Date post: | 04-Jan-2016 |
Category: |
Documents |
Upload: | prosper-wilkinson |
View: | 217 times |
Download: | 0 times |
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 P cells M cellsSize small largeConduction slow fast% ganglion cells 80% 20%Spatial resolution high lowTemporal resolution low highContrast sensitivity low good
P & M LGN cells
Characteristic P cells M cells
Color opponent yes no
Spatial resolution high low
Temporal Resolution 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.