Transcript ipsilateral
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 Fibers cross at the chiasm After the chiasm the structure is called the optic tract. Ipsilateral and contralateral • Varies across species. • Frontal eyes have crossing; lateral eyes do not. • In humans, ~50% of fibers cross the medial plane. 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. 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. The Superior Colliculus • Activity initiates eye movements. • Activity guides eye movements. 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. The Superior Colliculus • Thus, the superior colliculus appears to function to bring objects into fixation, rather than to analyze objects. 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. 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. Red-colored dye crystals (Dye-I) were inserted into the parvocellular layers and yellow/green-colored crystals into the magnocellular layers (Photo enhanced.) P & M ganglion Cells (cf. p. 90) Characteristic Size Conduction % ganglion cells Spatial resolution Temporal resolution Contrast sensitivity P cells small slow 80% high low low M cells large fast 20% low high good P & M LGN cells Characteristic Color opponent Spatial resolution Temporal Resolution P cells yes high low M cells no low high 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. 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.