Cortical column

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).

Human cerebral cortex

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.

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 (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.

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^[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.

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.^[4] There may be more if the columns can overlap, as suggested by Tsunoda et al..^[5]

See also

• Ocular dominance column
• Cortical minicolumn

References

1. ^ Kolb, Bryan; Whishaw, Ian Q. (2003). Fundamentals of human neuropsychology. New York: Worth. ISBN 0-7167-5300-6. 
2. ^ Horton JC, Adams DL (2005). "The cortical column: a structure without a function". Philos. Trans. R. Soc. Lond., B, Biol. Sci. 360 (1456): 837–862. doi:10.1098/rstb.2005.1623. PMC 1569491. PMID 15937015. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1569491. 
3. ^ "The Nobel Prize in Medicine 1981". http://nobelprize.org/medicine/laureates/1981/. Retrieved 2008-04-13. 
4. ^ Christopher Johansson and Anders Lansner (January 2007). "Towards cortex sized artificial neural systems". Neural Netw 20 (1): 48–61. doi:10.1016/j.neunet.2006.05.029. PMID 16860539. 
5. ^ Kazushige Tsunoda, Yukako Yamane, Makoto Nishizaki, and Manabu Tanifuji (August 2001). "Complex objects are represented in macaque inferotemporal cortex by the combination of feature columns". Nat. Neurosci. 4 (8): 832–838. doi:10.1038/90547. PMID 11477430. 

External links

• Mission to build a simulated brain begins "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 popsci book about column function by Jeff Hawkins
• Confusing cortical columns by Pasko Rakic in the Proceedings of the National Academy of Sciences of the United States of America summarizes what is known and corrects some misconceptions.