A cortical column, also called hypercolumn or sometimes cortical module,[1] is a group of neurons in the brain cortex 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".[2] A cortical module is defined as either synonymous with a hypercolumn (Mountcastle) or as a tissue block of multiple overlapping hypercolumns (Hubel&Wiesel).
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The human cerebral cortex is composed of 6 somewhat distinct layers; each layer identified by the nerve cell type and the destination of these nerve cell's axons (within the brain). The human cortex is a roughly 2.4 mm thick sheet of neuronal cell bodies that forms the external surface of the telencephalon. The dolphin cortical column is composed of 5 layers, the reptilian cortex is composed of three.
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 (Buxhoeveden 2002, Hubel 1977, Leise 1990, etc.). 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.
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[3] 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.
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.[4] There may be more if the columns can overlap, as suggested by Tsunoda et al..[5]