Place Field

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 6 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.

Remapping

Remapping refers to the change in the place field characteristics that occurs when a subject experiences a new environment, or the same environment in a new context. This phenomenon was first reported in 1987, and is thought to play a role in the memory function of the hippocampus. There are broadly two types of remapping: ''global'' remapping and ''partial'' remapping. When global remapping occurs, most or all of the place cells remap, meaning they lose or gain a place field, or their place field changes its location. Partial remapping means that most place fields are unchanged and only a small portion of the place cells remap. Some of the changes to the environment that have been shown to induce remapping include changing the shape or size of the environment, the color of the walls, the smell in the environment, or the relevance of a location to the task at hand.

Phase precession

The firing of place cells is timed in relation to local theta waves, a process termed phase precession. Upon entering a place field, place cells will fire in bursts at a particular point in the phase of the underlying theta waves. However, as an animal progresses through the place field, the firing will happen progressively earlier in the phase. It is thought that this phenomenon increases the accuracy of the place coding, and aids in plasticity, which is required for learning.

Directionality

In some cases place cells show directionality, meaning they will only fire in a location when the subject is travelling in a particular direction. However, they may also be omnidirectional, meaning they fire regardless of the direction the subject. The lack of directionality in some place cells might occur particularly in impoverished environments, whereas in more complicated environments directionality is enhanced. The radial arm maze is one such environment where directionality does occur. In this environment, cells may even have multiple place fields, of which one is strongly directional, while the others are not. In virtual reality corridors, the degree of directionality in the population of place cells is particularly high. The directionality of place cells has been shown to emerge as a result of the animal's behaviour. For example, the receptive fields become skewed when rats travel a linear track in a single direction. Recent theoretical studies suggest that place cells encode a successor representation which maps the current state to the predicted successor states, and that directionality emerges from this formalism. This computational framework also provides an account for the distortion of place fields around obstacles.

Sensory input

Place cells were initially believed to fire in direct relation to simple sensory inputs, but studies have suggested that this may not be the case. Place fields are usually unaffected by large sensory changes, like removing a landmark from an environment, but respond to subtle changes, like a change in color or shape of an object. This suggests that place cells respond to complex stimuli rather than simple individual sensory cues. According to the functional differentation model, sensory information is processed in various cortical structures upstream of the hippocampus before actually reaching the structure, so that the information received by place cells is a compilation, a functional derivative, of different stimuli.

Sensory information received by place cells can be categorized as either metric or contextual information, where metric information corresponds to where place cells should fire and contextual input corresponds to whether or not a place field should fire in a certain environment. Metric sensory information is any kind of spatial input that might indicate a distance between two points. For example, the edges of an environment might signal the size of the overall place field or the distance between two points within a place field. Metric signals can be either linear or directional. Directional inputs provide information about the orientation of a place field, whereas linear inputs essentially form a representational grid. Contextual cues allow established place fields to adapt to minor changes in the environment, such as a change in object color or shape. Metric and contextual inputs are processed together in the entorhinal cortex before reaching the hippocampal place cells. Visuospatial and olfactory inputs are examples of sensory inputs that are utilized by place cells. These types of sensory cues can include both metric and contextual information.

Visuospatial inputs

Spatial cues such as geometric boundaries or orienting landmarks are important examples of ''metric'' input. An example is the walls of an environment, which provides information about relative distance and location. Place cells generally rely on set distal cues rather than cues in the immediate proximal environment, though local cues can have a profound impact on local place fields. Visual sensory inputs can also supply important ''contextual'' information. A change in color of a specific object or the walls of the environment can affect whether or not a place cell fires in a particular field. Thus, visuospatial sensory information is critical to the formation and recollection of place field.

Olfactory inputs

Although place cells primarily rely on visuospatial input, some studies suggest that olfactory input may also affect the formation and stability of place fields. Olfaction may compensate for a loss of visual information, or even be responsible for the formation of stable place fields in the same was visuospatial cues are. This has been confirmed by a study in a virtual environment that was composed of odor gradients. Change in the olfactory stimulus in an environment may also cause the remapping of place cells.

Vestibular inputs

Stimuli from the vestibular system, such as rotations, can cause changes in place cells firing. After receiving vestibular input some place cells may remap to align with this input, though not all cells will remap and are more reliant on visual cues. Bilateral lesions of the vestibular system in patients may cause abnormal firing of hippocampal place cells as evidenced, in part, by difficulties with spatial tasks such as the radial arm maze and the Morris water navigation task.

Movement inputs

Movement can also be an important spatial cue. Mice use their self-motion information to determine how far and in which direction they have travelled, a process called path integration

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