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.