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Early Visual Processing


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What happens above the threshold?
the retina doesn't care about the number of photons (brightness) but is more intereted in contrast between two objects
What is positive contrast?
light from the object is brighter than the surroundings
What is negative contrast?
light from the object is dimmer than its surroundings
What do the optics of the eye exist for?
to focus light on the retina
T/F: The retina is a part of the CNS.
The retina is a ___ micron thick layer of nervous tissue with __ cell types and __ layers.
200 micron
5 types
10 layers
What is the retina?
a sensing organ that captures photons of light
__________ is important only at threshold.
What are the 5 cell types in the retina?
1.rods and cones
2.horizontal cells
3.bipolar cells
4.amacrine cells
5.ganglion cells
What type of cells are in the outermost layer of the retina? What is this layer called?
photoreceptors (rods and cones);
nuclear layer
How many layers does light have to pass through to get to the photoreceptors?
all of them (10)
What do photoreceptor cells synapse on 1st? 2nd?
bipolar cells in the outer plexiform layer;
ganglion cells
What do horizontal cells connect?
lateral connections between neighboring photreceptors
What do amacrine cells do?
make lateral connections between bipolar cell dendrites
What kind of photoreceptor system does the retina have?
dual (made of rods and cones)
Do rods and cones do different things?
no, they do the same thing under different conditions and they have different properties
How much does the dual system expand visual range?
10 orders of magnitude
What is the minimal pathway for a signal to leave the retina?
3 cells, 2 synapses;
photoreceptor --> bipolar cell --> ganglion cell
Where does a ganglion cell synapse?
lateral geniculate nucleus
What is the lateral circuit important for?
sensing contrast, both color contrast and light/dark contrast
What are the on/off pathways important for?
determining light intensity increases and decreases
How does the retina sense contrast (differences in edges)?
through the on/off pathway
T/F: There are two different pathways for determining light intensity increases and decreases.
How were rods and cones named?
for their shape
Rods contain a ____ concentration of what pigment?
high; rhodopsin
What is the pigment in cones? Do cones contain more or less pigment than rods?
cone opsin?;
about the same
What do the pigments in rods and cones do?
capture photons and allow them to undergo a conformational change which causes a signal to be sent
How do photoreceptors respond to light?
by hyperpolarizing
How do photoreceptors respond to dark?
by depolarizing (get excited)
Why are photoreceptors at the back of the retina?
to be near the blood supply of the choroid (behind the retina pigmented epithelium) so they have easy access to the O2
What is the optic nerve?
a collection of all the ganglion axons leaving the eye to make a connection to the brain
Are there photoreceptors on the optic nerve?
What does the fovea contain? What is it for?
a high density of cone receptors;
viewing of fine detail
Is damage in the fovea detrimental to vision?
yes (because it only covers 1/2 micrometer squared)
What percentage of photreceptors are rods?
T/F: Humans have a very tiny area of eye acuity.
How does the size of the cones change outside the fovea?
they become larger
How does the density of the rods change outside the fovea?
the density peaks just outside the fovea and levels off further in the periphery of the retina
Are rods sensitive? How many photons must fire for a rod to to generate a reliable electrical signal?
yes, highly;
How many rods synapse on a retinal bipolar cell?
What is the pathway when a rod is stimulated by a single photon of light?
light --> rod --> bipolar cell --> ganglion cell
Rods are have (good/poor) spatial localization?
Rods have (high/low) temporal resolution?
low (slow)
Cones have (high/low) temporal resolution?
high (fast)
A quickly flickering light in a dim room will appear as...
a steady light source
Cones have (high/low) sensitivity? Why?
low (lower than rods); it takes more than one photon to generate a signal in cones
What is responsible for color vision?
Do cones adapt quickly or slowly to changes in intensity of light (i.e. coming out of a dark movie theater)?
quickly (cones have a wide range of adaptation)
Explain the rod signaling cascade?
the ligand (photon of light) activates 11-cis-retinal which undergoes a conformational change to create an active rhodopsin enzyme -->
rhodopsin is coupled to transducin (a G protein) -->
transducin is activated to activated a phospohdieserase which degrades cGMP -->
the concentration of cGMP modulates the membrane potential
What happens when phosphodiesterase cleaves cGMP?
a membrane channel closes and prevents ions from entering and depolarizing the cell
What does dark cause? light?
dark: depolarization
light: hyperpolarization
Describe the protein rhodopsin
it is a 7 transmembrane domain G protein coupled receptor
How many transducin does rhodopsin activate before inactivation?
500 (high sensitivity, slow recovery)
How many transducin does opsin activate before inactivation?
50 (faster recovery but less sensitivity)
Photoreceptors fire _______ and ______ responses not _______.
graded; continuous; spikes
What is the NT of photoreceptors?
When is glutamate released by photoreceptors?
in the dark (light prevents release)
What causes photoreceptors to partially depolarize in the dark?
an influx of Na+ and Ca2+ ions
What do you get when you apply a dim light to a rod? A light 2x as bright? will this continue?
a small, slow response;
2x the signal;
no because increasing the brightness beyond the threshold only prevents the channels from reopening for longer
Do rods or cones generate a faster response?
cones (but more photons are required)
These receptors allow for the response to be over quicker but also allow the response to be enhanced with repeated stimulus.
Can both rods and cones be desensitized?
Are cones more sensitive to flickering light when there is bright or dim background lighting?
What is the Midget system?
highest visual acuity and fidelity of signals carrying that message requires a private line
What does convergent wiring do?
increases sensitivity
What does divergent wiring do?
increases contrast
What allows for the high fidelity and acuity of a visual signal?
each photoreceptor has a separate neuronal pathway to the CNS
A visual signal must transmit without ___________ in order to localize the origin of the signal.
What is convergent wiring? spatial resolution? sensitivity?
when several photoreceptors send signals to one photoreceptor;
What is divergent wiring?
when one photoreceptor signals to more than one bipolar cell
What are the three types of signaling pathways in vison?
1.private line
What is the Push-Pull system? Does it generate a larger or smaller signal?
Divergent wiring: when the light is off the cell depolarizes and stimulates a ganglion to tell it the light is off, when the light is on the cell hyperpolarizes and stimulates a different ganglion to signal that the light is on;
What are lateral pathways important for?
determining the edge of an object
What do feedback and feed forward synaptic interactions do?
add flexibility and complexity
What do spatial filters do?
lateral inhibition
What do temporal filters do?
directional selectivity
How does the lateral pathway allow for sharp edge detection?
one neuron inhibits its neighbor via the lateral pathways
What kind of wiring do On and Off pathways require?
What determines the response?
receptor type (not the NT)
In the On/Off system one of the bipolar cells has an _______ synapse. Which one?
the ON cell
What does an inverted synapse mean?
in the ON cell a secondary transmitter (cAMP) actually closes channels causing hyperpolarization
Which bipolar cell in the ON/OFF system responds to the NT by opening channels? What does this lead to?
the OFF cell;
What happens with light stimulation?
the photoreceptor hyperpolarizes and stops releasing NT
Why are they called midget bipolar cells?
because of their small dendritic arbor
What happens in the off pathway?
-light hyperpolarizes photoreceptor so less glutamate is released
-the off bipolar cell hyperpolarizes
-bipolar cell stops releasing NT so ganglion cell hyperpolarizes
-decrease in spike frequency
What happens in the On pathway?
the ON cell has an inverting synapse
Which system (on/off) has an inverting synapse?
On pathway
What kind of synapse does the amacrine cell have? What is it for?
inverted synapse;
to cancel out the stimulus from the bipolar cell
How do center and surround receptive fields work?
they involve a midget ganglion cell that receives a signal from one central bipolar cell; bipolar cells around the central bipolar cell stimulate amacrine cells which feed onto the ganglion cell in an inverted fashion
What happens if the bipolar cells around the central bipolar cell are also stimulated by light?
the signal to the ganglion would decrease
What does surround inhibition do to the spike frequency of the ganglion cell?
decreases it
How many different types of bipolar cells are there? How are they identified?
11; by morphology and if they respond to On or Off signals
How many types of amacrine cells are there?
more than 30
What determines if a bipolar cell is an On or Off cell?
where the bipolar cell dendrite resides in the sublamina
Where do On responses reside?
outermost ring of sublamina
Where do Off responses reside?
innermost ring of sublamina
What identifies Midget ganglion cells?
their small dendritic arbors
The bistratified ganglion cell responds when ___ light goes on or ____ light goes off.
T/F: Rods have specific ganglion
F, they have to piggyback their signals through the cone system using A II amacrine cells
What is it called when rods and cones are both active?
mesoptic lighting conditions
T/F: All photoreceptors have their own ganglia.
F, there is not enough space in the retina
Rod system has a high degree of ___________ summation and uses the _____ ganglion cell to transmit signals to the brain.
The rod photoreceptor stimulates the ___________ which transfers the signal to the ___________ which stimulates the _______.
rod bipolar cell;
A2 Amacrine cell;
cone system
What do spikes encode?
brightness of light by temporal frequency
**T/F: Contrast is more important than uniform light.
**The retina has a _______ photoreceptor system.
**What is the minimal pathway to the brain?
photoreceptor --> bipolar cell --> ganglion cell
**What two types of cells are important in lateral inhibition?
horizontal and amacrine
**What increases the differential between light increases and decreases?
diverging pathways

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