Direct observation of the crossed interhemispheric transfer of the left-right mirror-images in human vision

TL;DR

This study provides direct evidence of crossed interhemispheric transfer of mirror-images in human vision, revealing how symmetry and mirror-image perception are processed across brain hemispheres, especially in dyslexic individuals.

Contribution

It demonstrates that mirror-image transfer occurs via crossed callosal connections in the primary visual cortex, using a novel noise-activated afterimage method in dyslexic subjects.

Findings

Mirror-images are perceived through the opposite eye after monocular fixation.

Interhemispheric transfer of mirror-images occurs via crossed callosal pathways.

Primary and mirror images are spatially and temporally resolved in perception.

Abstract

Symmetry breaking is common in animal and human brains where the lack of asymmetry often perturbs behavioral and cognitive functions. In particular, the ubiquity of mirror-image confusion in young children, which often persists in dyslexia, is established. However, the very existence of these symmetric mirror-images and their perceptual or memory nature remain controversial. Here, using the noise-activated afterimage method, we demonstrate that a dyslexic with mirror-images is an ideal candidate for solving the mystery. Indeed, after a monocular fixation, the primary afterimages are perceived alone through this eye, while the mirror-images are also perceived alone, but exclusively through the other eye which has remained closed. We deduce that the callosal interhemispheric connections are necessarily projected on the dominance columns of layer 4 of the primary cortex, the only layer where segregation is strict, and are furthermore crossed. Our results show that perceived primary and mirror images are spatially and temporally resolved.