Place cell

A place cell is a kind of pyramidal neuron within the hippocampus that becomes active when an animal enters a particular place in its 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. 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 nearby fields as distant ones. 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. A place cell is a kind of pyramidal neuron within the hippocampus that becomes active when an animal enters a particular place in its 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. 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 nearby fields as distant ones. 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. Place cells are thought, collectively, to act 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, but the ways in which they function within the hippocampus are still being researched. Studies with rats have shown that place cells tend to fire quickly when a rat enters a new, open environment, but outside of a firing field, place cells tend to be relatively inactive. Together place cells are thought to form a 'cognitive map' in which they have localized firing patterns called place fields. 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'. 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. 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. 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. These cells were first discovered in the brain, and specifically in the hippocampus, by O'Keefe and Dostrovsky (1971). 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. 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 place 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. After O'Keefe and Dostrovsky first found the existence of place cells within the hippocampus in 1971, they conducted 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. This was one of the first indicators that place cells were related to spatial orientation. They also discovered that place 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). The firing of place cells is timed in relation to local theta waves, a process termed phase precession. 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. 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.