Vision
Stimulus
which
activates visual receptors: light waves in the visible
spectrum
Types
of Electromagnetic
Energy
Transparent
Cornea
Green
Muscular
Iris & Black Pupil Hole
Rods vs Cones
~120
million rods/eye
more
in periphery
very
sensitive (low threshold)
~100
rods share same optic nerve fiber to brain
night
vision (scotopic vision)
~6
million cones /eye
most
in center, especially in the fovea
need
bright light to reach threshold
have
1-to-1 lines to brain- good for detail vision or
“acuity”
3
different types - provide color vision
Layers
of the Retina
Optic
Nerve
Turning
Light
Waves Into Electrical Messages:
Rods
& cones have molecules of light sensitive photopigments
(11-cis-retinal+an
opsin) embedded in cell membrane.
Linked
to G-proteins like metabotropic neurotransmitter receptors,
except they
receive light!
Those Odd Rods
(& Cones)
They
don’t produce action potentials.
In
the dark they have open Na+ channels, constant depolarization
& transmitter
release!
Chemical
changes in response to light CLOSE Na+ channels, causing
hyperpolarization
& LESS release of transmitter (glutamate)!
Rods
signal bipolar cells by releasing less transmitter.
Cones
3
different types, absorbing different ranges of wavelengths
Types
of Color
“Blindness” or deficencies
How Do
We See
Color Under Unusual Lighting Conditions?
Retinex Theory
Cortex
uses input from various parts of the retina to determine
illumination conditions,
then generates color perceptions based on the relative
brightness of different
areas.
Visual
Fields
Each
half of your brain sees the opposite half of your visual world
The Richness
of Vision
The visual system
shows hierarchial organization, functional segregation, and
parallel processing
At
Least 2 Routes
Thru the
Primary Visual Pathway:
From
periphery of retina to the large ganglion & geniculate
cells (magnocellular)
- sensitive to location & depth and to movement
From
fovea of retina to the small (parvocellular) ganglion &
geniculate
cells - shape analysis
Also
a Mixed pathway for color& brightness
Many
Regions of
Cortex Involved in Visual Processing
Primary
visual cortex is just the first level of cortical processing
Secondary
“visual cortex” seems to have separate regions devoted to
color, location,
movement, shape, etc.
Parietal
& temporal association cortex also involved with higher
level processing.
The
Dorsal Stream
From
primary visual cortex dorsally thru prestriate cortex to
posterior parietal
lobe
Perceiving
and remembering/imagining the location & dimensions of
visual stimuli
This
allows visual/motor coordination whenever we interact with
objects, visual
search or scanning movements of eyes
Ventral
Stream
of Processing
From
primary visual cortex ventrally thru prestriate cortex to
inferior temporal
lobe
Recognizing,
describing
what objects or patterns are
Color
processing including remembering/imagining color of things
Object
perception- damage to inferior temporal cortex causes visual
agnosia
A
Middle Stream?
The
border between parietal, occipital and temporal lobes seems
necessary for
movement perception.
This
area receives input not only from visual cortex but also from
superior
colliculus and pulvinar of thalamus.
Visual
Agnosia
(not recognizing)
Because
different visual attributes seem to be processed in several
different brain
areas, different areas of brain damage lead to different kinds
of visual
agnosia (object agnosia, color agnosia, movement agnosia)
Prosopagnosia-
can’t
recognize individual faces (or similar members of other
complex classes
of visual stimuli)
One
Goal of visual system research:Visual
Prosthetic Devices