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Place cells are a type of neuron located in the hippocampus that play a role in spatial representation and self-location. Although place cells are part of a non-sensory cortical system, their firing behavior is strongly correlated to sensory input. Place cells fire in circuits known as place fields, which act as internal spatial representations of an environment. These circuits may have important implications for memory, as they provide the spatial context for memories and past experiences. Like many other parts of the brain, place cell circuits are dynamic. They are constantly adjusting and remapping to suit the current location and experience of the brain. Place cells do not work alone to create visuospatial representation; they are a part of a complex circuit that informs place awareness and place memory.

Table of Contents

Place Fields
Place cells fire in a specific region 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. A good depiction of place fields can be seen here. This animation shows place fields firing in succession as a rat moves along a linear track. Place fields are considered to be allocentric rather than egocentric, 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.

Sensory Input
Place cells were initially believed to fire in direct relation to simple sensory inputs, but recent studies suggest 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 a model known as the functional differentiation 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 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 cues
Spatial cues such as geometric boundaries or orienting landmarks are important examples of metric input. Place cells mainly rely on set distal cues rather than cues in the immediate proximal environment. Movement can also be an important spatial cue. The ability of place cells to incorporate new movement information is called path integration, and it is important for keeping track of self-location during movement. Path integration is largely aided by grid cells, which are a type of neuron in the entorhinal cortex that relay information to place cells in the hippocampus. Grid cells establish a grid representation of a location, so that during movement place cells can fire according to their new location while orienting according to the reference grid of their external environment. Visual sensory inputs can also supply important contextual information. A change in color of a specific object can affect whether or not a place field fires in a particular environment. Thus, visuospatial sensory information is critical to the formation and recollection of place field.

Olfactory cues
Although place cells primarily rely on visuospatial input, some studies suggest that olfactory input may also play a role in generating and recalling place fields. Relatively little is known about the interaction between place cells and non-visual sensory cues, but preliminary studies have shown that non-visual sensory input may have supplementary role in place field formation. A study by Save et al. found that olfactory information can be used to compensate for a loss of visual information. In this study, place fields in subjects exposed to an environment with no light and no olfactory signals were unstable; the position of the place field shifted abruptly and some of the constituent place cells stopped firing entirely. However, place cells in subjects exposed to a dark environment with olfactory signals remained stable despite a lack of visual cues. An additional study by Zhang et al. examined how the hippocampus uses olfactory signals to create and recall place fields. Similar to the Save et al. study, this study exposed subjects to an environment with a series of odors but no visual or auditory information. Place fields remained stable and even adapted to the rotation of the pattern of olfactory signals. Furthermore, the place fields would remap entirely when the odors were moved randomly. This suggests that place cells not only utilize olfactory information to generate place fields, but also use olfactory information to orient place fields during movement.

Hippocampal memory
The hippocampus plays an essential role in episodic memory. One important aspect of episodic memory is the spatial context in which the event occurred. Hippocampal place cells have been shown to exhibit stable firing patterns even when cues from a location are removed. Additionally, specific place fields begin firing when exposed to signals or a subset of signals from a previous location. This suggests that place cells provide the spatial context for a memory by recalling the neural representation of the environment in which the memory occurred. In other words, place cells prime a memory by differentiating the context for the event. By establishing spatial context, place cells can be used to complete memory patterns. Furthermore, place cells can maintain a spatial representation of one location while recalling the neural map of a separate location, effectively differentiating between present experience and past memory. Place cells are therefore considered to demonstrate both pattern completion and pattern separation qualities

Pattern Completion
Pattern completion is the ability to recall an entire memory from a partial or degraded sensory cue. Place cells are able to maintain a stable firing field even after significant signals are removed from a location, suggesting that they can recall a pattern from only some of the original input. Furthermore, pattern completion can be symmetric in that an entire memory can be retrieved from any part of it. For example, in an object-place association memory, spatial context can be used to recall an object and the object can be used to recall the spatial context.

Pattern Separation
Pattern separation is the ability to differentiate one memory from other stored memories. Pattern separation begins in the dentate gyrus, a section of the hippocampus involved in memory formation and retrieval. Granule cells in the dentate gyrus process sensory information using competitive learning, and relay a preliminary representation to form place fields. Place fields are extremely specific, as they are capable of remapping and adjusting firing rates in response to subtle sensory signal changes. This specificity is critical for pattern separation, as it distinguishes memories from one another.