Asymmetric connections, duplicate layers, and a vertically inverted map inthe primary visual system

The achiasmatic mutation is a remarkable and rare visual system mutation ca rried in a line of black sheepdogs. In affected animals, the optic chiasm i s missing, and each retina projects entirely to the ipsilateral hemisphere. As a result of this navigational error, maps of visual space in the latera l geniculate nucleus (LGN) have a unique structure with mirror reversals of field position across the A-A1 border. Animals also have a persistent and severe congenital nystagmus. In this report we analyze a novel variant of t he achiasmatic mutation, one in which retinal axons from only one eye succe ssfully cross midline and in which the great majority of fibers from both e yes terminate in a single lateral geniculate nucleus. The dominant optic tr act contains four times as many axons as the other tract. The hyperinnervat ed LGN has a lamination pattern consisting of duplicate and partly interwov en layers. A multiunit mapping study of visual cortex (primarily area 17 al ong the marginal gyrus) shows that receptive field topography and orientati on selectivity are normal. The size of central binocular visual space is ne arly normal and is flanked by monocular domains in the periphery. However, there is an inexplicable vertical inversion in the orientation of the corti cal representation: superior fields are located rostrally, and inferior fie lds are located caudally. Despite a host of drastic abnormalities at all le vel of the visual system, from retina to cortex, this animal was behavioral ly indistinguishable from normal dogs and did not have any detectable oculo motor abnormalities.