Cortical column
Template:Short description A cortical column is a group of neurons forming a cylindrical structure through the cerebral cortex of the brain perpendicular to the cortical surface.<ref name="Mountcastle1957">Template:Cite journal</ref> The structure was first identified by Vernon Benjamin Mountcastle in 1957. He later identified minicolumns as the basic units of the neocortex which were arranged into columns.<ref>Template:Cite journal</ref> Each contains the same types of neurons, connectivity, and firing properties.<ref name="Bennett2020">Template:Cite journal</ref> Columns are also called hypercolumn, macrocolumn,<ref name=":0">Template:Cite journal</ref> functional column<ref name=":1">Template:Cite journal</ref> or sometimes cortical module.<ref>Template:Cite book</ref> Neurons within a minicolumn (microcolumn) encode similar features, whereas a hypercolumn "denotes a unit containing a full set of values for any given set of receptive field parameters".<ref name="horton">Template:Cite journal</ref> A cortical module is defined as either synonymous with a hypercolumn (Mountcastle) or as a tissue block of multiple overlapping hypercolumns.<ref>Template:Cite journal</ref>
Cortical columns are proposed to be the canonical microcircuits for predictive coding,<ref>Template:Cite journal</ref> in which the process of cognition is implemented through a hierarchy of identical microcircuits.<ref name="Bennett2020" /> The evolutionary benefit to this duplication allowed human neocortex to increase in size by almost 3-fold over just the last 3 million years.<ref name="Bennett2020" />
The columnar hypothesis states that the cortex is composed of discrete, modular columns of neurons, characterized by a consistent connectivity profile.<ref name=":1" /> The columnar organization hypothesis is currently the most widely adopted to explain the cortical processing of information.<ref>Template:Cite journal</ref>
Mammalian cerebral cortexEdit
{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} The mammalian cerebral cortex, the grey matter encapsulating the white matter, is composed of layers. The human cortex is between 2 and 3 mm thick.<ref name="Saladin11">Template:Cite book</ref> The number of layers is the same in most mammals, but varies throughout the cortex. 6 layers can be recognized in the neocortex, although many regions lack one or more layers. For example, fewer layers are present in the archipallium and the paleopallium.<ref>Template:Cite book</ref>
Columnar functional organizationEdit
The columnar functional organization, as originally framed by Vernon Mountcastle,<ref name="Mountcastle1957" /> suggests that neurons that are horizontally more than 0.5 mm (500 μm) from each other do not have overlapping sensory receptive fields, and other experiments give similar results: 200–800 μm.<ref name=":0" /><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> Various estimates suggest there are 50 to 100 cortical minicolumns in a hypercolumn, each comprising around 80 neurons. Their role is best understood as 'functional units of information processing.'
An important distinction is that the columnar organization is functional by definition, and reflects the local connectivity of the cerebral cortex. Connections "up" and "down" within the thickness of the cortex are much denser than connections that spread from side to side.
Hubel and Wiesel studiesEdit
David Hubel and Torsten Wiesel followed up on Mountcastle's discoveries in the somatic sensory cortex with their own studies in vision. A part of the discoveries that resulted in them winning the 1981 Nobel Prize<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> was that there were cortical columns in vision as well, and that the neighboring columns were also related in function in terms of the orientation of lines that evoked the maximal discharge. Hubel and Wiesel followed up on their own studies with work demonstrating the impact of environmental changes on cortical organization, and the sum total of these works resulted in their Nobel Prize.
Number of cortical columnsEdit
There are about 200 million (2×108) cortical minicolumns in the human neocortex with up to about 110 neurons each,<ref>Template:Cite journal</ref> and with estimates of 21–26 billion (2.1×1010–2.6×1010) neurons in the neocortex. With 50 to 100 cortical minicolumns per cortical column a human would have 2–4 million (2×106–4×106) cortical columns. There may be more if the columns can overlap, as suggested by Tsunoda et al.<ref>Template:Cite journal</ref> Jeff Hawkins claims that there are only 150,000 columns in the human neocortex, based on research made by his company Numenta.<ref name="Hawkins J. 2021">Template:Cite book</ref>
There are claims that minicolumns may have as many as 400 principal cells,<ref>Template:Cite book</ref> but it is not clear if that includes glia cells.
Some contradict the previous estimates,<ref>Template:Cite journal</ref> claiming the original research is too arbitrary.<ref name="Rakic 12099–12100">Template:Cite journal</ref> The authors propose a uniform neocortex, and choose a fixed width and length to calculate the cell numbers. Later research pointed out that the neocortex is indeed not uniform for other species,<ref>Template:Cite journal</ref> and studying nine primate species they found that "the number of neurons underneath 1 mm2 of the cerebral cortical surface ... varies by three times across species." The neocortex is not uniform across species.<ref name="Rakic 12099–12100"/><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> The actual number of neurons within a single column is variable, and depends on the cerebral areas and thus the function of the column.
See alsoEdit
ReferencesEdit
External linksEdit
- Template:Cite news
- The Blue Brain Project aims to simulate a cortical column
- On Intelligence—a popular science book about column function by Jeff Hawkins
- Template:Cite journal Summarizes what is known and corrects some misconceptions.