of field CA1 passing through the longitudinal association bundle sends offshoots to visual, auditory, somatosensory, gustatory, main and accessory olfactory, and visceral areas-as well as the basolateral amygdalar complex and the agranular insular and orbital areas; and a cortical-subcortical-cortical pathway through the fornix from the whole longitudinal extent of field CA1 innervates rather strongly a rostral region that includes the tenia tecta along with the anterior cingulate, prelimbic, infralimbic, and orbital areas. The functional consequences of long-term potentiation in field CA1 projection neurons remain to be explored.

Cytoarchitectonic and chemoarchitectonic subdivisions of the perirhinal and parahippocampal cortices in macaque monkeys

Although the perirhinal and parahippocampal cortices have been shown to be critically involved in memory processing, the boundaries and extent of these areas have been controversial. To produce a more objective and reproducible description, the architectonic boundaries and structure of the perirhinal (areas 35 and 36) and parahippocampal (areas TF and TH) cortices were analyzed in three macaque species, with four different staining methods [Nissl and immunohistochemistry for parvalbumin, nonphosphorylated neurofilaments (with SMI-32), and the m2 muscarinic acetylcholine receptor]. We further correlated the architectonic boundary of the parahippocampal cortex with connections to and from different subregions of anterior area TE and with previously published connections with the prefrontal cortex and temporal pole (Kondo et al. [2005] J. Comp. Neurol. 493:479-509). Together, these data provided a clear delineation of the perirhinal and parahippocampal areas, although it differs from previous descriptions. In particular, we did not extend the perirhinal cortex into the temporal pole, and the lateral boundaries of areas 36 and TF with area TE were placed more medially than in other studies. The lateral boundary of area TF in Macaca fuscata was located more laterally than in Macaca fascicularis or Macaca mulatta, although there was no difference in architectonic structure. We recognized a caudal, granular part of the parahippocampal cortex that we termed "area TFO." This area closely resembles the laterally adjacent area TE and the caudally adjacent area V4 but is clearly different from the more rostral area TF. These areas are likely to have distinct functions.

Comparative analysis of layer-specific genes in Mammalian neocortex

We examined the expression patterns of 4 layer-specific genes in monkey and mouse cortices by fluorescence double in situ hybridization. Based on their coexpression profiles, we were able to distinguish several subpopulations of deep layer neurons. One group was characterized by the expression of ER81 and the lack of Nurr1 mRNAs and mainly localized to layer 5. In monkeys, this neuronal group was further subdivided by 5-HT2C receptor mRNA expression. The 5-HT2C(+)/ER81(+) neurons were located in layer 5B in most cortical areas, but they intruded layer 6 in the primary visual area (V1). Another group of neurons, in monkey layer 6, was characterized by Nurr1 mRNA expression and was further subdivided as Nurr1(+)/connective tissue growth factor (CTGF)(-) and Nurr1(+)/CTGF(+) neurons in layers 6A and 6B, respectively. The Nurr1(+)/CTGF(+) neurons coexpressed ER81 mRNA in monkeys but not in mice. On the basis of tracer injections in 3 monkeys, we found that the Nurr1(+) neurons in layer 6A send some corticocortical, but not corticopulvinar, projections. Although the Nurr1(+)/CTGF(-) neurons were restricted to lateral regions in the mouse cortex, they were present throughout the monkey cortex. Thus, an architectonic heterogeneity across areas and species was revealed for the neuronal subpopulations with distinct gene expression profiles.

Distinctive morphology of hippocampal CA1 terminations in orbital and medial frontal cortex in macaque monkeys

The fine morphology of hippocampal connections to the orbital and medial frontal cortex (OMFC) was investigated by placing injections of anterograde tracers in the CA1 in two monkeys. The axons terminated mainly in layers 2 and 3, of areas 11, 13, 14c, 25, and 32, and were widely divergent in these layers, traversing 2-4 mm. Boutons were scattered along the main axon, but also occurred as distinctive small, spherical clusters of terminations ("mini-clusters"; diameter <50 microm). Occasional larger terminal arbors were observed in layer 3 and these were often unusually tortuous or convoluted. These features may imply a specialized microcircuitry of hippocampal-OMFC connections, including an origin from a particular subpopulation of CA1 projection neurons.

Zinc-enriched amygdalo- and hippocampo-cortical connections to the inferotemporal cortices in macaque monkey

Synaptic zinc (Zn), a co-factor in some glutamatergic synapses, has been implicated in plasticity effects, as well as in several excitotoxic and other pathophysiological conditions. In this study, we provide information about the distribution of Zn in inferotemporal cortex, a region at the interface of the visual and hippocampal networks. In brief, we found a lateral to medial increase in Zn, where TEad, a unimodal visual area, showed low levels of Zn; TEav, intermediate levels; and perirhinal cortex, a multimodal limbic area, high levels. The distribution of parvalbumin, a calcium binding protein, showed a reverse gradient to that of Zn. The neurons of origin of the Zn+ termination were identified by making intracortical injections sodium selenite (Na2SeO3). This substance interacts with Zn to form precipitates of ZnSe and in this form is transported retrogradely to the soma. A mixed population of labeled neurons was visualized, which included Zn+ neurons in CA1 of the hippocampus and in several amygdala subnuclei. In CA1, Zn+ neurons were restricted to the upper part of stratum pyramidale. Zn is thought to contribute to activity-dependent synaptic plasticity. The specifically high level in perirhinal cortex, and its origin from neurons in CA1 and the amygdala, may relate to cellular events involved in visual long-term memory formation.

Direct projections from CA1 to the superior temporal sulcus in the monkey, revealed by single axon analysis

Anterograde tracer injections in the middle sector of CA1 in macaque monkeys demonstrate a direct projection to the fundus of the anterior superior temporal sulcus, in area IPa. Terminations are predominantly in layer 3. With regard to both terminal and arbor configuration, these hippocampal-cortical connections are morphologically similar to corticocortical connections to temporal association cortex. This report provides additional evidence of direct CA1 connections to particular multimodal cortical areas.

Functional connectivity with the hippocampus during successful memory formation

Although it is well established that the hippocampus is critical for episodic memory, little is known about how the hippocampus interacts with cortical regions during successful memory formation. Here, we used event-related functional magnetic resonance imaging (fMRI) to identify areas that exhibited differential functional connectivity with the hippocampus during processing of novel objects that were subsequently remembered or forgotten on a postscan test. Functional connectivity with the hippocampus was enhanced during successful, as compared with unsuccessful, memory formation, in a distributed network of limbic cortical areas-including perirhinal, orbitofrontal, and retrosplenial/posterior cingulate cortex-that are anatomically connected with the hippocampal formation. Increased connectivity was also observed in lateral temporal, medial parietal, and medial occipital cortex. These findings demonstrate that successful memory formation is associated with transient increases in cortico-hippocampal interaction.