Chapter 10 Notes - Chapter 10 Visual System Cortical...

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Chapter 10: Visual System: Cortical Processing and Object Perception Visual processing begins in the occipital lobe and works its way through all 4 lobes along two streams 1. The dorsal stream: proceeds through the parietal stream and deals with spatial information. 2. Ventral stream: proceeds through the temporal stream and deals with details A. The Retinal Projection to the Brain 1. General layout of the retinal projection Subcortical targets of the retinal output The visual system contains a sort of thalamus on each side of the brain, known as the LGN ( lateral geniculate nucleus ). Plays a role in transfer of visual information to the visual cortex The other major target is the superior colliculus, which is paired also, and sits closer to the midline just below the LGN Cortical projection of retinal signals
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Retinal signals must pass the LGN before they hit the visual cortex. Retinal fibers synapse onto the LGN and the axons from the LGN go to the visual cortex ( optic radiation goes from LGN to cortex) There’s no crossover at this level!!! The LGN’s project to the cortex on their respective sides. Signal splitting at the optic chiasm The optic nerves from each eye meet at the optic chiasm, but once there, the nasal and temporal fibers take different routes. The fibers from the temporal half project to the LGN on the SAME side The fibers from the nasal half project to the LGN on the OTHER side Once the nerves leave the optic chiasm, the nerve bundle is called the optic tract and each goes to the LGN on the same side. Both our eyes are fixated on an object. Object L (on the left visual field) falls on the right side of both retinas, and Object R (on the right visual field) falls on the left side of both retinas. Because of this crossing, they both end up on the nasal side of the retina of each eye, and since each eye’s nasal retina crosses to the LGN on the other side of the head, all images from the left visual field are processed by the right side of the brain and all images from the right visual field are processed by the left side of the brain. The foveal representation Since the left and right retinal fields are projected to opposite sides, there has to be very precise splitting at the level for the fovea. But we don’t know this for sure, and it’s highly unlikely that we’re biologically able to make this precise splitting, so it’s been suggested that around the fovea, the splitting is a little blurry. 2. The Lateral Geniculate Nucleus
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The LGN has 6 layers. Fibers are projected from the optic chiasm optic tract (90%) LGN primary visual cortex Structural and functional properties of the LGN 90% of the fibers from the optic tract go to the LGN. Two types of “neurons” in the 6 layers of the LGN Magnocellular: have large cell bodies, found in the two lowest layers (1 & 2) Parvocellular: have smaller cell bodies, found in the 4 upper layers (3 to 6) Just as ganglion cells have a receptive field, so do LGN neurons. All LGN neurons have
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