t  Asymmetric Connections, Duplicate Layers, and a Vertically Inverted Map in the Primary Visual System

Abstract

The achiasmatic mutation is a remarkable and rare visual system mutation carried in a line of black sheepdogs. In affected animals, the optic chiasm is missing, and each retina projects entirely to the ipsilateral hemisphere. As a result of this navigational error, maps of visual space in the lateral 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 the achiasmatic mutation, one in which retinal axons from only one eye successfully cross midline and in which the great majority of fibers from both eyes terminate in a single lateral geniculate nucleus. The dominant optic tract contains four times as many axons as the other tract. The hyperinnervated LGN has a lamination pattern consisting of duplicate and partly interwoven layers. A multiunit mapping study of visual cortex (primarily area 17 along the marginal gyrus) shows that receptive field topography and orientation selectivity are normal. The size of central binocular visual space is nearly normal and is flanked by monocular domains in the periphery. However, there is an inexplicable vertical inversion in the orientation of the cortical representation: superior fields are located rostrally, and inferior fields are located caudally. Despite a host of drastic abnormalities at all level of the visual system, from retina to cortex, this animal was behaviorally indistinguishable from normal dogs and did not have any detectable oculomotor abnormalities.

Congenital Achiasma and See-Saw Nystagmus in VACTERL Syndrome

“…A study of 2 patients has suggested that achiasmatic patients have deranged retinotopic maps in the occipital cortex despite relatively normal visual fields and perception (15). This type of altered anatomy and physiology has also been reported in the lateral geniculate nucleus and visual cortex of achiasmatic Belgian sheepdogs, with dramatic discontinuity of receptive field representations and proximity of neurons that respond to visual stimuli on opposite sides of the visual field (9,10,16)…”

The effects of 1 week of REM sleep deprivation on parvalbumin and calbindin immunoreactive neurons in central visual pathways of kittens

“…The effects of the loss of endogenous activity may be buffered at higher levels of the visual pathway (Weliky and Katz 1999). An example of such buffering is found in the hemichiasmatic dog (Hogan et al . 1999a).…”

How do features of sensory representations develop?

“…In one type, most of the retinal ganglion cells of both eyes project to the lateral geniculate nucleus of one hemisphere and not the other. (57) In the other phenotype, retinal axons of each eye project only to the ipsilateral hemisphere. (58) Nevertheless, topological maps of the altered inputs were found in visual cortex.…”