Optic Chiasm. Credit: glaucoma-eye-info.com
One way to increase our understanding of bilateral brains, like our own, is to inspect their paired sensory systems. In our visual system, the optic nerves normally combine at a place called the optic chiasm. Here half the fibers from each eye cross over to the opposite hemisphere. When this natural partition fails to develop normally, the system compensates in different ways. In people with albinism, for example, almost all of the fibers fully cross at the chiasm. As a result, images are combined in the brain in such a way that full depth of vision is limited. Their eyes also may move slightly independent of each other, or dart back-and-forth in a condition known as nystagmus. When the opposite situation occurs, that in which the optic nerves do not cross at all during their development, it is called congenital achiasma. An individual with this rare condition was recently studied with different forms MRI. The results, reported in the journal Neuropsychologia, show that achiasma can occur as an isolated defect, lacking any structural abnormalities in other pathways that cross the midline. The study also demonstrated that the part of the cortex that first receives the visual input, the primary visual cortex, does not rely on information from the opposite side to perform its immediate functions.
When input to the two halves of the brain is parsed according to according to the eye, rather than to the visual field, binocularity is typically affected in some way or another. The eyes may a slightly crossed configuration, and nystagmus occurs more readily as the visual system updates. The subject of the present study, henceforth known as GB, additionally displayed an eye effect known as seesaw nystagmus. In this type of nystagmus, the eyes alternately move up and down, out of phase with each other. When initial MRI scans failed to show an optic chiasm in patient GB, researchers subsequently verified that it was completely absent by tracing the nerves with diffusion tensor imaging (DTI). He was also given a series of tests during a functional MRI scan (fMRI), to see how the visual field mapped to his cortex.
More on this interesting subject here Congenitally absent optic chiasm: Making sense of visual pathways.