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The Retina has layers of cells
•
•
photoreceptors transduce incoming light
ganglion cells send signals along to the brain
Two kinds of Photoreceptors
• 2 types of photoreceptors: rods and
cones
• rods are very sensitive - useful in dim
light
•
The Retina
•
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
Neurons “collect” information
• each ganglion cell integrates
information from a particular spot on
the retina called its receptive field
Receptive Fields
Action potentials
Stimulus is in
receptive field
Stimulus is
near receptive
field
Stimulus is
outside receptive
field
Visual Pathways
• 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
Stimulus
Cortical Activity
Visual Pathways
• signals are separated according to the type of
information
Dorsal “Where”
Pathway:
Motion and
Location
Primary Visual
Cortex (V1)
Ventral “What” Pathway: Color and Form
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