In many regions of the mammalian cerebral cortex, cells that share a common receptive field property are grouped into columns (Mountcastle 1997). Despite intensive study, the function of the cortical column remains unknown. In the squirrel monkey, the expression of ocular dominance columns is variable, with columns present in some animals and not in others. By searching for differences between animals with and without columns, it should be possible to infer how columns contribute to visual processing. Single cell recordings outside layer 4C were made in 9 squirrel monkeys, followed by labeling of ocular dominance columns in layer 4C. In the squirrel monkey, compared with the macaque, cells outside layer 4C were more likely to respond to stimulation of either eye, whether ocular dominance columns were present or not. In three animals lacking ocular dominance columns, single cells were recorded from layer 4C. Remarkably, 20% of cells in layer 4C were monocular, despite the absence of columns. This observation means that ocular dominance columns are not necessary for monocular cells to occur in striate cortex. In macaques, each row of cytochrome oxidase (CO) patches is aligned with an ocular dominance column and receives koniocellular input serving one eye only. In squirrel monkeys this was not true: CO patches and ocular dominance columns had no spatial correlation and the koniocellular input to CO patches was binocular. Thus, even when ocular dominance columns occur in the squirrel monkey, they do not transform the functional architecture to resemble that of the macaque.