Orientation cell

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Orientation cells are specialized neurons in the brain that form the basis for spatial orientation . They are located in both hemispheres in the enlarged hippocampus . The orientation ability arises from the interplay of local cells (place cells), head direction cells (head direction cells), grid cells (grid cells) and boundary cells (boundary cells). Orientation cells were mainly researched on the model organism rat , research on mice is also important. Other vertebrates, including humans, have been poorly researched, but previous results suggest that the corresponding units are likely to be organized in a similar way.

For their research on orientation cells, John O'Keefe , May-Britt Moser and Edvard Moser jointly received the Nobel Prize in Physiology or Medicine in 2014 .

Basics

Two different systems exist in the brain as the basis for spatial orientation. The first determines spatial and positional relationships relative to one's own body or certain parts of the body. The second serves as a correlate for the location or the direction related directly to the outer space. This reference to an external frame of reference is called allocentric . It enables the individual to create a cognitive map of his environment, whereby places, routes and the associated properties and possibly dangers can be effectively remembered over the long term.

anatomy

The allocentric orientation system is located within the brain in the enlarged hippocampus. These include above all the three fields of the dentate gyrus , the so-called Ammon's horn (Cornu ammonis) and the subiculum (with pre- and parasubiculum) within the hippocampus and adjacent parts of the entorhinal cortex . Within these structures it is bound to specialized nerve cells (neurons). In internal processing, there are loop-like circuits between different pyramid cells inside the structure. These make it possible to activate an entire associated structure, even with incomplete input, for example from the sensory cells. Orientation performance is primarily tied to certain types of neurons; These were investigated by deriving potentials from individual nerve cells using measuring electrodes with a hair-thin glass capillary as the tip.

Cell types

Place cells

Areas within a labyrinth are coded in different colors, in each of which a certain place cell assigned to this location, in the field CA1 of the Ammon's horn of a laboratory rat, fires. The individual colored fields together form a mental map of the labyrinth

The place cells as the fundamental basis of cognitive maps were first discovered in the early 1970s. They are located in the actual hippocampus and in the dentate gyrus. Place cells usually fire at a constant, low rate. If the test animal moves into a certain spatial area, for example a certain section of a test labyrinth already known to the animal , the frequency increases significantly. Each spatial area known to the test animal is linked to a specific location cell. If the test animal wanders through the labyrinth, the associated place cells are excited one after the other; in their interplay they form a mental map. Location cells are not homogeneous with one another and receive their location information in completely different ways; In addition to landmarks, remembered paths and borders play a role, with the assignment often remaining remarkably stable when individual landmarks are removed. Place cells are active independent of certain contexts and activities, in open space the direction of movement or head or body posture does not matter. However, they sometimes react sensitively to changes in sensory stimuli such as smell or color. When you visit new places, place cells are imprinted on them extremely quickly; they enable stable memories the next time you visit the same place. Through learning, your information can later be changed and adapted more slowly.

Head direction cells

The head direction cells, which were only discovered in the early 1990s, fire regardless of their location. Each head direction cell reacts specifically to a certain orientation of the head, so that a direction field like a compass rose can be made out around it . The input of the head direction cells seems to be very important for learning the location information of the location cells. Head direction cells were found not only in the enlarged hippocampus, but also partially in other structures of the cerebral cortex.

Grid cells

Grid cells divide the environment into a regular triangular grid of fields that are each assigned to a cell

Grid cells were discovered in 2005 as a further type of place cell. They are mainly located in the median entorhinal cortex, i.e. outside the hippocampus in the narrower sense. Grid cells process location information by depicting specific locations in a grid-like grid of triangular fields that cover the surroundings like tiles. This grid is anchored in a certain place, for example by a landmark. Unlike place cells, each grid cell has an associated field that identifies a certain point in a space around a place, so it fires, depending on the orientation, at numerous different spatial locations. The field is also dependent on the scale (which controls the size of the individual tiles), on the orientation, i.e. H. the direction of the grid. Different-scale grids, for large and small room sections, and those with different orientations can overlap. The input of the grid fields does not seem to be absolutely necessary for the formation of specific location cells, since their function is retained even if they fail. Possibly they form more of a mobile orientation frame for the planning of routes and movements.

Border cells

The way in which place cells record and process position information led to the hypothesis that there must be special cells that represent the position of a place in relation to borders, i.e. barriers that cannot be overcome for movement. Such limits are, for example in the labyrinth, both its walls and the edges of downward falls. Such cells, called border cells, were discovered very soon after the grid cells in the subiculum of the (enlarged) hippocampus. As far as is known, border cells fire primarily in the direct near field of such borders, i.e. they do not represent an extended spatial unit with these as a reference. In the meantime, "inverse" border cells have also been discovered that fire at all locations that are not close to such a border.

Other cell types

The orientation performance of an organism is based on the integration of the information provided by the respective orientation cells. The place cells play a key role in remembering spaces. The cells actually present in the hippocampus, however, cannot all be assigned to one of the types in the manner of a textbook; numerous cells show mixed properties, for example location cells, which are also sensitive to directional information. Other cells are sensitive to temporal information, such as places in the labyrinth that can only be passed with a delay.

Individual evidence

  1. ↑ a b James J. Knierim (2015): The Hippocampus. Current Biology 25 (23): R1116-R1121. doi: 10.1016 / j.cub.2015.10.049
  2. ↑ a b c d e Tom Hartley, Colin Lever, Neil Burgess, John O'Keefe (2014): Space in the brain: how the hippocampal formation supports spatial cognition. Philosophical Transactions of the Royal Society B 369: 20120510. doi: 10.1098 / rstb.2012.0510
  3. ^ A b Daniel Bush, Caswell Barry Neil Burgess (2014): What do grid cells contribute to place cell firing? Trends in Neuroscience 37 (3): 136-145. doi: 10.1016 / j.tins.2013.12.003
  4. ↑ Torkel hafting, Marianne Fyhn, Sturla Molden, May-Britt Moser, Edvard I. Moser (2005): Microstructure of a spatial map in the entorhinal cortex. Nature 436: 801-806. doi: 10.1038 / nature03721
  5. ↑ Sarah Stewart, Ali Jeewajee, Thomas J. Wills, Neil Burgess, Colin Lever (2014): Boundary coding in the rat subiculum. Philosophical Transactions of the Royal Society B 369: 20120514. doi: 10.1098 / rstb.2012.0514
  6. ↑ Mark P. Brandon, Julie Koenig, Stefan Leutgeb (2013): Parallel and convergent processing in grid cell, head-direction cell, boundary cell, and place cell networks. WIRE's Cognitive Science 5 (2): 207-219. doi: 10.1002 / wcs.1272

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