Neuroscience lecture notes 12.2.09 From the Eye to the Striate Cortex The retinofugal projection The lateral geniculate nucleus Anatomy and physiology of striate cortex. The problems of vision Vision involves much more than just a projection of the image onto a “screen” or a piece of film Visual stimuli often have many different “qualities”, like motion, color, form and texture The “visual scene in our head” is vivid and rich in detail How do the neurons in our brain “produce” this visual scene? Visual information is analyzed, processed and interpreted by the visual brain Major parts of the visual brain are the retina, the lateral geniculate nucleus, and visual cortex The retinofugal projection Structures forming the retinofugal projection Optic nerve Optic chiasm Optic tract Right and left visual hemifield Central portion of the visual field is viewed by both eyes Optic nerve fibers cross in the optic chiasm such that the left visual hemifield projects onto the right hemisphere and the right hemifield to the left hemisphere The superior colleculus Some (10%) of optic tract axons project to the hypothalamus and the superior colliculus The superior colliculus is involved in orienting responses, for example eye and head movements toward visual stimuli. The lateral geniculate nucleus Most optic tract axons innervate the lateral geniculate nucleus, a part of the dorsal thalamus LGN neurons project to the primary visual cortex through the optic radiation The LGN consists of 6 cell layers. The layers are numbered 1 through 6, from ventral to dorsal. Both left and right LGN receive projections from the ipsilateral and contralateral retina The left visual field is mapped onto the right LGN M-type and P-type retinal ganglion cells project to separate layers in the LGN They form parallel pathways to visual cortex. Primary visual (striate) cortex Striate cortex = primary visual cortex = V1 = Brodmann’s area 17 Cortex (including striate cortex) consists of 6 layers Two principal cells types: stellate cells and pyramidal cells Only pyramidal cells send axons to other parts of the brain LGN connections to striate cortex LGN axons terminate mainly in layer IVC The LGN projection preserves retinotopy Magnocellular and parvocellular LGN inputs remain segregated Left and right eye inputs to layer IVC also remains segregated Blobs: “spots” of elevated cytochrome oxidase There are differences in the inputs to blobs and so-called “interblob” regions Other cells in the M channel show direction selectivity Retinotopy Neighboring locations in the retina project to neighboring locations in the LGN and in the striate cortex. Physiology of striate cortex Physiology: there are three relatively independent channels of visual information processing Magnocellular pathway Parvo-interblob pathway Blob pathway The physiology of these visual channels is largely the result of the underlying anatomy (cells and connections) Disruptions of the magnocellular pathway may be associated with dyslexia, a learning disability characterized by problems with oral or written language processing.