Ocular dominance columns in V1 are more susceptible than associated callosal patches to imbalance of eye input during precritical and critical periods.

In Long Evans rats, ocular dominance columns (ODCs) in V1 overlap with patches of callosal connections. Using anatomical tracers, we found that ODCs and callosal patches are present at postnatal day 10 (P10), several days before eye opening, and about 10 days before the activation of the critical period for ocular dominance plasticity (~P20). In rats monocularly enucleated at P10 and perfused ~P20, ODCs ipsilateral to the remaining eye desegregated, indicating that rat ODCs are highly susceptible to monocular enucleation during a precritical period. Monocular enucleation during the critical period exerted significant, although smaller, effects. Monocular eye lid suture during the critical period led to a significant expansion of the ipsilateral projection from the nondeprived eye, whereas the contralateral projection invaded into, and intermixed with, ipsilateral ODCs innervated by the deprived eye. We propose that this intermixing allows callosal connections to contribute to the effects of monocular deprivation assessed in the hemisphere ipsilateral to the nondeprived eye. The ipsilateral and contralateral projections from the deprived eye did not undergo significant shrinkage. In contrast, we found that callosal patches are less susceptible to imbalance of eye input. In rats monocularly enucleated during either the precritical or critical periods, callosal patches were maintained in the hemisphere ipsilateral to the remaining eye, but desegregated in the hemisphere ipsilateral to the enucleated orbit. Callosal patches were maintained in rats binocularly enucleated at P10 or later. Similarly, monocular deprivation during the critical period had no significant effect on callosal patches in either hemisphere.

Influence of ocular dominance columns and patchy callosal connections on binocularity in lateral striate cortex: Long Evans versus albino rats.

In albino rats, it has been reported that lateral striate cortex (V1) is highly binocular, and that input from the ipsilateral eye to this region comes through the callosum. In contrast, in Long Evans rats, this region is nearly exclusively dominated by the contralateral eye even though it is richly innervated by the callosum (Laing, Turecek, Takahata, & Olavarria, 2015). We hypothesized that the inability of callosal connections to relay ipsilateral eye input to lateral V1 in Long Evans rats is a consequence of the existence of ocular dominance columns (ODCs), and of callosal patches in register with ipsilateral ODCs in the binocular region of V1 (Laing et al., 2015). We therefore predicted that in albino rats input from both eyes intermix in the binocular region, without segregating into ODCs, and that callosal connections are not patchy. Confirming our predictions, we found that inputs from both eyes, studied with the transneuronal tracer WGA-HRP, are intermixed in the binocular zone of albinos, without segregating into ODCs. Similarly, we found that callosal connections in albino rats are not patchy but instead are distributed homogeneously throughout the callosal region in V1. We propose that these changes allow the transcallosal passage of ipsilateral eye input to lateral striate cortex, increasing its binocularity. Thus, the binocular region in V1 of albino rats includes lateral striate cortex, being therefore about 25% larger in area than the binocular region in Long Evans rats. Our findings provide insight on the role of callosal connections in generating binocular cells.

The Effect of Onset Age of Visual Deprivation on Visual Cortex Surface Area Across-Species.

Blindness early in life induces permanent alterations in brain anatomy, including reduced surface area of primary visual cortex (V1). Bilateral enucleation early in development causes greater reductions in primary visual cortex surface area than at later times. However, the time at which cortical surface area expansion is no longer sensitive to enucleation is not clearly established, despite being an important milestone for cortical development. Using histological and MRI techniques, we investigated how reductions in the surface area of V1 depends on the timing of blindness onset in rats, ferrets and humans. To compare data across species, we translated ages of all species to a common neuro-developmental event-time (ET) scale. Consistently, blindness during early cortical expansion induced large (~40%) reductions in V1 surface area, in rats and ferrets, while blindness occurring later had diminishing effects. Longitudinal measurements on ferrets confirmed that early enucleation disrupted cortical expansion, rather than inducing enhanced pruning. We modeled the ET associated with the conclusion of the effect of blindness on surface area at maturity (ETc), relative to the normal conclusion of visual cortex surface area expansion, (ETdev). A final analysis combining our data with extant published data confirmed that ETc occurred well before ETdev.

Visual Interhemispheric and Striate-Extrastriate Cortical Connections in the Rabbit: A Multiple Tracer Study.

Previous studies in rabbits identified an array of extrastriate cortical areas anatomically connected with V1 but did not describe their internal topography. To address this issue, we injected multiple anatomical tracers into different regions in V1 of the same animal and analyzed the topography of resulting extrastriate labeled fields with reference to the patterns of callosal connections and myeloarchitecture revealed in tangential sections of the flattened cortex. Our results extend previous studies and provide further evidence that rabbit extrastriate areas resemble the visual areas in rats and mice not only in their general location with respect to V1 but also in their internal topography. Moreover, extrastriate areas in the rabbit maintain a constant relationship with myeloarchitectonic borders and features of the callosal pattern. These findings highlight the rabbit as an alternative model to rats and mice for advancing our understanding of cortical visual processing in mammals, especially for projects benefiting from a larger brain.