Blockade of retinal or cortical activity does not prevent the development of callosal patches normally associated with ocular dominance columns in primary visual cortex

Callosal patches in primary visual cortex of Long Evans rats, normally associated with ocular dominance columns, emerge by postnatal day 10 (P10), but they do not form in rats monocularly enucleated a few days before P10. We investigated whether we could replicate the results of monocular enucleation by using tetrodotoxin (TTX) to block neural activity in one eye, or in primary visual cortex. Animals received daily intravitreal (P6–P9) or intracortical (P7–P9) injections of TTX, and our physiological evaluation of the efficacy of these injections indicated that the blockade induced by a single injection lasted at least 24 h. Four weeks later, the patterns of callosal connections in one hemisphere were revealed after multiple injections of horseradish peroxidase in the other hemisphere. We found that in rats receiving either intravitreal or cortical injections of TTX, the patterns of callosal patches analyzed in tangential sections from the flattened cortex were not significantly different from the pattern in normal rats. Our findings, therefore, suggest that the effects of monocular enucleation on the distribution of callosal connections are not due to the resulting imbalance of afferent ganglion cell activity, and that factors other than neural activity are likely involved.

Early life maturation of human visual system white matter is altered by monocular enucleation

Monocular enucleation early in life and the resultant lack of binocular visual input during visual development results in functional and structural brain changes in adulthood, including alterations in white matter microstructure. However, the time courses of these neurodevelopmental changes are unknown. Here, we investigated whether structural brain changes were present at 8 to 12 years of age in a group of children with a history of monocular enucleation prior to 3 years of age (the ME group) relative to control participants with normal binocular vision (the BC group). Structural connectivity was measured using diffusion tensor imaging (DTI). Relative to the BC group, the ME group exhibited significantly increased radial and mean diffusivity in the optic radiation contralateral to the enucleated eye, the bidirectional interhemispheric V1 to V1 tracts and the V1 to MT tract ipsilateral to the enucleated eye. These changes indicate abnormal myelinization and reduced axonal density in subcortical and cortical visual pathway white matter structures following unilateral enucleation and loss of binocular vision. Our findings are broadly consistent with those recently reported for older uniocular individuals suggesting that these effects are present in childhood and persist into adulthood.