Plasticity of Visual Interhemispheric Transfer to Areas 17 and 18 in Cats With Convergent Unilateral Early Strabismus

Abstract

Abstract: : Purpose: Unilateral convergent squint significantly modifies interhemispheric transfer to cortical areas 17 and 18 when activated through the stimulation of the non deviated eye. This was established in cats with surgically induced squint performed early after birth. It was shown that the location of transcallosally activated neurons of the visual areas 17 and 18 in the hemisphere ipsilateral to the deviated eye is not restricted to the 17/18 border (as in normal conditions) but expands to large portions of area 17 and area 18. It results of the stabilisation of exuberant juvenile callosals terminals. The aim of this study is to assess morphologically those exuberances. Methods: We studied interhemispheric connections in case of early unilateral convergent strabismus in cats through an anatomical approach : callosal axons were labelled with biocytin and 3–D reconstructed with the Neurolucida software. Results: Reconstructing 6 axons confirmed the exuberance of callosal terminals in the hemisphere ipsilateral to the deviated eye. They originate not only from the border 17/18 but also from areas 17 and 18. The originating zone in terms of anterior or posterior localisation differs from the terminal zone. Conclusions: Early convergent strabismus modifies visual interhemispheric transfer to areas 17 and 18 through the corpus callosum. Homotopic connections occur as in normal animals, but also heterotopic ones. Those anatomical results agree with functional data. Callosal tangential exuberance would be a vain attempt to correct the disparity induced by strabismus. Development and stabilisation of callosal projections therefore depend on visual binocular experience, and not only uniocular.

Keywords: strabismus • plasticity • binocular vision/stereopsis