The entorhinal cortex (EC) is an area of the brain's allocortex, located in the medial temporal lobe, whose functions include being a widespread network hub for memory, navigation, and the perception of time. The EC is the main interface between the hippocampus and neocortex. The EC-hippocampus system plays an important role in declarative (autobiographical/episodic/semantic) memories and in particular spatial memories including memory formation, memory consolidation, and memory optimization in sleep. The EC is also responsible for the pre-processing (familiarity) of the input signals in the reflex nictitating membrane response of classical trace conditioning; the association of impulses from the eye and the ear occurs in the entorhinal cortex.
The hippocampus is a major component of the brain of humans and other vertebrates. Humans and other mammals have two hippocampi, one in each side of the brain. The hippocampus is part of the limbic system, and plays important roles in the consolidation of information from short-term memory to long-term memory, and in spatial memory that enables navigation. The hippocampus is located in the allocortex, with neural projections into the neocortex in humans, as well as primates. The hippocampus, as the medial pallium, is a structure found in all vertebrates. In humans, it contains two main interlocking parts: the hippocampus proper, and the dentate gyrus.
The dentate gyrus (DG) is part of the hippocampal formation in the temporal lobe of the brain, which also includes the hippocampus and the subiculum. The dentate gyrus is part of the hippocampal trisynaptic circuit and is thought to contribute to the formation of new episodic memories, the spontaneous exploration of novel environments and other functions.
Episodic memory is the memory of everyday events that can be explicitly stated or conjured. It is the collection of past personal experiences that occurred at particular times and places; for example, the party on one's 7th birthday. Along with semantic memory, it comprises the category of explicit memory, one of the two major divisions of long-term memory.
In cognitive psychology and neuroscience, spatial memory is a form of memory responsible for the recording and recovery of information needed to plan a course to a location and to recall the location of an object or the occurrence of an event. Spatial memory is necessary for orientation in space. Spatial memory can also be divided into egocentric and allocentric spatial memory. A person's spatial memory is required to navigate around a familiar city. A rat's spatial memory is needed to learn the location of food at the end of a maze. In both humans and animals, spatial memories are summarized as a cognitive map.
A place cell is a kind of pyramidal neuron in the hippocampus that becomes active when an animal enters a particular place in its environment, which is known as the place field. Place cells are thought to act collectively as a cognitive representation of a specific location in space, known as a cognitive map. Place cells work with other types of neurons in the hippocampus and surrounding regions to perform this kind of spatial processing. They have been found in a variety of animals, including rodents, bats, monkeys and humans.
Head direction (HD) cells are neurons found in a number of brain regions that increase their firing rates above baseline levels only when the animal's head points in a specific direction. They have been reported in rats, monkeys, mice, chinchillas and bats, but are thought to be common to all mammals, perhaps all vertebrates and perhaps even some invertebrates, and to underlie the "sense of direction". When the animal's head is facing in the cell's "preferred firing direction" these neurons fire at a steady rate, but firing decreases back to baseline rates as the animal's head turns away from the preferred direction.
Theta waves generate the theta rhythm, a neural oscillation in the brain that underlies various aspects of cognition and behavior, including learning, memory, and spatial navigation in many animals. It can be recorded using various electrophysiological methods, such as electroencephalogram (EEG), recorded either from inside the brain or from electrodes attached to the scalp.
In the brain, the perforant path or perforant pathway provides a connectional route from the entorhinal cortex to all fields of the hippocampal formation, including the dentate gyrus, all CA fields, and the subiculum.
A grid cell is a type of neuron within the entorhinal cortex that fires at regular intervals as an animal navigates an open area, allowing it to understand its position in space by storing and integrating information about location, distance, and direction. Grid cells have been found in many animals, including rats, mice, bats, monkeys, and humans.
The perirhinal cortex is a cortical region in the medial temporal lobe that is made up of Brodmann areas 35 and 36. It receives highly processed sensory information from all sensory regions, and is generally accepted to be an important region for memory. It is bordered caudally by postrhinal cortex or parahippocampal cortex and ventrally and medially by entorhinal cortex.
The retrosplenial cortex (RSC) is a cortical area in the brain comprising Brodmann areas 29 and 30. It is secondary association cortex, making connections with numerous other brain regions. The region's name refers to its anatomical location immediately behind the splenium of the corpus callosum in primates, although in rodents it is located more towards the brain surface and is relatively larger. Its function is currently not well understood, but its location close to visual areas and also to the hippocampal spatial/memory system suggest it may have a role in mediating between perceptual and memory functions, particularly in the spatial domain. However, its exact contribution to either space or memory processing has been hard to pin down.
The trisynaptic circuit, or trisynaptic loop is a relay of synaptic transmission in the hippocampus. The circuit was initially described by the neuroanatomist Santiago Ramon y Cajal, in the early twentieth century, using the Golgi staining method. After the discovery of the trisynaptic circuit, a series of research has been conducted to determine the mechanisms driving this circuit. Today, research is focused on how this loop interacts with other parts of the brain, and how it influences human physiology and behaviour. For example, it has been shown that disruptions within the trisynaptic circuit leads to behavioural changes in rodent and feline models.
Boundary cells are neurons found in the hippocampal formation that respond to the presence of an environmental boundary at a particular distance and direction from an animal. The existence of cells with these firing characteristics were first predicted on the basis of properties of place cells. Boundary cells were subsequently discovered in several regions of the hippocampal formation: the subiculum, presubiculum and entorhinal cortex.
Memory consolidation is a category of processes that stabilize a memory trace after its initial acquisition. A memory trace is a change in the nervous system caused by memorizing something. Consolidation is distinguished into two specific processes. The first, synaptic consolidation, which is thought to correspond to late-phase long-term potentiation, occurs on a small scale in the synaptic connections and neural circuits within the first few hours after learning. The second process is systems consolidation, occurring on a much larger scale in the brain, rendering hippocampus-dependent memories independent of the hippocampus over a period of weeks to years. Recently, a third process has become the focus of research, reconsolidation, in which previously consolidated memories can be made labile again through reactivation of the memory trace.
Sharp waves and ripples (SWRs) are oscillatory patterns produced by extremely synchronised activity of neurons in the mammalian hippocampus and neighbouring regions which occur spontaneously in idle waking states or during NREM sleep. They can be observed with a variety of imaging methods, such as EEG. They are composed of large amplitude sharp waves in local field potential and produced by tens of thousands of neurons firing together within 30-100 ms window. They are some of the most synchronous oscillations patterns in the brain, making them susceptible to pathological patterns such as epilepsy.They have been extensively characterised and described by György Buzsáki and have been shown to be involved in memory consolidation in NREM sleep and the replay of memories acquired during wakefulness.
John O'Keefe, is an American-British neuroscientist, psychologist and a professor at the Sainsbury Wellcome Centre for Neural Circuits and Behaviour and the Research Department of Cell and Developmental Biology at University College London. He discovered place cells in the hippocampus, and that they show a specific kind of temporal coding in the form of theta phase precession. He shared the Nobel Prize in Physiology or Medicine in 2014, together with May-Britt Moser and Edvard Moser; he has received several other awards. He has worked at University College London for his entire career, but also held a part-time chair at the Norwegian University of Science and Technology at the behest of his Norwegian collaborators, the Mosers.
The hippocampus participates in the encoding, consolidation, and retrieval of memories. The hippocampus is located in the medial temporal lobe (subcortical), and is an infolding of the medial temporal cortex. The hippocampus plays an important role in the transfer of information from short-term memory to long-term memory during encoding and retrieval stages. These stages do not need to occur successively, but are, as studies seem to indicate, and they are broadly divided in the neuronal mechanisms that they require or even in the hippocampal areas that they seem to activate. According to Gazzaniga, "encoding is the processing of incoming information that creates memory traces to be stored." There are two steps to the encoding process: "acquisition" and "consolidation". During the acquisition process, stimuli are committed to short term memory. Then, consolidation is where the hippocampus along with other cortical structures stabilize an object within long term memory, which strengthens over time, and is a process for which a number of theories have arisen to explain the underlying mechanism. After encoding, the hippocampus is capable of going through the retrieval process. The retrieval process consists of accessing stored information; this allows learned behaviors to experience conscious depiction and execution. Encoding and retrieval are both affected by neurodegenerative and anxiety disorders and epilepsy.
Phase precession is a neurophysiological process in which the time of firing of action potentials by individual neurons occurs progressively earlier in relation to the phase of the local field potential oscillation with each successive cycle. In place cells, a type of neuron found in the hippocampal region of the brain, phase precession is believed to play a major role in the neural coding of information. John O'Keefe, who later shared the 2014 Nobel Prize in Physiology or Medicine for his discovery that place cells help form a "map" of the body's position in space, co-discovered phase precession with Michael Recce in 1993.
Lisa Giocomo is an American neuroscientist who is associate professor in the Department of Neurobiology at Stanford University School of Medicine. Giocomo probes the molecular and cellular mechanisms underlying cortical neural circuits involved in spatial navigation and memory.
