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Unformatted text preview: Central Visual Processing Optic Chiasm- Optic nerves project from eyeball into brain thru blind spot - Each retina ‘sees’ both visual hemifelds (visual world)- Must be coalesced (aligned) in brain- Half of retinal efferents must join up w/ efferents from other eyeball -> cross at chiasm- Each half of brain processes only contralateral Feld- Half-brain concerned w/ opposite half of visual Feld- L half brain is concerned w/ R hemiFeld- R half brain concerned w/ L hemiFeld- Half of afferents in each optic nerve cross at optic chiasm- The rest project to same side of brain- After chiasm, optic nerve becomes optic tract- Diagram: black arrows = visual info from R hemiFeld- R hemiFeld inputs -> L retina on both eyes - Efferents travel to L brain hemisphere- Inputs from R eye must cross chiasm to join w/ inputs from L eye- Vice versa for L hemiFeld inputs (gray lines)- Causes motor system to also cross -> cerebral cortex looks at opposite side of body to match w/ visual system Visual Nuclei- Afferents in optic tract have many targets: 1) Thalamus: interface btwn sensory systems + cerebral cortex- lateral geniculate nucleus (LGN):- “Geniculate” refers to way it sticks out of thalamus like a knee, on top of SC- Responsible for guiding eye-head mvts- Concentrates on M- & P-stream afferents 2) Superior colliculus (SC) : on brainstem; one of the bumps on midbrain- Directly receives retinal afferents- Concentrates on rapidly adapting afferents ( M-stream /magnocellular pathways) 3) Pretectal nuclei : “in front of colliculus” on brainstem- Responsible for autonomic fxns, i.e. pupillary light re¡ex, and accessory optic system for motion detection (M-stream); used for postural corrections 4) Suprachiasmatic nucleus (SCN): in hypothalamus, sits on top of optic chiasm- Target for small group of afferents -> responsible for circadian cycle- “Master clock” of body- Also , V1 cortical sensory region in cortex: receives signals from LGN - Right of back of occipital lobe, medial wall of cortex btwn hemispheres- About the farthest you can get from eyeball in cortex! PSL302Y: Lecture 19, by Prof. MacKay Wed., Oct.27, 2010 1 of 7 Lateral geniculate nucleus (LGN)- 6 layers (like cortex): align neuronal r.f.’s- 1-2 for M-stream - 3-6 for P-stream- Alternate layers receive afferents from different sides of retina (of one eyeball)- We don’t merge inputs from both eyeballs yet- So: 3 layers from 1 eyeball, 3 for other eyeball- Organize mappings of retina->contiguous btwn layers- R.f.’s of neurons in all layers, lying on radial axis, are the same (see “ common r.f. ” line)- All refer to same pt in visual Feld- Bigger r.f.’s than they are for ganglion cells of retina, due to synaptic convergence- General rule for all somatosensory systems: primary afferents have tiny r.f.’s, but at cortex have much larger r.f.’s- Note: Ipsi-> “same side”, vs. contra-> “opposite side”- Ipsilateral retinal afferents -> layers 2,3,5- Contralateral retinal afferents -> layers 1,4,6...
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- Fall '11
- Prof. MacKay, Central Visual Processing