Spatial firing patterns of 8 place cells recorded from the
CA1 layer of a rat. The rat ran back and forth along an elevated track, stopping at each end to eat a small food reward. Dots indicate positions where action potentials were recorded, with color indicating which neuron emitted that action potential.
A
place cell is a type of
pyramidal neuron within the
hippocampus that becomes active when the animal enters a particular place in the environment; this place is known as the
place field. A given place cell will have only one, or a few, place fields in a typical small laboratory environment, but more in a larger region.
[1] There is no apparent topography to the pattern of place fields, unlike other brain areas such as visual cortex - neighboring place cells are as likely to have distant fields as neighboring ones.
[2] In a different environment, typically about half the place cells will still have place fields, but these will be in new places unrelated to their former locations.
[3]
The place cells are thought, collectively, to act as a cognitive representation of a specific location in space, known as a
cognitive map.
[4] Place cells work with other types of neurons in the hippocampus and surrounding regions to perform this kind of spatial processing.
[5] but the ways in which they function within the hippocampus are still being researched.
[6]
Studies with rats have shown that place cells tend to fire quickly when a rat enters a new, open environment, however outside of a firing field, place cells tend to be relatively inactive.
[7] Together place cells are thought to form a "
cognitive map" in which they have localized firing patterns called place fields.
[8]Place cell firing patterns are often determined by external sensory information and the local environment. Place cells have proven to have the ability to suddenly change their firing pattern from one pattern to another, a phenomenon known as “re-mapping” and though place cells do change according to the external environment, they are stabilized by attractor dynamics which “enable the system to resist small changes in sensory input but respond collectively and coherently to large ones."
[8]
Although place cells are part of a non-sensory cortical system, their firing behavior is strongly correlated to sensory input. Place cells fire when an animal is located in parts of the environment known as
place fields.
[9] These circuits may have important implications for memory, as they provide the spatial context for memories and past experiences.
[9] 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.
[9]
Background[edit]
These cells were first discovered in the brain, and specifically in the hippocampus, by O’Keefe and Dostrovsky (1971).
[11] Though the hippocampus plays a role in learning and memory, the existence of place cells within the hippocampus demonstrates the role it plays with spatial adaptation and awareness. There have been recorded increases in firing patterns of rats in open environments and recorded spatial learning and awareness impairments after damage to the hippocampus and the place cells within.
[11] Studies with rats have shown that place cells are very responsive to spatial surroundings. For example a study by John O'Keefe and Lynn Nadel found that space cells would fire more rapidly when rats ran past places in the environment. when a new item was added to the environment, or when an item that is usually there is not present.
[12]
After O'Keefe and Dostrovsky first found the existence of place cells within the hippocampus in 1971, they conduced a study five years later with rats that demonstrated these place cells would fire whenever the rat was within a certain place in the environment.
[13] This was one of the first indicators that place cells were related to spatial orientation. They also discovered that space cells fired in different areas of the hippocampus depending on where the rat went, and this whole firing network made up the rat’s environment (O’Keefe 1976, Wilson & McNaughton 1993). As environments changed, the same place cells would fire, but the relationship and dynamic between firing fields would change (O’Keefe & Conway 1978). Therefore place cells are thought to give humans and animals a guide to the environment it is navigating and its position in that environment. Place cells are generally observed through recorded action potentials. As humans or animals navigate large environments and then arrive at a particular location, there is a notable increase in the place cell firing rate once that specific location has been reached (Eichenbaum, Dudchencko Wood, Shaprio and Tanila, 1999). For more information on studies with rats, "Place Cells and Aging".
There has been much debate as to whether hippocampal place cells function based upon
landmarks in the environment or on environmental boundaries or an interaction between the two.
[14] There has also been much study as to whether hippocampal pyramidal cells (mostly in rats) signal non-spatial information as well as spatial information. According to the cognitive map theory, the hippocampus's primary role in the rat is to store spatial information through place cells and the rat hippocampus was biologically designed to provide the rat with spatial information.
[15]
However, there have been investigations as to whether the hippocampus may store other non-spatial information as well.
[15] These other explanations in favor of non-spatial components of the hippocampus argue that the hippocampus has "flexible" functions in that it can apply memory in circumstances different from those under which these relationships were learned. There are also views that claim that the hippocampus has functions altogether removed from time and space.
[15] However, other explanations of data that prematurely support the existence of non-spatial functions in the hippocampus must be considered. Evidence against this flexibility theory comes in the form of using the
delayed non-match-to sample task. This task uses flexibility in that the rat is first presented with a visual representation such as a block. After a delay, when presented with the block and a novel object, the rat must choose the novel object in order to obtain a reward. Its completion of this task requires flexibility. However, during this task, hippocampal activity does not sufficiently increase and
lesioning (induced
trauma) in the hippocampus does not change the rat's performance on this task.
[15]
Place cells fire in different, often widespread, hippocampal locations at the same time, which some interpret as their having different functions in different locations. A rat's representation of its environment is constructed by the firing of groups of place cells that are widely distributed in the hippocampus, however, this does not necessarily mean that each location serves a different purpose. When recording the firing fields of certain hippocampal cells in an open field environment, firing fields prove to be similar even when the rat travels in different directions, exhibiting
omnidirectionality. However, when limitations are placed in the aforementioned environment, fields prove to be directional and fire in one direction but not in another.
The same directionality occurs when rats participate in the
radial arm maze. The radial arm maze consists of a central circle from which several arm-like projections radiate. These projections either contain food or do not. Some consider the firing or lack of firing of place cells depending on the arm to be a function of
goal-oriented behavior. However, when moving from one arm to another when they both contain food, place cells only fire in one direction, meaning that one cannot attribute firing purely to a goal-approach. A directionality component must be added: for example, a North goal as opposed to a South goal.
[15]
When visual cues in an environment such as visibility of a line where the wall meets the floor, height of the wall, and width of the wall are available to the rat to discern distance and location of the wall, the rat internalizes this external information to register its surroundings. However, when these visual
cues are unavailable, the rat registers wall location by colliding with the wall and then place cell firing rate after the collision provides information to the rat about its distance from the wall based on the direction and speed of its movements after the collision. In this situation, the firing of place cells is due to motor inputs.
[15]
There are both simple place cells with purely locational correlates and also complex place cells that increase their firing rate when the rat encounters a particular object or experience. Others fire when a rat's expectations in a particular location are not met or when they encounter novelty along their path: the cells that fire in these situations are known as misplace cells.
The place cells that appear to operate based solely on
non-spatial memory seem to have spatial components. Many lesioning experiments attempting to inflict non-spatial memory
deficits in the hippocampus have been unsuccessful. In some cases, lesioning has been successful in inflicting non-spatial memory deficits, however, other structures besides the hippocampus were affected by lesioning. Therefore, the rat’s non-spatial memory deficits could have been unrelated to place cells.
[15] Thus, based on information from studies thus far, the cognitive map theory seems to be most supported and non-spatial theories may fail to take spatial components into account.
[15]
Place cells are located in the hippocampus, a brain structure located in the medial temporal lobe of the brain.
Function[edit]
Place fields[edit]
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[16] This animation shows place fields firing in succession as a rat moves along a linear track. Place fields are thus 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.
[17]
Sensory input[edit]
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.
[17] 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.
[18]This suggests that place cells respond to complex stimuli rather than simple individual sensory cues. According to a model known as the
functional differentiationmodel, 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.
[17]
