User:Paul C. Foster/Ocular Dominance Column Outline

Ocular dominance columns are stripes of neurons in the visual cortex of certain mammals (including humans) that respond preferentially to input from one eye or the other. The columns span multiple cortical layers, and are laid out in a striped pattern across the surface of the striate cortex. The stripes lie perpendicular to the orientation columns.

Ocular dominance columns were important in early studies of cortical plasticity, as it was found that monocular deprivation causes the columns to degrade, with the non-deprived eye assuming control of more of the cortical cells.

It is believed that ocular dominance columns must be important in binocular vision. Surprisingly, however, many animals, such as squirrel monkeys, are known to often be missing ocular dominance columns and still have binocular vision. This has led some to question whether they serve a purpose, or are just a byproduct of development.

Discovery
Ocular dominance columns were discovered in the 1960s by Hubel and Wiesel as part of their Nobel prize winning work on the structure of the visual cortex in cats. They have since been found in many animals, such as ferrets, macaques, and humans. Notably, they are also absent in many animals with binocular vision, such as rats.

Structure
Ocular dominance columns are stripe shaped regions that lie perpendicular to the orientation columns in V1. They are enervated by input from the lateral geniculate nucleus (LGN) into layer 4 and have mostly reciprocal projections to many other parts of the visual cortex.

Relation to Other features of V1
Whole section can be reffed from also great images in that reference, but too many for this article The ocular dominance columns are stripe shaped and cover the primary (striate) visual cortex. If the columns corresponding to one eye were colored, a pattern similar to that shown in the accompanying figure would be visible when looking at the surface of the cortex. However, the same region of cortex could also be colored by the direction of edge that it responds to, giving the orientation columns.
 * On top of orientation columns
 * Perpendicular to orientation columns at edges
 * Registered:
 * Centered on cytox blobs
 * Centered on pinwheels
 * Contrary to prior belief, centers of pinwheels and cytox are not coincident
 * Striate visual cortex can be divided into "modules" or "hypercolumns" however, due to irregular shape, they do not form a regular mosaic. also the division of modules is artificial, as the boundaries are fuzzy.

Formation
There is no consensus yet as to how ODCs are initially developed. One possibility is that they develop through Hebbian learning triggered by spontaneous activity in the eyes of the developing fetus, or in the LGN. Another possibility is that axonal guidance cues may guide the formation, or a combination of mechanisms may be at work.


 * ODCs form before birth
 * spontaneous waves of discharge in retina may direct ODC growth before birth through plastic mechanism
 * these waves have been shown to direct eye specific segregation in LGN
 * there is evidence that these retinal waves also induce the formation of ODCs before birth

Sensitive periods
[Mention work of hubel and wiesel, stryker and shatz,Crair et al]
 * Critical period, now called sensitive period, exists during which the ODCs are modified by plasticity
 * If both eyes are closed, removed ,or silenced during CP its ODCs shrink or are completely eliminated.
 * If one eye is closed ("monocular deprivation"), removed, or silenced during CP its ODCs corresponding to the affected eye shrink.

Models
Many models have been proposed to explain the development and plasticity of the ODCs. In general these models can be split into two categories, those that posit formation via chemotaxis and those that posit a Hebbian activity dependent mechanism.
 * Generally chemotaxis models assume activity independent formation via the action of axon guidance molecules with the structures only later being refined by activity
 * But there are now known to be activity dependent and activity modifying  guidance molecules.

Modified Hebbian learning

 * One major model of the formation of the stripes seen in ODCs is that they form by hebbian competition between axon terminals.
 * The ODCs look like Turing patterns, which can be formed by Hebbian mechanisms when incoming activity is locally excitatory and long range inhibitory.
 * Some models are pure excitatory, some inhibitory, some mixed

Chemotaxis
Chemotactic models posit the existence of axon guidance molecules that direct the initial formation of the ODCs
 * Models tend not to be very specific since no axon guidance molecule has ever been found.
 * Most chemotaxis models require an activity dependent process to take over after formation
 * Work in achiasmatic Belgian sheepdogs makes activity dependent formation unlikely and shows that the columns are probably temporal vs. nasal rather than contra vs ipsilateral
 * Sperry believed there were axon guidance molecules that distinguish temporal from nasal retina

Turing Mechanism
This section can be entirely reffed from the great book on self organization:
 * Turing patterns or reaction diffusion patterns happen in systems with local positive feedback and long range negative feedback.
 * Turing patterns are common in biology, ex. zebra stripes, leopard spots, almost all coat patterns.
 * Grid cells may be formed by turing patterns
 * ODCs are identical to turing patterns seen in simulation
 * These patterns could be caused by chemical interactions between axon guidance molecules or by hebbian activity dependent mechansm
 * Turing patterns thus don't settle the chem vs hebbian debate.

Function
It was has long been believed that ocular dominance columns play some role in binocular vision. However, it has been reported that squirrel monkeys, which lack often ocular dominance stripes, have a stereoacuity comparable to that of human observers. Furthermore, several observations indicate that species with no clear ocular dominance columns still display excellent visual capabilities[]. Another candidate function for ocular dominance columns (and for columns in general) is the minimization of connection lengths and processing time, which could be evolutionarily important .Many believe that the ocular dominance columns serve no function

Notes/Quotes
1."In the squirrel monkey, no relationship exists between ocular dominance columns (when present) and CO patches (Horton & Hocking 1996a)"in The cortical column: a structure without a function 2."Orientation pinwheels tend to be situated in the middle of ocular dominance columns (Bartfeld & Grinvald 1992; Blasdel 1992b; Crair et al. 1997)."ibid 3."Columns are often regarded as a special feature of the cortex, but retinal input to the superior colliculus is segregated into parallel stripes that resemble closely ocular dominance columns (Hubel et al. 1975)." 4."The only salient point to emerge is that species with ocular dominance columns are predators. Among mammals, efficient predation requires high-grade stereopsis, but as outlined in the previous paragraph, disparity-tuned cells appear to have no systematic relationship with ocular dominance columns."

Questions for Shatz
-In (Stryker and Harris 1986) it is mentioned that binocular blockade completely removes ocular dominance columns, but in papers with monocular manipulations it seems that the destruction is only partial. How should I interpret this, and should I address it in the article? -Are retinal waves different nasally from temporally? If so, than nasal-temporal activity dependent segregation is unsurprising, contradicting the surprise at mirror symmetry found in (Adams & Horton 2003). I have included the Adams and Horton paper as strong evidence for the chemotaxis camp, but I'm unsure if I should. -What do you make of the Crowley and Katz claim of ODC-like patches in enucleated animals? -Is there any evidence for the MHC mechanisms of plasticity in V1? -Do firing patterns similar to retinal waves occur from any other source? Should I be including any other sources of experience independent activity in my description of prenatal ODC formation? -Is the critical/sensitive period for experience dependent column plasticity distinct from the period when ODCs form initially before birth? I'm thinking of adding a representative development timeline to the article, assuming such a thing exists. -Considering the audience ("The interested lay reader"), should I include information about the relation to the subplate? -I have so far included little on inhibitory plasticity effects, because I have had trouble finding information on it. Should I add more, and, if so, where can I find a description of it? -What is cytochrome oxidase? There are apparently blobs of it, and it is correlated to activity, but other than that I can't figure out why it seems to be so important to everyone. -Do you know of any major developments in the ODC field in the last five years? I haven't found much, and I wonder if I'm missing something.