User:Rtang3/sandbox

Orientation Columns are organized regions of neurons in the primary visual cortex that are specific for locating and contrasting edges are various angles in the visual field. These orientation columns are found in slabs that are perpendicular to the surface of the visual cortex.

Background and History
In 1958, David Hubel and Torsten Wiesel discovered cells in the visual cortex that had orientation selectivity. This was found through an experiment by giving a cat specific visual stimuli and measuring the corresponding excitation of the neurons in striate cortex (V1). The experimental set up was of a slide projector, a cat, electrodes, and an electrode monitor. They discovered this orientation selectivity when changing slides on the projector. The act of changing the slides produced a faint shadow line across the projector, and excited the neuron they were measuring. At the time of this experiment it was not conclusive that these orientation selective cells were in a "columnar" structure but the possibility of this structure was considered by research conducted by Vernon Mountcastle in 1956 about the topographic properties of the somatosensory system.

In 1974 Hubel and Wiesel wrote a paper about the geometry of orientation columns. They recorded 1410 cells in 45 penetrations into the striate cortex. Through this 1 dimensional technique they conceptualized that the orientation columns are not columns but slabs. In 1985 Gary Blasdel discovered a technique to visualize these orientation columns in 2D. His technique used photodiodes to detect optical changes in the visual cortex with the metabolic marker, 2-deoxyglucose, which labels active neurons. This confirmed Hubel and Wiesel's studies and also brought to light the swirls and pinwheel formations in the striate cortex.

Hubel and Wiesel received the Nobel Prize in Physiology and Medicine in 1981 for their contributions to the development of the visual system.

Physiology
Orientation columns are located in the primary visual cortex also known as the striate cortex. These orientation columns are not cylindrical in shape as the word column implies but are flat slabs that are parallel to each other. The slabs are perpendicular to the surface of the visual cortex and are lined up similar to slices of bread. These neurons are highly discriminatory for visual orientations and their motion.

Most of the cells in orientation columns are complex cells. Complex cells will respond to a properly orientated line in any location of the receptive field, whereas simple cells have a more narrow receptive field where a properly oriented line will excite it. Simple cells have distinct subdivisions of excitatory and inhibitory regions. It is proposed that complex cells receive input from many simple cells, which explains why the complex cells have a slightly wider receptive field.

There are possible biological advantages to the highly ordered structures of orientation columns. First possible advantage is that orientation selectivity may be intensified with lateral inhibition from neighboring cells of a slightly different preferred orientation. This would provide an efficient system for wiring between the striate cortex and the lateral geniculate nucleus (LGN). The second possible advantage is the ordered structure aids in development. By guaranteeing all orientations are represented throughout the visual field with minimal redundancy and no deficiencies. The third possible advantage is that if columns with similar orientation selectivity are close together, fewer afferents from the LGN are needed. This allows for efficient wiring. So by removing a few LGN inputs and adding a few, the orientation selectivity can be changed marginally.

Ocular dominance columns are also found in the striate cortex. These columns lie perpendicularly to orientation columns. During microelectrode experiments, it is normal to see penetrations where eye dominance changes between the contralateral eye and ipsilateral eye but this does not interrupt the orientation sequence.

Fractures
Fractures are breaks in the sequence of orientation selectivity from microelectrode studies. In these studies the fractures occur randomly during trials. Optical imaging have allowed for an explanation for these fractures, they are due to the local discontinues that the pinwheel formations create (also known as singularities).

Development
Orientation maps are innately determined at birth. Although like other parts of the brain the visual cortex goes through a critical period where the visual environment can change the orientation maps due to its plastic nature during this period. Visual deprivation during this period will cause a deterioration of these innate connections. Also if the visual environment is confined to only vertical or horizontal lines during this critical period the distribution of the preferred orientation of cells in the striate cortex become abnormal. This is probably due to cells maturing their preferred orientation to that of the most common type of visual stimulus.

Moire Interference
Moire interference (also known as Moire Patterns) from retinal ganglion cells (RGCs) is one model for the origin of orientation maps. The ideal case takes two layers of perfect hexagonal lattices of the on-center and off-center receptive fields of the RGCs. These two layers are superimposed on each other in such a way that produces a periodic interference pattern. This pattern produces a mosaic where the nearest off-center cell is an on-center cell and vice versa. This mosaic produces periodic orientation maps that can explain the origin of the orientation maps in the visual cortex.

Orientation Scotomas
The theory of Moire interference patterns governing the orientation map predicts the existence of orientation scotomas. This is because the receptive fields of the RGCs are not perfectly hexagonal and therefore, at some locations, representation of specific orientations will be missing. Currently there is research that is testing this hypothesis by "mapping human orientation discrimination thresholds of very small stimuli in the far periphery."

External Links for Further Reading

 * Eye, Brain and Vision By Hubel
 * Nobel Lecture 1981 By David Hubel