The Visual Cortex

Abstract

In order to understand the organization of visual spatial organization within the occipital cortex, it is necessary to review basic functional aspects of the retina and its projection to the lateral geniculate. Retinal photoreceptors actually hyperpolarize when light hits the 11-cis retinal molecule and induces molecular motion within the photoreceptor membrane from the isomerization into all-trans retinal; constant fine microsaccades of the eye prevent extinction of the visual pigments. The output for the temporal half of each retina joins with the same for the nasal hemi-retina from the opposite eye to unite at the optic chiasm and project back to innervate the laminated lateral geniculate body of the thalamus, where the optic radiations are formed which sweep back around the occipital horns of the lateral ventricles to reach the primary occipital cortex. Ocular dominance columns form a basic organizational theme to the occipital cortex, where serpiginous strips from one eye alternate with the same for the other eye. Superimposed on this anatomic parcelization are functional columns tuned for orientation of the visual stimulus where neurons selectively fire according to the angle of movement yet other showing more complex properties of selectively firing according to sharp changes in contrast (edge detection). Clinical considerations with regards to diseases of the visual system include Leber’s hereditary optic neuropathy that affects the retinal ganglion cells; degenerative illness affecting the primary visual cortex as well as other areas of cerebral cortex include Lewy body dementia. With regards to migraine, posterior waves of spreading depression affecting the occipital cortex generate migrainous visual symptoms in advance of the typically severe, debilitating headache.