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Perception is constructive

Perception is constructive. Perception is context dependent. Perception involves interpretation. Perception groups into meaningful units. Optic Nerve. Carries ~1.25 million RGCs from retina. Is myelinated (unlike retinal nerve cells) About 2 inches (5 cm) to optic chiasm

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Perception is constructive

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  1. Perceptionis constructive

  2. Perception is context dependent

  3. Perception involves interpretation

  4. Perception groups into meaningful units

  5. Optic Nerve • Carries ~1.25 million RGCs from retina. • Is myelinated (unlike retinal nerve cells) • About 2 inches (5 cm) to optic chiasm • Fibers cross at the chiasm • After the chiasm the structure is called the optic tract.

  6. Ipsilateral and contralateral • Varies across species. • Frontal eyes have crossing; lateral eyes do not. • In humans, ~50% of fibers cross the medial plane.

  7. The first bifurcation: • In some fish and amphibians, the majority of LGN cells project to the superior colliculus. • In mammals and reptiles, the majority of LGN cells project to the cortex. • In humans, ~80 of LGN cells project to the cortex; ~20% of LGN cells project to the superior colliculus.

  8. The Superior Colliculus • Cells in the superior colliculus are position sensitive, but have ill-defined ON and OFF regions. • Thus, probably not concerned with what is present, just with where something is.

  9. The Superior Colliculus • Activity initiates eye movements. • Activity guides eye movements.

  10. The Superior Colliculus • Cells are multi-modal or multisensory. • They respond to apparently co-occurring sound and light. • They respond more vigorously when both stimuli are present than when just one is present.

  11. The Superior Colliculus • Thus, the superior colliculus appears to function to bring objects into fixation, rather than to analyze objects.

  12. Structure of the LGN • Six layers bent in the middle. • Layers 1-2 are magnocellular; layers 3-6 are parvocellular. • Layers 1, 4, & 6 are contralateral; layers 2, 3, & 5 are ipsilateral.

  13. Layers 1, 4, and 6 respond to information from the contralateral eye, whereas layers 2, 3, and 5 respond to information from the ipsilateral eye.

  14. Red-colored dye crystals (Dye-I) were inserted into the parvocellular layers and yellow/green-colored crystals into the magnocellular layers (Photo enhanced.)

  15. P & M ganglion Cells (cf. p. 90) Characteristic P cells M cells Size small large Conduction slow fast % ganglion cells 80% 20% Spatial resolution high low Temporal resolution low high Contrast sensitivity low good

  16. P & M LGN cells Characteristic P cells M cells Color opponent yes no Spatial resolution high low Temporal Resolution low high

  17. Color opponency in the LGN • Most parvocellular cells display color opponency. • Some centers are excited by one color, but inhibited by others. • For example, red/green and blue/yellow.

  18. Function of the LGN • LGN appears to preserve retinal information about the presence of edges. • LGN receives information from both the retina and the reticular activating system. Thus, the LGN could be modulated by the overall level of arousal. • K cells may be involved in suppressing cortical information processing during saccades. • LGN also receives input from the cortex.

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