The corpus callosum and the visual cortex: plasticity is a game for two

Abstract

Throughout life, experience shapes and selects the most appropriate brain functional connectivity to adapt to a changing environment. An ideal system to study experience-dependent plasticity is the visual cortex, because visual experience can be easily manipulated. In this paper, we focus on the role of interhemispheric, transcallosal projections in experience-dependent plasticity of the visual cortex. We review data showing that deprivation of sensory experience can modify the morphology of callosal fibres, thus altering the communication between the two hemispheres. More importantly, manipulation of callosal input activity during an early critical period alters developmental maturation of functional properties in visual cortex and modifies its ability to remodel in response to experience. We also discuss recent data in rat visual cortex, demonstrating that the corpus callosum plays a role in binocularity of cortical neurons and is involved in the plastic shift of eye preference that follows a period of monocular eyelid suture (monocular deprivation) in early age. Thus, experience can modify the fine connectivity of the corpus callosum, and callosal connections represent a major pathway through which experience can mediate functional maturation and plastic rearrangements in the visual cortex.

Figure 1

Silencing callosal input during the early critical period impairs maturation of visual acuity. (a) Schematics of the experimental protocol. BoNT/E was unilaterally injected into the visual cortex in P14 rat pups to cause a prolonged silencing of one hemisphere. The contralateral, uninjected side has a normal visual experience through the retinogeniculate pathway and only lacks callosal input activity. (b) Bilateral impairments in visual acuity at P35 after unilateral injection of BoNT/E at P14: summary of visual acuities in naïve rats (NOR), rats injected with vehicle (VEHICLE) and in the hemisphere ipsilateral (IPSI) and contralateral (CONTRA) to BoNT/E infusion. Each circle represents one animal. Mean visual acuity (diamonds) is significantly reduced in both hemispheres of BoNT/E rats in comparison with that in normal or vehicle-injected animals. Error bars indicate SE. Data are from [54].

Figure 2

Relative contribution of callosal and thalamic pathways to cortical binocularity. (a, b) Effects of acute callosal silencing on OD in rat primary visual cortex. Binocularity was measured in one side before and after acute pharmacological inactivation of the opposite cortex (a). The results indicate a significant shift of cortical OD towards the contralateral eye, due to the reduction of responses driven by the ipsilateral eye (b). (c, d) Effects of acute geniculate silencing on OD in rat primary visual cortex. The OD histogram shifts towards the ipsilateral eye following acute geniculate inactivation (d). This effect is mainly due to a loss of contralateral eye-driven input. Data are from [32, 55].