activates visual receptors: light waves in the visible spectrum
Types of Electromagnetic
Iris & Black Pupil Hole
Rods vs Cones
sensitive (low threshold)
rods share same optic nerve fiber to brain
vision (scotopic vision)
million cones /eye
in center, especially in the fovea
bright light to reach threshold
1-to-1 lines to brain- good for detail vision or “acuity”
different types - provide color vision
of the Retina
Waves Into Electrical Messages:
& cones have molecules of light sensitive photopigments (11-cis-retinal+an
opsin) embedded in cell membrane.
to G-proteins like metabotropic neurotransmitter receptors, except they
Those Odd Rods
don’t produce action potentials.
the dark they have open Na+ channels, constant depolarization & transmitter
changes in response to light CLOSE Na+ channels, causing hyperpolarization
& LESS release of transmitter (glutamate)!
signal bipolar cells by releasing less transmitter.
different types, absorbing different ranges of wavelengths
Types of Color
“Blindness” or deficencies
How Do We See
Color Under Unusual Lighting Conditions?
uses input from various parts of the retina to determine illumination conditions,
then generates color perceptions based on the relative brightness of different
half of your brain sees the opposite half of your visual world
– that part of the visual field to which a particular cell in the visual
circular group of receptors feed into a single ganglion cell
are wired up such that the receptors in middle of circle have an opposite
effect on firing rate than those in edge.
receptive fields of other ganglion cells are the opposite (- in the middle,
+ on the edge)
Fields – bar shaped instead of circular, respond to lines or edges
– active cell inhibits neighboring cells. Lateral inhibition increases
the difference in neuron activity on the light and dark sides of a contour
or border. Makes sure our visual system responds to edges.
The visual system
shows hierarchial organization, functional segregation, and parallel processing
At Least 2 Routes
Primary Visual Pathway:
periphery of retina to the large ganglion & geniculate cells (magnocellular)
- sensitive to location & depth and to movement
fovea of retina to the small (parvocellular) ganglion & geniculate
cells - shape analysis
a Mixed pathway for color& brightness
Many Regions of
Cortex Involved in Visual Processing
visual cortex is just the first level of cortical processing
“visual cortex” seems to have separate regions devoted to color, location,
movement, shape, etc.
& temporal association cortex also involved with higher level processing.
The Dorsal Stream
primary visual cortex dorsally thru prestriate cortex to posterior parietal
and remembering/imagining the location & dimensions of visual stimuli
allows visual/motor coordination whenever we interact with objects, visual
search or scanning movements of eyes
primary visual cortex ventrally thru prestriate cortex to inferior temporal
describing what objects or patterns are
processing including remembering/imagining color of things
perception- damage to inferior temporal cortex causes visual agnosia
A Middle Stream?
border between parietal, occipital and temporal lobes seems necessary for
area receives input not only from visual cortex but also from superior
colliculus and pulvinar of thalamus.
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)
can’t recognize individual faces (or similar members of other complex classes
of visual stimuli)
Goal of visual system research:Visual