A cortical column, also called hypercolumn or sometimes cortical module, 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.
Human cerebral cortex
The human cerebral cortex is composed of 6 layers; each layer receives and sends signals to different parts of the brain. The human cortex is a roughly 2.5 mm thick sheet of neuronal cell bodies that forms the external surface of the telencephalon. The dolphin cortical column is composed of only 5 layers. The reptilian cortex has only three layers.
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
Hubel and 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 2×106 function columns in humans . There may be more if the columns can overlap, as suggested by Tsunoda et al .
- Kolb & Whishaw, Fundamentals of Human Neuropsychology, 2003
- Towards cortex sized artificial neural systems, Christopher Johansson and Anders Lansner, Neural Networks, Vol. 20 #1, pp48–61, Elsevier, January 2007
- Complex objects are represented in macaque inferotemporal cortex by the combination of feature columns, Kazushige Tsunoda, Yukako Yamane, Makoto Nishizaki, and Manabu Tanifuji, Nature Neuroscience Vol. 4 #8, pp832–838, Nature, August 2001
- 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"
- Blue Brain Project Aims to simulate a cortical column
- On Intelligence -- a popsci book about column function by Jeff Hawkins