What Are Place Cells
Place cells are neurons found in the hippocampus that trigger an action potential when an animal is in a certain location within an environment. Thus, these cells constitute a "place-field" and are thought to be pyramidal cells (excitatory neurons) in the hippocampus. Several studies confirmed that place cells reside in the hippocampus, because when the hippocampus was removed or destroyed in rats, they display significantly impaired place navigation when swimming through the Morris water maze.
O'Keefe and Dostrovsky were the first scientists to describe the phenomenon of place cells. From their observations, they hypothesized that the (rat) hippocampus serves to form a cognitive map of the environment.
Place Cells As Pointers in the Cognitive Map
When investigations of animal learning first began, advocates of the dominant "stimulus-response" (SR) theory argued that maze learning is achieved by direct associations between stimuli and rewarded behavioral responses. In 1948, however, Tolman demonstrated that rats can actually navigate mazes using shortcuts, suggesting that attributes of rat learning behavior could not necessarily be explained by SR theory. As a result of his experiments, Tolman concluded that rats may create and then utilize cognitive maps--or global representations of the environment--to localize directional cues in mazes.
The proposal of cognitive maps was first critiques for its lack of compelling evidence for an underlying neural mechanism. Upon the discovery of place cells by O'Keefe and Dostrovsky, scientists began to think that these neurons might be the building blocks of the long-hypothesized cognitive map.
As a rat explores the environment at large, we can readily correlate significant increases in a place cell's firing rate with the specific location of the animal. The firing rate can exceed 100 Hz or not even fire at all. O'Keefe and colleagues observed that hippocampal neurons even maintained their spatial firing pattern when certain environmental cues used in maze tests were removed, but not when the shape of the environment was altered. This indicated that there is a threshold of cues required for rats to sufficiently define their "cognitive maps."
What's in a Place Cell "Map"?
One problem with the place map hypothesis is the lack of evidence indicating how place cells are able to communicate one's "location" with respect to "other locations." Likewise, how many place cells are required to create a cognitive map? To identify one's location in an environment requies that the entire area be represented, which would be most simply represented by place fields that are uniformly distributed throughout the environment. In contrast to this logic, early studies show that "space" in the hippocampus is not represented in this fashion; place fields are not continuous or topographically organized. Rather, there is evidence that place fields are made up of clusters of neighboring cells. It seems taht hippocampal representation does not involve a homogeneous representation of the entire area of physical space.
Recommended reading: Bures J, Fenton AA, Kaminsky Y, Zinyuk L. Place cells and place navigation. Proc Natl Acad Sci U S A. 1997 Jan 7;94(1):343-50.
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Place Cells, sometimes called Space Cells are a collection of neurons, found in the Hippocampus, that activate in response to an animal's position in it's physical environment. Together, a set of place cells form an abstract map of the geometry of an animal's environment. Each Place Cell has a place field, or mapping of areas to which it is most sensitive to. The firing of many different place cells, according to their place fields and the physical postion of the animal, specify the animals location and possibly it's orientation. When an animal is placed in a new environment, it's cognitive spatial map is likely to be quite disorganized. Experimentaly, this can be observed as the erratic firing of place cells, without regard to the physical location of the animal. A typical experiment might consist of a freely moving rodent implanted with extracellular electrodes and placed in a novel enclosure. In this way, the firing of many place cells can be recorded in conjunction with the animals location. Over time, an organized map with distinct place fields for different cells will emerge. Indeed, it has been found that through repeated exposures animals can progressively separate and fine tune their representations of two similar environments. Once they are tuned by experience, these spatial representations are robust and can remain stable for months. The precise mechanisms of plasticity that tune and maintain the cognitive spatial map are not known. However, it is likely that well studied mechanisms of plasticity such as LTP and LTD are involved. Animals in which molecules involved in LTP and LTD (NMDAR, CaMKII, etc.) have been perturbed genetically, often show deficits in the formation of organized place fields.
A schematic of an early place cell experiment and some data demonstrating the direction specific nature of this place cell's response. (From: O'Keefe and Dostrovsky)
The evolution of the place fields of 7 different cells as a mouse is repeatedly exposed to two different environments. (From: Wills, O'Keefe, et al.)
Wills TJ, Lever C, Cacucci F, Burgess N, O'Keefe J. Attractor dynamics in the hippocampal representation of the local environment.Science. 2005 May 6;308(5723):873-6.
Independent rate and temporal coding in hippocampal pyramidal cells. Nature. 2003 Oct 23;425(6960):828-32.
Lever C, Wills T, Cacucci F, Burgess N, O'Keefe J. Long-term plasticity in hippocampal place-cell representation of environmental geometry. Nature. 2002 Mar 7;416(6876):90-4.
The hippocampus as a spatial map. Preliminary evidence from unit activity in the freely-moving rat. Brain Res. 1971 Nov;34(1):171-5.