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 (V1). 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 squirrel monkeys either lack or partially lack ocular dominance columns, which would not be expected if they are useful. This has led some to question whether they serve a purpose, or are just a byproduct of development.
History
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. Ocular dominance columns 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 of the primary visual cortex that lie perpendicular to the orientation columns, as can be seen in the accompanying figure. Different species have somewhat different morphologies and levels of organization. For example, humans, cats, ferrets, and macaques all have fairly well defined columns, while squirrel monkeys have quite variable columns. There is even variation in expression in individuals of the same species and in different parts of the cortex of the same individual. The columns are innervated by input from the lateral geniculate nucleus (LGN) into cortical layer 4 and have mostly reciprocal projections to many other parts of the visual cortex.
Relation to other features of V1
The ocular dominance columns cover the primary (striate) visual cortex, with the exception of monocular regions of the cortical map corresponding to peripheral vision and the blind spot. 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, resulting in the orientation columns, which are laid out in a characteristic pinwheel shape. Similarly, there are columns in the cortex that have high levels of the protein cytochrome oxidase. These are called cytochrome oxidase "blobs" because of their scattered blob-like appearance.
All three types of column are present in the visual cortex of humans and macaques, among other animals. In macaques, it was found that both blobs and pinwheel centers tend to lie in the center of ocular dominance columns, but no particular relation has been found between pinwheel centers and blobs. In humans, the layout of the columns is similar; however, humans have somewhat variable column expression with at least one subject having disordered columns similar to those commonly found in squirrel monkeys.
Most early models of the columns supposed that there were discrete "modules" or "hypercolumns" tiling the cortex, consisting of a repeating unit containing a full set of orientation and ocular dominance columns. While such units can be constructed, the map of columns is so distorted that there is no repeating structure and no clear boundaries between modules. Additionally, practically every combination of having or not having orientation, dominance, and cytochrome oxidase columns has been observed in one species or another. Further confusing the issue, squirrel monkeys don't always express columns, and even when they do the cytochrome oxidase blobs are not in register with the ocular dominance columns.