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• photoreceptors transduce incoming light
• ganglion cells send signals along to the brain
The Retina has layers of cells
• 2 types of photoreceptors: rods and cones
• rods are very sensitive - useful in dim light
•
Two kinds of Photoreceptors
• Rods and cones are distributed differently across the retina
The Retina
• visual acuity (ability to see detail) depends on cones - thus acuity varies across the visual field
The Retina
• Why don’t you notice your blind spot?
The Retina
• Why don’t you notice your blind spot?– Blindspots don’t overlap!
– Your brain “fills in” the missing information
– The specific information in the blindspot isn’t much more missing than the rest of the periphery!
The Retina
• three types of cones: short, medium, and long
• different absorptions enable color vision
The Retina
• each ganglion cell integrates information from a particular spot on the retina called its receptive field
Neurons “collect” information
Receptive Fields
Stimulus is in receptive
fieldStimulus is
near receptive
fieldStimulus is outside
receptive field
Action potentials
• Ganglion cells project to the brain via the optic nerve
• information is projected to contralateral cortex!
Visual Pathways
• the retina is mapped onto primary visual cortex
• called a retinotopic or spatiotopic map
Visual Pathways
Stimulus Cortical Activity
• signals are separated according to the type of information
Visual Pathways
Dorsal Dorsal “Where” “Where” Pathway: Pathway:
Motion and Motion and LocationLocation
Ventral “What” Pathway: Color and FormVentral “What” Pathway: Color and Form
Primary Primary Visual Cortex Visual Cortex
(V1)(V1)
Seeing
Seeing
• Discussion of Gregory’s Article on Visual Illusions - Tuesday Oct 23
Seeing
• Seeing is the process of converting electromagnetic radiation into a conscious mental event
Seeing
• visual system faces many challenges– too much information– too little information– conflicting information– ambiguous information
Seeing
• too much information
Seeing
• too little information
Seeing
• Conflicting Information
Seeing
• Ambiguous information
Seeing
• We will consider two aspects of vision to explore how the brain overcomes these challenges:
– Seeing depth– Seeing in color