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.