This study describes the overall arrangement of geniculocortical input representing the system of cortical ocular dominance bands in layer IV of striate cortex in the adult cat. The pattern of ocular dominance bands was revealed by transneuronal transport of the intraocularly injected tracer wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). Our data indicate that this procedure does not damage the retina and that it results in relatively uniform uptake and transport of the tracer. Using previously published techniques (Olavarria and Van Sluyters, 1983, 1985), both cortical hemispheres of each cat were unfolded, flattened and tangentially sectioned. Analysis of the WGA-HRP labeling patterns in these sections revealed a relatively continuous network of irregularly branching bands in layer IV of area 17 in both hemispheres. Because of a systematic difference in the level of interband labeling, ocular dominance bands appear less distinct in the hemisphere contralateral to the injected eye. There is also a tendency for interband labeling to be greater in cortical regions that represent the more peripheral aspects of the binocular portion of the visual field. The width of an individual ocular dominance band in the cat fluctuates, so that it appears to be made up of a series of uniformly sized, roughly circular beads of label. The diameter of these beads averages 667 micron, and preliminary counts indicate that there are 650-675 beads in each striate cortex. Contrary to earlier suggestions, in 4 out of 6 hemispheres analyzed quantitatively there was no tendency for ocular dominance bands to be oriented along a preferred axis in cat striate cortex, including an axis orthogonal to the border between areas 17 and 18. Ocular dominance bands in area 18 appear to be broader than those in area 17, and they seem to have a greater tendency to be oriented orthogonal to the 17/18 border than those in area 17. Compared with the ocular dominance pattern in monkey striate cortex, the ocular dominance pattern in the cat is much less regular. In general, cat ocular dominance bands appear to fluctuate more in width, to change direction more often, and to be less likely to run orthogonal to the 17/18 border. The greater regularity of the primate ocular dominance pattern may be related to differences in the way in which the visual hemifield is mapped onto the striate cortex in these 2 species.

Cytochrome-oxidase blobs are central to two of the most influential ideas in contemporary visual neuroscience--cortical modularity and parallel processing pathways. In particular, the regular 2D array of cytochrome-oxidase-rich blobs in primate visual cortex is arguably the most compelling evidence for cortical modularity and has been hypothesized to mark a separate processing stream through the visual cortex. Although previously a variety of mammals have been studied, blobs have only been demonstrated in the visual cortex of primates, which has led to the conclusion that blobs represent a primate-specific feature of visual cortical organization. Here we demonstrate the presence of cytochrome-oxidase blobs in a nonprimate species. Throughout the full tangential extent of layers II-III in cat visual cortex the cytochrome-oxidase staining pattern is distinctly patchy, with the darkly stained blobs forming a regular 2D array. In addition, the blobs in cat visual cortex are functionally related to the underlying ocular dominance columns. The presence of cytochrome-oxidase blobs in the cat clearly demonstrates that they no longer can be considered a primate-specific feature of visual cortical organization.

In layer IV of the primary visual cortex, in both the macaque monkey and the cat, geniculocortical terminals representing the two eyes are segregated into alternating zones known as ocular dominance bands. Viewed tangentially, in the monkey these bands take the form of a series of branching parallel stripes that run roughly perpendicular to the border of striate cortex. In the cat, the overall ocular dominance pattern consists of irregularly branching, beaded bands that exhibit no predominant orientation. If the striking differences in the appearance of these two patterns reflect important differences in the basic rules governing cortical ocular dominance, then this poses a problem for attempts to formulate general principles of visual cortical organization. However, it has been suggested that the differences in the appearance of the ocular dominance patterns in these two species could result simply from known differences in the boundary conditions of their geniculocortical pathways. This article describes the formulation and testing of a single computational model that accurately predicts the quite dissimilar ocular dominance patterns in cats and monkeys. This model also generalizes to predict the different ocular dominance patterns observed in young and old three-eyed frogs, supporting the notion that the overall pattern of ocular dominance is governed by a common set of rules. The significance of these results is discussed in terms of previous models, which have focused largely on local processes underlying the development of ocular dominance segregation. Although the present model is not a developmental one, it does shed some light on potential mechanisms for establishing retinotopy in striate cortex and on possible developmental relationships between the geniculostriate pathway and intrinsic modularity of the striate cortex.