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. Place-cell firing patterns are often determined by stimuli in the environment such as visual landmarks, and olfactory and vestibular stimuli. Place cells have the ability to suddenly change their firing pattern from one pattern to another, a phenomenon known as remapping. This remapping may occur in either some of the place cells or in all place cells at once. It may be caused by a number of changes, such as in the odor of the environment. Place cells are thought to play an important role in episodic memory. They contain information about the spatial context a memory took place in. And they seem to perform consolidation by exhibiting replay – the reactivation of the place cells involved in a certain experience at a much faster timescale. Place cells show alterations with age and disease, such as Alzheimer's disease, which may be involved in a decrease of memory function. The 2014 Nobel Prize in Physiology or Medicine was awarded to John O'Keefe for the discovery of place cells, and to Edvard and May-Britt Moser for the discovery of grid cells.
Background Place cells were first discovered by John O'Keefe and Jonathan Dostrovsky in 1971 in rats' hippocampuses. They noticed that rats with impairments in their hippocampus performed poorly in spatial tasks, and thus hypothesised that this area must hold some kind of spatial representation of the environment. To test this hypothesis, they developed chronic electrode implants, with which they could record the activity of individual cells extracellularly in the hippocampus. They noted that some of the cells showed activity when a rat was "situated in a particular part of the testing platform facing in a particular direction". These cells would later be called place cells.
In 1976, O'Keefe performed a follow-up study, demonstrating the presence of what they called place units. These units were cells that fired in a particular place in the environment, the place field. They are described as having a low resting firing rate (<1 Hz) when a rat is not in its place field, but a particularly high firing rate, which can be over 100 Hz in some cases, within the place field. Additionally, O'Keefe described six special cells, which he called misplace units, which also fire only in a particular place, but only when the rat performed an additional behaviour, such as sniffing, which was often correlated with the presence of a novel stimulus, or the absence of an expected stimulus. The findings ultimately supported the cognitive map theory, the idea that the hippocampus hold a spatial representation, a cognitive map of the environment. There has been much debate as to whether hippocampal place cells function depends on landmarks in the environment, on environmental boundaries, or on an interaction between the two. Additionally, not all place cells rely on the same external cues. One important distinction in cues is local and distal, where local cues appear in the immediate vicinity of a subject, whereas distal cues are far away, and act more like landmarks. Individual place cells have been shown to follow either or rely on both. Additionally, the cues on which the place cells rely may depend on previous experience of the subject and the saliency of the cue. There has also been much debate as to whether hippocampal pyramidal cells truly encode non-spatial information as well as spatial information. According to the cognitive map theory, the hippocampus's primary role is to store spatial information through place cells and the hippocampus was biologically designed to provide a subject with spatial information. Recent findings, such as a study showing that place cells respond to non-spatial dimensions, such as sound frequency, disagree with the cognitive map theory. Instead, they support a new theory saying that the hippocampus has a more general function encoding continuous variables, and location just happens to be one of those variables. This fits in with the idea that the hippocampus has a predictive function.
Relationship to grid cells It has been proposed that place cells are derivatives of grid cells, pyramidal cells in the entorhinal cortex. This theory suggests that the place fields of the place cells are a combination of several grid cells, which have hexagonal grid-like patterns of activity. The theory has been supported by computational models. The relation may arise through Hebbian learning. But grid cells may perform a more supporting role in the formation of place fields, such as path integration input. Another non-spatial explanation of hippocampal function suggests that the hippocampus performs clustering of inputs to produce representations of the current context – spatial or non-spatial.
Properties
Place fields Place cells fire in a specific region of an environment, known as a place field. Place fields are roughly analogous to the receptive fields of sensory neurons, in that the firing region corresponds to a region of sensory information in the environment. However, unlike receptive fields, place cells show no topography, meaning that two neighboring cells do not necessarily have neighboring place fields. Place cells fire spikes in bursts at a high frequency inside the place field, but outside of the place field they remain relatively inactive. Place fields are allocentric, meaning that they are defined with respect to the outside world rather than the body. By orienting based on the environment rather than the individual, place fields can work effectively as neural maps of the environment. A typical place cell will have only one or a few place fields in a small laboratory environment. However, in larger environments, place cells have been shown to contain multiple place fields which are usually irregular. Place cells may also show directionality, meaning they will only fire in a certain location when travelling in a particular direction.
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