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Lec 11

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University of Toronto St. George
Doug Mac Kay

11 Central visual processingOctober 03 2007All visual information lightdark contrast is conveyed by ganglion cells which are the only cells with axons that produce action potentials that project to the brainOptic ChiasmEach retina sees both visual hemifields but each half of brain processes only contralateral fieldoexceptionthere is an extreme part of the leftright hemisphere that the rightleft retina cannot see only the ipsilateral leftright eye can seetherefore half of afferents in each optic nerve cross in optic chiasmothe rest project to same side of brainafter chiasm optic nerve PNS becomes optic tract CNSVisual nucleiAfferents in optic tract have many targetso1 lateral geniculate nucleus thalamuso2 superior colliculusM stream only rapidly adapting cellso3 pretectal nucleipupillary light reflex and accessory optic system for motion detection M streamautonomic zonenucleus in front superior colliculuso4 suprachiasmatic nucleuscircadian cyclein hypothalamus on top of optic chiasmLateral geniculate nucleus LGNback of thalamusvisual nucleusoreceives information from both M and P streams but terminate in different layersoimportant for visual perceptionosends signals to primary visual cortex6 layers2 for M stream bottom 4 for P stream topodoubled because of the two eyesinformation from both eyes are kept separate in the thalamus because fusion of inputs from both eyes need to be done carefullyalternate layers receive afferents from different retinareceptive fields of neurons in all layers lying on a radial axis are the sameoLGN aligns all receptive fields so cells match upinformation is sent up to the visual cortex where fusion occurshave centersurround receptive fields but bigger receptive fields than ganglion due to convergencePrimary visual cortex V1LGN organizes inputs to visual cortexV1 lies along calcarine sulcus in medial wall of occipital lobe at back of brainretinotopic organizationolayout on retina is preserved in LGN and V1largest area devoted to fovea and macula where photoreceptors are most numerous small receptive fields more packed in1
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