Chapter 10

Chapter 10 - Chapter 10- The Visual System: Cortical...

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Chapter 10- The Visual System: Cortical Processing and Object Perception A. The Retinal Projection to the Brain General layout of the retinal projection - Visual info is processed in one way or another in all 4 lobes, starting in the occipital. - Beyond the occipital lobe, visual processing occurs along 2 major cortical pathways: the dorsal stream proceeds to the parietal lobe and the ventral stream into the temporal lobe. - In earlier times it was believed that the ventricles played an integral part of visual processing; da Vinci’s anatomical sketches showed optic nerves end directly in ventricles. - A correct understanding began w/ the anatomist Padua in the Renaissance. Subcortical targets of the retinal output - The thalamus is a major site for the relay of sensory information. - The visual system contains a major thalamic structure on each hemisphere = the LGN . - LGN is the principal subcortical target of the retinal projection; relays info to visual cortex. - Other major target of the retinal projection = superior colliculus, sitting just below the LGN. Cortical projection of retinal signals - Retinal fibers synapse onto LGN neurons; axons of LGN neurons carry visual signals to the cortex- the massive fiber projection that transmits these signals is the optic radiation . - At this level there is no crossover of fibers- right and left LGN project to respective sides. Signal splitting at the optic chiasm - General rule governing sensory projections = that a crossover takes place at some point. - When the two optic nerves come together at the optic chiasm , the fibers associated with the nasal retina and those with the temporal retina begin to follow different routes. - Temporal retinal inputs from either eye continue to project to their respective sides’ LGNs. o After the OC, optic nerve bundle contains fibers from the ipsalateral (same) side . - Nasal retinal inputs from either eye cross over at the OC to the opposite side’s LGN. o After the OC, optic nerve bundle contains fibers from the contralateral side as well. - Central retinal input is bilaterally represented through its projection to both sides; this is critical because it binds together the two halves of the visual field into a seamless image. - Because the fiber bundle after crossover at the optic chiasm contains information of a different nature than the optic nerve, it is called the optic tract to distinguish it. - Why the visual system exclusively is patterned in this way o Info from the left visual field falls on the right retinal side of each eye; info from the right visual field falls on the left retinal side of each eye. o The right optic tract transmits signals from the right retinal halves (L-visual field). o
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This note was uploaded on 12/18/2011 for the course PSYC 212 taught by Professor Shahin during the Fall '11 term at McGill.

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Chapter 10 - Chapter 10- The Visual System: Cortical...

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