Visual System
Serial Processing—sensory information is processed in a sequence; at each stage some parts of the stimulus are analyzed and other parts are “passed through” and become lost/ignored as “background noise.”
-Stimulus (photons)  retina  more central processes such as thalamus and V-I
-Central Processes = Thalamus location:Lateral Geniculate Nucleus (LGN); and Primary Visual Cortex (V-I)
Parallel Processing—at each level of the serial chain, certain classes of neurons are specialized to process a certain aspect of the signal and ignore all other features. (Not every cell handles all information types) Different functions occurring at the same time
Example: a car assembly line where at one station a worker is attaching the steering wheel and another worker is attaching the breaks, then the car moves onto the next station, where a worker is attaching the
tie rods and another is attaching the wheels. (serial & parallel processing)
*Value of Parallel Processing—simultaneous analysis of color and shape information (or other info) at more than one retinal location.
Example: looking at a stoplight (paying attention to color change at the center of the visual field) while paying attention to a traffic monitoring cameral on the side of the visual field (using simultaneous processing of what is appearing at edge of field).
ORGANIZATION OF THE EYE
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Fovea—central area of retina; tightly packed small-sized cones; no BVs
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Macula—fovea + surrounding area; larger cones and contains rods; few BV’s
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Periphery of Retina—few cones, mostly rods, all cell types within layers get larger in size as you
 move out from macula  periphery
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Blind Spot—optic disk/optic nerve head; no rods/cones, only axons of ganglion cells and BV’s
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Retina extends more on the nasal side
Macula is on the temporal side of optic disk
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Thickness of the layers increases going toward central retina, and is greates right before reaching fovea (about 5deg)
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Each eye views more than half of the visual field, left and right eye visual fields overlap significantly
RODS & CONES
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Outer segment—visual pigment location
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Inner segment—metabolic machinery, nissl substance
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Rods—internal membrane “pancake” disks within by inner outersegment that are shed at surface (phagocytozed cells in epithelial layer) and new disks made in RER in segment replace them
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Cones—infoldings in plasma membrane (not disks) with attached pigment molecules; and they are not shed, but instead their pigment molecules are transported into/out of plasma membrane
PHOTOTRANSDUCTION
Human-sensitive enough to detect single photon

Light energy  Chemical Energy  Electrical Energy  Transmitter release
Rods—within the membranous disks is RHODOPSIN: 7TM protein of Opsin Family containing RETINAL
Retinal—vit A aldehyde; chromophore; absorbs certain wavelengths
Retinal Isomerization: Inactive Retinal (cis) + Light energy = Active Retinal (trans)
1. Rhodopsin absorbs light and becomes activated (retinal goes from cis  trans)
2. Retinal activates G-protein,
Transducin
3. Transducin actives cGMP
Phosphodiesterase (↓[cGMP])
4. Na+ channel closes (needs cGMP to stay open)
5. Rod member hyperpolarizes
Normal membrane mV ~-40mV (depolarized)  continuous synaptic release of GLUTAMATE
Presence of Light: cell membrane is hyperpolarized; Glutamate release decreases
Amplification present (retinal can interact w/multiple transducins; cGMP phosphodiesterase hydrolyzes thousands of cGMPs.
Cones have faster response than Rods.
Rods have greater sensitivity (can perceive a single photon)
RODS & CONES SPECTRA
S-Cones/Blue Cones : ~420nm
M-Cones/Green Cones : ~530 nm
L-Cones/Red Cones : ~560 nm
Rods: ~500nm; blue-green light
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Principle of Univariance—given wavelength of light is not uniquely absorbed by single pigment type
 overlap
 output pattern of all three cone types working together is what determines color
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Rods don’t participate in color—not bc of their absorbance-but bc their signals are not sent to the “cone output analyzers” that perceive color.
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All 3 types are intermixed on the retina
Color Blindness
All 3 types are needed to distinguish any one color; one cone type alone cannot distinguish color (like rods).
Cone pigments can be entirely missing, or have incorrect AA sequences from genetic mutation.

Trichromates: needs a combination of the 3 pure color (R, G, B) lights to match any visible color (needs all 3 cones)
Can have “anomalous color vision” if all 3 pigments are present but one has deviant AA sequences
Dichromates: have 2 functional types of photopigments
Monochromates: have 1 functional type of photopigments
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Rods maximize light sensitivity at the expense of high spatial resolution (fine detail vision)— opposite is true for cones.
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Cones can adapt to wide intensity range of ambient light (in very bright situations-midday sunny beach-cones can continue to function; Rods have much more limited range and cannot change their output further once the background light level is super high.
“ANALYZER” STAGES
Daylight (cone) & Nocturnal (rod) Pathways
On & Off pathways
Color-coded cone pathways
M and P pathways (start with ganglion cells)
ON and OFF Pathways—some bipolar cells depolarize when there is light (on) and others hyperpolarize under the same conditions (off).
Bipolar Cells—use graded potentials, not AP’s
Cone Bipolars—connect to 1 or more cones (of same type usually); emphasize high acuity daylight vision
Rod Bipolars—connect to rods only; emphasize high sensitivity to light (less acuity)

Central Retina-mostly cone bipolars (cones dominant in macula)
Peripheral Retina-mixture of rods and cones and mixture of pathways
Convergence—many rods go to one rod bipolar
Peripheral ganglion cells can receive mixed rod and cone input
Ex) Amacrine A-II cell needed for rod pathway but also has direct contact with cone bipolar
Low Light Levels  rod bipolar + A-II circuit
High Light Levels  cone bipolars dominates
(rods are inactivated at high light levels)
Receptive field increases going across the layers (photoreceptors  retinal ganglion cells) due to convergent inputs (receptive field of bipolar is aggregate of individual photoreceptor receptive fields that input into the bipolar)
Receptive field is proportional to the radial spread of dendrites fanning out from the cell body (# of synaptic connections)
Cones in fovea are super compact and make synapses to only a few cells at each layer, making receptive field small and preserving the extremely high resolution of individual cones (restricts the divergence)
ON AND OFF PATHWAYS-in detail
On-Pathways—activated by light, suppressed by dark
(depolarize with light)
Off-Pathways—activated by dark, suppressed by light
(hyperpolarize with light)
On and Off Bipolar cells keep the signals separate
Bipolar synapse to Retinal Ganglion Cells = always excitatory

Panel A: Center Response
-based on the type of Bipolar cell (on or off) that is contacting the Retinal
Gangion Cell
Panel B: horizontal cells activated from surround inputs suppress/cancel the photoreceptors direct response to illumination. (does so by causing greater Ca2+ influx into the cone terminals, making more glutamate released (inhibitory), causing bipolars to hyperpolarize reducing their excitatory signal to the on-center Retinal Ganglion Cells)
The image on the retina is degraded due to light scatter away from the image center. It is the job of neurons connected to the photoreceptors and even further
‘upstream’ to reduce the influence of light scatter and this is done through lateral interactions (horizontal cells) which suppress signals arising off target.
Thus the antagonistic interaction b/w center and surrounds suppresses the effects of off-axis light scattering in the retinal image.
-Improves contrast and enhances edges
Boundaries and Edges (when just the center or just the surround is lit) are strong signal activators and Uniform
Edges (all bright or all dark) are not.
Retinal Ganglion Cells
1.) On-Center or Off-Center
2.) M or P type
M (Magno)-Big
Larger Receptive Fields
Lack color sensitivity
Mixed rod & cone inputs (Day & Night)
Poor Resolution & Visual Acuity
Good for movement detection, fast flickering sensitivity, in periphery of visual field
P (Parvo)-Small
Small Receptive Fields
Color-Selective
Strongly cone-dominant (Daylight)
High Visual Acuity