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) CharacteristicP cellsM cells Sizesmalllarge Conductionslowfast % ganglion cells80%20% Spatial resolutionhighlow Temporal resolutionlow high Contrast sensitivitylow good

P & M LGN cells CharacteristicP cellsM cells Color opponentyes no Spatial resolutionhighlow Temporal Resolutionlowhigh

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.