A cortical column, also called hypercolumn, macrocolumn, functional column or sometimes cortical module, is a group of neurons in the cortex of the brain that 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 or as a tissue block of multiple overlapping hypercolumns. The columnar hypothesis states that the cortex is composed of discrete, modular columns of neurons, characterized by a consistent connectivity profile. 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.
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. The number of layers is the same in most 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.
The columnar functional organization, as originally framed by Vernon Mountcastle, suggests that neurons that are horizontally more than 0.5 mm 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. 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 studies
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.
Number of Cortical columns
There are about 100,000,000 cortical minicolumns in the neo-cortex with up to 110 neurons each, giving 1,000,000–2,000,000 cortical columns. There may be more if the columns can overlap, as suggested by Tsunoda et al. Updated information: it's not a scientific consensus now that "There are about 100,000,000 cortical minicolumns in the neo-cortex with up to 110 neurons each," as the original research is too arbitrary in many ways: the authors just chose a fixed width and length to calculate the cell numbers. They mainly use the number to propose that the neocortex is uniform, but later research pointed out that the neocortex is indeed not uniform by studying nine primate species and found that " the number of neurons underneath 1 mm2 of the cerebral cortical surface" "varies by three times across species." So now it is common sense that the neocortex is not uniform. The number of neurons within a single column is a variable, which depends on the existence of cortical columns in certain brain areas and on the definition of the column.
