Multiplexing of temporal and spatial information in the lateral entorhinal cortex

Contrary to the common view that brain regions in the "what" and "where" streams distinctly process object and spatial cues, respectively, we found that both streams encode both object and spatial information but distinctly organize memories for objects and space. Specifically, perirhinal cortex and lateral entorhinal cortex represent objects and, within the object-specific representations, the locations where they occur. Conversely, medial entorhinal cortex represents relevant locations and, within those spatial representations, the objects that occupy them. Furthermore, these findings reach beyond simple notions of perirhinal cortex and lateral entorhinal cortex neurons as object detectors and MEC neurons as position detectors, and point to a more complex organization of memory representations within the medial temporal lobe system.

The medial temporal lobe (MTL) is involved in mnemonic processing. The perirhinal cortex (PRC) plays a role in object recognition memory, while the hippocampus is required for certain forms of spatial memory and episodic memory. The lateral entorhinal cortex (LEC) receives direct projections from PRC and is one of the two major cortical inputs to the hippocampus. The transformations that occur between PRC and LEC neural representations are not well understood. Here, we show that PRC and LEC had similarly high proportions of neurons with object-related activity (PRC 52/94; LEC 72/153), as expected from their locations in the "what" pathway into the hippocampus. However, LEC unit activity showed more spatial stability than PRC unit activity. A minority of LEC neurons showed stable spatial firing fields away from objects; these firing fields strongly resembled hippocampal place fields. None of the PRC neurons showed this place-like firing. None of the PRC or LEC neurons demonstrated the high firing rates associated with interneurons in hippocampus or medial entorhinal cortex, further dissociating this information processing stream from the path-integration based, movement-related processing of the medial entorhinal cortex and hippocampus. These results provide evidence for nonspatial information processing in the PRC-LEC pathway, as well as showing a functional dissociation between PRC and LEC, with more purely nonspatial representations in PRC and combined spatial-nonspatial representations in LEC.

LATERAL ENTORHINAL CORTEX HYPOACTIVATION IN AMNESTIC MILD COGNITIVE IMPAIRMENT

The human entorhinal-hippocampal circuitry is characterized by an information-specific functional organization where two routes, that are preferentially connected to the parahippocampal cortex or the perirhinal and retrosplenial cortices, divide the entorhinal cortex as well as hippocampal subiculum and CA1 subregions.

In this review, we aim to reappraise the organization of intrinsic and extrinsic networks of the entorhinal cortex with a focus on the concept of parallel cortical connectivity streams. The concept of two entorhinal areas, the lateral and medial entorhinal cortex, belonging to two parallel input-output streams mediating the encoding and storage of respectively what and where information hinges on the claim that a major component of their cortical connections is with the perirhinal cortex and postrhinal or parahippocampal cortex in, respectively, rodents or primates. In this scenario, the lateral entorhinal cortex and the perirhinal cortex are connectionally associated and likewise the postrhinal/parahippocampal cortex and the medial entorhinal cortex are partners. In contrast, here we argue that the connectivity matrix emphasizes the potential of substantial integration of cortical information through interactions between the two entorhinal subdivisions and between the perirhinal and postrhinal/parahippocampal cortices, but most importantly through a new observation that the postrhinal/parahippocampal cortex projects to both lateral and medial entorhinal cortex. We suggest that entorhinal inputs provide the hippocampus with high-order complex representations of the external environment, its stability, as well as apparent changes either as an inherent feature of a biological environment or as the result of navigating the environment. This thus indicates that the current connectional model of the parahippocampal region as part of the medial temporal lobe memory system needs to be revised.

Recent models of episodic memory propose a division of labor among medial temporal lobe cortices comprising the parahippocampal gyrus. Specifically, perirhinal and lateral entorhinal cortices are thought to comprise an object/item information pathway, whereas parahippocampal and medial entorhinal cortices are thought to comprise a spatial/contextual information pathway. Although several studies in human subjects have demonstrated a perirhinal/parahippocampal division, such a division among subregions of the human entorhinal cortex has been elusive. Other recent work has implicated pattern separation computations in the dentate gyrus and CA3 subregions of the hippocampus as a mechanism supporting the resolution of mnemonic interference. However, the nature of contributions of medial temporal lobe cortices to downstream hippocampal computations is largely unknown. We used high-resolution fMRI during a task selectively taxing mnemonic discrimination of object identity or spatial location, designed to differentially engage the two information pathways in the medial temporal lobes. Consistent with animal models, we demonstrate novel evidence for a domain-selective dissociation between lateral and medial entorhinal cortex in humans, and between perirhinal and parahippocampal cortex as a function of information content. Conversely, hippocampal dentate gyrus/CA3 demonstrated signals consistent with resolution of mnemonic interference across domains. These results provide insight into the information processing capacities and hierarchical interference resolution throughout the human medial temporal lobe.

A prominent theory in the neurobiology of memory processing is that episodic memory is supported by contextually gated spatial representations in the hippocampus formed by combining spatial information from medial entorhinal cortex (MEC) with non-spatial information from lateral entorhinal cortex (LEC). However, there is a growing body of evidence from lesion and single-unit recording studies in rodents suggesting that LEC might have a role in encoding space, particularly the current and previous locations of objects within the local environment. Landmarks, both local and global, have been shown to control the spatial representations hypothesized to underlie cognitive maps. Consequently, it has recently been suggested that information processing within this network might be organized with reference to spatial scale with LEC and MEC providing information about local and global spatial frameworks respectively. In the present study, we trained animals to search for food using either a local or global spatial framework. Animals were re-tested on both tasks after receiving excitotoxic lesions of either the MEC or LEC. LEC lesioned animals were impaired in their ability to learn a local spatial framework task. LEC lesioned animals were also impaired on an object recognition (OR) task involving multiple local features but unimpaired at recognizing a single familiar object. Together, this suggests that LEC is involved in associating features of the local environment. However, neither LEC nor MEC lesions impaired performance on the global spatial framework task.

The encoding of time and its binding to events are crucial for episodic memory, but how these processes are carried out in hippocampal-entorhinal circuits is unclear. Here we show in freely foraging rats that temporal information is robustly encoded across time scales from seconds to hours within the overall population state of the lateral entorhinal cortex. Similarly pronounced encoding of time was not present in the medial entorhinal cortex or in hippocampal areas CA3-CA1. When animals' experiences were constrained by behavioural tasks to become similar across repeated trials, the encoding of temporal flow across trials was reduced, whereas the encoding of time relative to the start of trials was improved. The findings suggest that populations of lateral entorhinal cortex neurons represent time inherently through the encoding of experience. This representation of episodic time may be integrated with spatial inputs from the medial entorhinal cortex in the hippocampus, allowing the hippocampus to store a unified representation of what, where and when.

The Mechanism of Rate Remapping in the Dentate Gyrus

Principal neurons in different medial entorhinal cortex (MEC) layers show variations in spatial modulation that stabilize between 15 and 30 days postnatally. These in vivo variations are likely due to differences in intrinsic membrane properties and integrative capacities of neurons. The latter depends on inputs and thus potentially on the morphology of principal neurons. In this comprehensive study, we systematically compared the morphological and physiological characteristics of principal neurons in all MEC layers of newborn rats before and after weaning. We recorded simultaneously from up to four post-hoc morphologically identified MEC principal neurons in vitro. Neurons in L(ayer) I-LIII have dendritic and axonal arbors mainly in superficial layers, and LVI neurons mainly in deep layers. The dendritic and axonal trees of part of LV neurons diverge throughout all layers. Physiological properties of principal neurons differ between layers. In LII, most neurons have a prominent sag potential, resonance and membrane oscillations. Neurons in LIII and LVI fire relatively regular, and lack sag potentials and membrane oscillations. LV neurons show the most prominent spike-frequency adaptation and highest input resistance. The data indicate that adult-like principal neuron types can be differentiated early on during postnatal development. The results of the accompanying paper, in which principal neurons in the lateral entorhinal cortex (LEC) were described (Canto and Witter,2011), revealed that significant differences between LEC and MEC exist mainly in LII neurons. We therefore systematically analyzed changes in LII biophysical properties along the mediolateral axis of MEC and LEC. There is a gradient in properties typical for MEC LII neurons. These properties are most pronounced in medially located neurons and become less apparent in more laterally positioned ones. This gradient continues into LEC, such that in LEC medially positioned neurons share some properties with adjacent MEC cells.

The present report deals with the projections from the entorhinal and perirhinal cortices to subcortical forebrain structures and the brainstem in the cat. By using anterograde and retrograde tracing techniques, it could be demonstrated that the entire mediolateral extent of the parahippocampal cortex issues prominent projections to the dorsal and ventral striatum, the amygdala, and the claustrum. In addition, the entorhinal cortex sends projections to the septum and the diagonal band of Broca. Only the perirhinal cortex gives rise to a weak projection to the dorsolateral periaquaductal gray and the ventral pontine region. The major proportion of the subcortical projections originates in the perirhinal cortex and the lateral entorhinal cortex, whereas the medial entorhinal cortex has a much sparser output and sends no fibers to the amygdala. The subcortical projections from both the entorhinal cortex and the perirhinal cortex arise mostly from their deep layers. It was further found that these projections are topographically organized along the mediolateral axis of the parahippocampal cortex. This mediolateral axis is related to a ventrolateral to dorsomedial axis in the septum, a mediolateral axis in the amygdala and the ventral striatum, and a ventrodorsal coordinate in the dorsal striatum and the claustrum. A further topography was observed in the projections from the perirhinal cortex to the lateral amygdaloid nucleus. A rostrocaudal axis in the perirhinal cortex corresponds to a mediolateral axis in the lateral amygdaloid nucleus. The present observations are compared with data concerning the connectivity of the parahippocampal cortex with the hippocampal formation and other cortical structures. It is suggested that the parahippocampal cortex in the cat may be conceptualized as an interface between the hippocampal formation and several subcortical structures in the realm of the limbic and motor systems.

Episodic memory, the conscious recollection of past events, is typically experienced from a first-person (egocentric) perspective. The hippocampus plays an essential role in episodic memory and spatial cognition. Although the allocentric nature of hippocampal spatial coding is well understood, little is known about whether the hippocampus receives egocentric information about external items. We recorded in rats the activity of single neurons from the lateral entorhinal cortex (LEC) and medial entorhinal cortex (MEC), the two major inputs to the hippocampus. Many LEC neurons showed tuning for egocentric bearing of external items, whereas MEC cells tended to represent allocentric bearing. These results demonstrate a fundamental dissociation between the reference frames of LEC and MEC neural representations.

There is context-related temporal and spatial information in video. How to make better use of these information is the key to improving the accuracy and speed of video object detection. The method of processing spatial and temporal information through 3D convolution is better than that of processing spatial information first and then processing time dimension information. There are two reasons for this assertion. One is that the 3D convolution can process spatial information and temporal information at the same time which will not divide them into two parts in feature extraction. The other is that the human visual derivation mechanism needs to memorize multi frames and process a video as a whole. After the training test on NFS dataset, it is verified that 3D convolution has stronger ability to extract temporal and spatial information. In addition, this paper also proposes a downward sampling method by connecting the low-resolution layer and the high-resolution layer, which can effectively improve the FPN model and enrich its high-level feature representation. After training and testing with different number of downward sample modules on MOT16 dataset, the improvement of model performance by down-sampling module is verified. Finally, this paper combines the downward sample module with the 3D network, and obtains the best test results on the NFS dataset.

Ce rapport analyse l'organisation anatomique du systeme limbique, ses principales connexions avec les autres compartiments du cerveau et sa physiologie en rapport avec le comportement de l'animal

In the context of automotive driving assistance, this paper describes a generic (i.e. applicable to both vehicle interior and exterior scenes) vision based approach for scene content analysis. It makes use of temporal and spatial information from a stereoscopic sequence of images to localize objects and estimate their position and motion. The proposed method is divided into three steps. First, image features are selected, tracked and reconstructed in the 3D world space. Second, a clustering step is processed in the 5D space made of the positions and 2D motions parameters. The last step is devoted to clusters interpretation: it is out of the scope of the paper, however orientations are given to illustrate the capabilities of the proposed approach. The paper is organized as follows: first, the use of temporal and spatial information from a stereoscopic sequence is investigated. A state of the art of existing methods is presented. Then, a generic approach for object segmentation is proposed. Lastly, experimental results are presented.