A cortical column, also called hypercolumn, macrocolumn or sometimes cortical module, is a group of neurons in the cortex of the brain which can be successively penetrated by a probe inserted perpendicularly to the cortical surface, and which have nearly identical receptive fields. Neurons within a minicolumn encode similar features, whereas a hypercolumn "denotes a unit containing a full set of values for any given set of receptive field parameters". A cortical module is defined as either synonymous with a hypercolumn (Mountcastle) or as a tissue block of multiple overlapping hypercolumns.
It is still unclear what precisely is meant by the term, and it does not correspond to any single structure within the cortex. It has been impossible to find a canonical microcircuit that corresponds to the cortical column, and no genetic mechanism has been deciphered that designates how to construct a column. However, the columnar organization hypothesis is currently the most widely adopted to explain the cortical processing of information.
Mammalian cerebral cortex
The mammalian cerebral cortex, the grey matter encapsulating the white matter, is composed of layers. The human cortex is roughly 2.4 mm thick. The number of layers is the same in all mammals, but varies throughout the cortex. In the neocortex 6 layers can be recognized although many regions lack one or more layers, fewer layers are present in the archipallium and the paleopallium.
Columnar functional organization
The columnar functional organization, as originally framed by Vernon Mountcastle, 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. Various estimates suggest there are 50 to 100 cortical minicolumns in a hypercolumn, each comprising around 80 neurons.
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 studies
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 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.
Size of cortex
From the size of the cortex and the typical size of a column, it can be estimated that there are about two million function columns in humans.[contradictory] There may be more if the columns can overlap, as suggested by Tsunoda et al..
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- Graham-Rowe, Duncan (6 June 2005). "Mission to build a simulated brain begins". New Scientist. "[...] the initial phase of Blue Brain will model the electrical structure of neocortical columns - neural circuits that are repeated throughout the brain. These are the network units of the brain, says Markram. Measuring just 0.5 millimetres by 2 mm, these units contain between 10 and 70,000 neurons, depending upon the species. Once this is complete, the behaviour of columns can be mapped and modelled [...]"
- The Blue Brain Project aims to simulate a cortical column
- On Intelligence—a popular science book about column function by Jeff Hawkins
- Rakic, P. (August 2008). "Confusing cortical columns". Proceedings of the National Academy of Sciences 105 (34): 12099–12100. doi:10.1073/pnas.0807271105. Summarizes what is known and corrects some misconceptions.