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109 Cards in this Set
- Front
- Back
- 3rd side (hint)
What happens above the threshold?
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the retina doesn't care about the number of photons (brightness) but is more intereted in contrast between two objects
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What is positive contrast?
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light from the object is brighter than the surroundings
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What is negative contrast?
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light from the object is dimmer than its surroundings
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What do the optics of the eye exist for?
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to focus light on the retina
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T/F: The retina is a part of the CNS.
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true
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The retina is a ___ micron thick layer of nervous tissue with __ cell types and __ layers.
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200 micron
5 types 10 layers |
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What is the retina?
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a sensing organ that captures photons of light
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__________ is important only at threshold.
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Brightness
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What are the 5 cell types in the retina?
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1.rods and cones
2.horizontal cells 3.bipolar cells 4.amacrine cells 5.ganglion cells |
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What type of cells are in the outermost layer of the retina? What is this layer called?
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photoreceptors (rods and cones);
nuclear layer |
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How many layers does light have to pass through to get to the photoreceptors?
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all of them (10)
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What do photoreceptor cells synapse on 1st? 2nd?
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bipolar cells in the outer plexiform layer;
ganglion cells |
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What do horizontal cells connect?
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lateral connections between neighboring photreceptors
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What do amacrine cells do?
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make lateral connections between bipolar cell dendrites
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What kind of photoreceptor system does the retina have?
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dual (made of rods and cones)
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Do rods and cones do different things?
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no, they do the same thing under different conditions and they have different properties
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How much does the dual system expand visual range?
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10 orders of magnitude
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What is the minimal pathway for a signal to leave the retina?
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3 cells, 2 synapses;
photoreceptor --> bipolar cell --> ganglion cell |
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Where does a ganglion cell synapse?
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lateral geniculate nucleus
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What is the lateral circuit important for?
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sensing contrast, both color contrast and light/dark contrast
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What are the on/off pathways important for?
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determining light intensity increases and decreases
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How does the retina sense contrast (differences in edges)?
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through the on/off pathway
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T/F: There are two different pathways for determining light intensity increases and decreases.
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True
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How were rods and cones named?
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for their shape
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Rods contain a ____ concentration of what pigment?
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high; rhodopsin
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What is the pigment in cones? Do cones contain more or less pigment than rods?
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cone opsin?;
about the same |
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What do the pigments in rods and cones do?
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capture photons and allow them to undergo a conformational change which causes a signal to be sent
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How do photoreceptors respond to light?
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by hyperpolarizing
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How do photoreceptors respond to dark?
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by depolarizing (get excited)
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Why are photoreceptors at the back of the retina?
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to be near the blood supply of the choroid (behind the retina pigmented epithelium) so they have easy access to the O2
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What is the optic nerve?
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a collection of all the ganglion axons leaving the eye to make a connection to the brain
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Are there photoreceptors on the optic nerve?
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no
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What does the fovea contain? What is it for?
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a high density of cone receptors;
viewing of fine detail |
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Is damage in the fovea detrimental to vision?
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yes (because it only covers 1/2 micrometer squared)
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What percentage of photreceptors are rods?
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95%
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T/F: Humans have a very tiny area of eye acuity.
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True
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How does the size of the cones change outside the fovea?
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they become larger
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How does the density of the rods change outside the fovea?
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the density peaks just outside the fovea and levels off further in the periphery of the retina
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Are rods sensitive? How many photons must fire for a rod to to generate a reliable electrical signal?
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yes, highly;
one |
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How many rods synapse on a retinal bipolar cell?
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1000s
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What is the pathway when a rod is stimulated by a single photon of light?
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light --> rod --> bipolar cell --> ganglion cell
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Rods are have (good/poor) spatial localization?
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poor
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Rods have (high/low) temporal resolution?
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low (slow)
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Cones have (high/low) temporal resolution?
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high (fast)
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A quickly flickering light in a dim room will appear as...
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a steady light source
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Cones have (high/low) sensitivity? Why?
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low (lower than rods); it takes more than one photon to generate a signal in cones
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What is responsible for color vision?
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cones
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Do cones adapt quickly or slowly to changes in intensity of light (i.e. coming out of a dark movie theater)?
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quickly (cones have a wide range of adaptation)
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Explain the rod signaling cascade?
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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 |
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What happens when phosphodiesterase cleaves cGMP?
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a membrane channel closes and prevents ions from entering and depolarizing the cell
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What does dark cause? light?
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dark: depolarization
light: hyperpolarization |
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Describe the protein rhodopsin
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it is a 7 transmembrane domain G protein coupled receptor
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How many transducin does rhodopsin activate before inactivation?
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500 (high sensitivity, slow recovery)
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How many transducin does opsin activate before inactivation?
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50 (faster recovery but less sensitivity)
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Photoreceptors fire _______ and ______ responses not _______.
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graded; continuous; spikes
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What is the NT of photoreceptors?
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glutamate
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When is glutamate released by photoreceptors?
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in the dark (light prevents release)
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What causes photoreceptors to partially depolarize in the dark?
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an influx of Na+ and Ca2+ ions
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What do you get when you apply a dim light to a rod? A light 2x as bright? will this continue?
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a small, slow response;
2x the signal; no because increasing the brightness beyond the threshold only prevents the channels from reopening for longer |
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Do rods or cones generate a faster response?
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cones (but more photons are required)
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These receptors allow for the response to be over quicker but also allow the response to be enhanced with repeated stimulus.
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cones
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Can both rods and cones be desensitized?
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yes
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Are cones more sensitive to flickering light when there is bright or dim background lighting?
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dim
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What is the Midget system?
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highest visual acuity and fidelity of signals carrying that message requires a private line
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What does convergent wiring do?
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increases sensitivity
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What does divergent wiring do?
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increases contrast
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What allows for the high fidelity and acuity of a visual signal?
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each photoreceptor has a separate neuronal pathway to the CNS
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A visual signal must transmit without ___________ in order to localize the origin of the signal.
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summation
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What is convergent wiring? spatial resolution? sensitivity?
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when several photoreceptors send signals to one photoreceptor;
low; sensitive |
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What is divergent wiring?
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when one photoreceptor signals to more than one bipolar cell
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What are the three types of signaling pathways in vison?
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1.private line
2.divergence 3.convergence |
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What is the Push-Pull system? Does it generate a larger or smaller signal?
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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;
larger |
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What are lateral pathways important for?
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determining the edge of an object
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What do feedback and feed forward synaptic interactions do?
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add flexibility and complexity
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What do spatial filters do?
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lateral inhibition
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What do temporal filters do?
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directional selectivity
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How does the lateral pathway allow for sharp edge detection?
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one neuron inhibits its neighbor via the lateral pathways
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What kind of wiring do On and Off pathways require?
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divergent
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What determines the response?
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receptor type (not the NT)
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In the On/Off system one of the bipolar cells has an _______ synapse. Which one?
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inverting;
the ON cell |
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What does an inverted synapse mean?
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in the ON cell a secondary transmitter (cAMP) actually closes channels causing hyperpolarization
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Which bipolar cell in the ON/OFF system responds to the NT by opening channels? What does this lead to?
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the OFF cell;
depolarization |
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What happens with light stimulation?
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the photoreceptor hyperpolarizes and stops releasing NT
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Why are they called midget bipolar cells?
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because of their small dendritic arbor
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What happens in the off pathway?
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-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 |
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What happens in the On pathway?
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the ON cell has an inverting synapse
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-light hyperpolarizes the photoreceptor so it stops releasing NT
-the bipolar cells depolarize and stimulate the ganglion cell -the ganglion cell depolarizes -spike frequency increases |
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Which system (on/off) has an inverting synapse?
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On pathway
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What kind of synapse does the amacrine cell have? What is it for?
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inverted synapse;
to cancel out the stimulus from the bipolar cell |
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How do center and surround receptive fields work?
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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
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What happens if the bipolar cells around the central bipolar cell are also stimulated by light?
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the signal to the ganglion would decrease
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What does surround inhibition do to the spike frequency of the ganglion cell?
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decreases it
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How many different types of bipolar cells are there? How are they identified?
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11; by morphology and if they respond to On or Off signals
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How many types of amacrine cells are there?
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more than 30
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What determines if a bipolar cell is an On or Off cell?
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where the bipolar cell dendrite resides in the sublamina
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Where do On responses reside?
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outermost ring of sublamina
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Where do Off responses reside?
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innermost ring of sublamina
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What identifies Midget ganglion cells?
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their small dendritic arbors
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The bistratified ganglion cell responds when ___ light goes on or ____ light goes off.
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blue;
yellow |
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T/F: Rods have specific ganglion
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F, they have to piggyback their signals through the cone system using A II amacrine cells
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What is it called when rods and cones are both active?
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mesoptic lighting conditions
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T/F: All photoreceptors have their own ganglia.
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F, there is not enough space in the retina
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Rod system has a high degree of ___________ summation and uses the _____ ganglion cell to transmit signals to the brain.
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spatial;
cone |
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The rod photoreceptor stimulates the ___________ which transfers the signal to the ___________ which stimulates the _______.
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rod bipolar cell;
A2 Amacrine cell; cone system |
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What do spikes encode?
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brightness of light by temporal frequency
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**T/F: Contrast is more important than uniform light.
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T
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**The retina has a _______ photoreceptor system.
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dual
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**What is the minimal pathway to the brain?
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photoreceptor --> bipolar cell --> ganglion cell
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**What two types of cells are important in lateral inhibition?
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horizontal and amacrine
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**What increases the differential between light increases and decreases?
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diverging pathways
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