Reorganization of Visual Callosal Connections Following Alterations of Retinal Input and Brain Damage

Abstract

Vision is a very important sensory modality in humans. Visual disorders are numerous and arising from diverse and complex causes. Deficits in visual function are highly disabling from a social point of view and in addition cause a considerable economic burden. For all these reasons there is an intense effort by the scientific community to gather knowledge on visual deficit mechanisms and to find possible new strategies for recovery and treatment. In this review, we focus on an important and sometimes neglected player of the visual function, the corpus callosum (CC). The CC is the major white matter structure in the brain and is involved in information processing between the two hemispheres. In particular, visual callosal connections interconnect homologous areas of visual cortices, binding together the two halves of the visual field. This interhemispheric communication plays a significant role in visual cortical output. Here, we will first review the essential literature on the physiology of the callosal connections in normal vision. The available data support the view that the callosum contributes to both excitation and inhibition to the target hemisphere, with a dynamic adaptation to the strength of the incoming visual input. Next, we will focus on data showing how callosal connections may sense visual alterations and respond to the classical paradigm for the study of visual plasticity, i.e., monocular deprivation (MD). This is a prototypical example of a model for the study of callosal plasticity in pathological conditions (e.g., strabismus and amblyopia) characterized by unbalanced input from the two eyes. We will also discuss the findings of callosal alterations in blind subjects. Noteworthy, we will discuss data showing that inter-hemispheric transfer mediates recovery of visual responsiveness following cortical damage. Finally, we will provide an overview of how callosal projections dysfunction could contribute to pathologies such as neglect and occipital epilepsy. A particular focus will be on reviewing noninvasive brain stimulation techniques and optogenetic approaches that allow to selectively manipulate callosal function and to probe its involvement in cortical processing and plasticity. Overall, the data indicate that experience can potently impact on transcallosal connectivity, and that the callosum itself is crucial for plasticity and recovery in various disorders of the visual pathway.

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

“…Therefore down-regulation of interhemispheric tracts could affect a variety of visual functions including those known to be disrupted by early life enucleation such as face perception and motion integration. Our results are consistent with a large literature involving non-human animals demonstrating that a range of manipulations including early monocular enucleation or eyelid suture alter the development of intracortical primary visual cortex connections (Nys et al, 2015, Restani & Caleo, 2016.…”

“…Reduced FA is consistent with increased RD and MD, although FA is a more general measure of white matter microstructure . Intracortical connectivity within the normal visual pathway facilitates binocular vision, enables integrated processing across the left and right visual hemifields (Peiker, Wunderle, Eriksson, Schmidt & Schmidt, 2013, Restani & Caleo, 2016, Song & Rees, 2018 and facilitates interhemispheric synchronization that supports sparse encoding of visual information (Knyazeva, 2013). Therefore down-regulation of interhemispheric tracts could affect a variety of visual functions including those known to be disrupted by early life enucleation such as face perception and motion integration.…”

Intact hemisphere and corpus callosum compensate for visuomotor functions after early visual cortex damage

“…Evidence about restructuring of callosal connections following cortical damage in humans and animals is compelling ( 46 ). We speculate that the functional meaning of interhemispheric communication at posterior callosal sites is that of transferring sensory-based information.…”

Homotopic Commissural Projections of Area Prostriata in Rat and Mouse: Comparison With Presubiculum and Parasubiculum

“…Commissural connections participate in processing and integration of information between two hemispheres (Restani and Caleo, 2016 ). It was reported that two of prostriata's adjoining areas, PrS and PaS, have strong commissural connections with contralateral medial entorhinal cortex (MEC) in rat (van Groen and Wyss, 1990 ; Honda et al, 2008 ; Preston-Ferrer et al, 2016 ).…”