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Lecture 11

Lecture 11 - Central Visual Processing

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Michelle French

PSL300H1F L 11; Oct 05, 2011 Afferents in optic tract have many targets: Optic Chiasm 1) lateral geniculate nucleus (thalamus) Each retina sees both visual hemifields, but 2) superior colliculus; M stream only (rapidly each half of brain processes only contralateral adapting info, important for orienting eyes field and head towards stimuli, so motor structure) therefore, half of afferents in each optic 3) pretectal nuclei: more autonomic functions nerve cross in optic chiasm; the rest project to like pupillary light reflex, (right in front of same side of brain SC/tectum in midbrain), and accessory optic after chiasm, optic nerve becomes optic tract system for motion detection (a lot of M stream input) 2&3 side by simple, very simple system for detecting motion 4) suprachiasmatic nucleus(in hypothalamus, essential, master clock): circadian cycle govern 24/7 cycles, day/night, input from optic tract Two eyeballs looking at visual field, pretty much looking at entire field which is revealed since left field is on right side of each retina and vice [email protected] of thalamus, SCN right on top of optic versa chiasm, SC&pretectal also receive branches from So to analyzed entire right field in left axons going to the LGN hemisphere and left field in right hemisphere Primary visual cortex V1 1 station in cerebral cortex receiving visual info from the thalamus, easiest way of collecting these axons right at back of occipital lobe of the brain (as far But means all the info of the right hemifield of the right eye has to cross to the other side and as possible from eyeballs) vice versa And all info of left hemifield in left eye must Lateral geniculate nucleus (LGN) cross to other side 6 layers: 2 for M stream, 4 for P stream (slowly adapting) Other half stays on same side Similar to cortex but dif organizations Joining together on one side all info of contralateral hemifield alternate layers receive afferents/input from the About half of axons crossing dif retina (the two eyes) Blind spot at medial part of retina, fovea away so LGN not attempting to fuse the info, just collecting the info and trying to organize in a from blind spot methodical manner Blind spot near bottom outer corner of eyeballs Retinotopic manner so receptive field of in the pic Always one eye covering the blind spot neurons in all the layers lying along the radial 1 place in brain where info collected: Lateral axis are the same geniculate nucleus in thalamus layered Note bigger r.f.s than ganglion cells in retina, due to convergence since more synapses gone structure, sticks out like a bump in back of thru; same centre organization (light in centre thalamus, some of the axons also terminate in and dark in surround or vice versa) the SC at top of midbrain rapidly-adapting (M pathway) Visual nuclei like in S1 w magnification of areas of fingers w more sensitivity and mechano receptors Photoreceptors squished into fovea blown up out of proportion in V1, since most receptors in fovea and surrounding macula periphery (largest surface of retina) not getting proportional share of V1 3 layers for input from same side (white,ipsilateral) and 3 layers for opposite side (grey, contralateral) Point is dividing up layers btwn the 2 eyes First 2 layers for magnocellular input Last 4 for parvocellular input But wonder why need 2 sets for p? Colour-related info rather than just black/white! If take radial axis thru all layers, the neurons in each layer along it will respond to same spot in Calcarine (curving) sulcus in medial wall of contrlateral visual field occipital lobe Ex. Right LGN mapping left half of visual world all nerons along radial axis respond V1 line the banks at the bottom of the calcarine to same place in visual field sulcus, extends to back of occipital pole (comes out at gyrus at back), most in calcarine sulcus Organization - Retinotopic mapping, each Fovea, central 3 degrees (actually 1.5 since neuron has place in layer, lined up across the hemifield) of physical visual field layers blown up disproportionately in v1 over a Same retinotopic mapping in each layer makes easier to gather the info to send to third of V1 devoted to this central portion of the same place in the cortex to V1 since all visual field macula has 6.5 degrees per hemifield (13 from same point in visual field
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