Term
| What portion of the retina is responsible for low light situations? High light situations? |
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Definition
Low light- rods
High light- cones |
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Term
| What are the different areas the retina projects onto and what are their individual purposes? |
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Definition
-Hypothalamus- allows for synchronization of our internal clock
-Superior colliculi- provides input for visually targeted eye movements
-Sensory input for pupillary reflex
-Lateral geniculate and striate cortex for detailed analysis of form, motion, and color |
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Term
| What layers of the retina must light travel through before reaching the retina? |
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Definition
-Retinal Ganglion cells
-Inner plexiform layer
-Inner nuclear layer
-outer plexiform layer
-Outer nuclear layer
-Lastly reaches the photoreceptor outer segments |
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Term
| What forms the optic nerve? |
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Definition
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Term
| Where is the first synaptic zone and what cells does it connect? |
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Definition
| The synaptic zone is fount in the inner plexiform layer and it contains contacts between bipolar cells, amacrine cells, and retinal ganglion cells |
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Term
| What does the inner nuclear layer contain? |
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Definition
| Cell bodies of the amacrine cells, bipolar cells,and horizontal cells |
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Term
| Other than the inner plexiform layer, what is another synaptic layer? |
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Definition
| The outer plexiform layer which contains contacts between bipolar cells, horizontal cells and the photoreceptor. |
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Term
| Where are the cell bodies of the photoreceptors located? |
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Definition
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Term
| Where does phototransduction take place? |
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Definition
| On the photoreceptor outer segments. |
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Term
| What advantage is there in having the photoreceptors embedded in a pigmented epithelium? |
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Definition
-it absorbs light that might make it down through the retina and prevents it from scattering within the eye and causing blurring
-biochemical and metabolic roles including renewing photopigments and phagocytosing photoreceptor disks |
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Term
| What cells make up the vertical pathway? |
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Definition
| Photoreceptors, bipolar cells, and retinal ganglion cells. |
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Term
| What is the neurotranmitter that cells of the vertical pathway use? |
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Definition
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Term
| What cells within the vertical pathway fire action potentials? |
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Definition
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Term
| What is the job of photoreceptors and bipolar cells? |
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Definition
| Photoreceptors and bipolar cells generate graded potentials only (receptor potentials and synaptic potentials) with neurotransmitter being released in proportion to membrane depolarization. |
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Term
| What cells make up the lateral pathway? |
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Definition
| Horizontal cells and amacrine cells |
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Term
| What is the function of horizontal cells and what is the nuerotransmitter that allows it to do this? |
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Definition
| Horizontal cells mediate lateral inhibition between photoreceptors which is important for circular, center-surround, antagonistic receptive fields of the bipolar and ganglion cells. The neurotransmitter for humans is unknown though in some primates it is GABA. |
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Term
| What is the function of amacrine cells? |
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Definition
| Amacrine cells exact function in the lateral pathway is unknown. They have an influence on the function of retinal ganglion cells and their horizontal orientation suggests a role in shaping the circular receptive field of ganglion cells. |
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Term
| What part of the retina contains a higher ratio of rods to cones and what is the result of this? |
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Definition
| There are a greater number of rods in the peripheral retina which makes it more sensitive to low light. |
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Term
| Describe the physical characteristics of the fovea. What advantages do these provide? |
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Definition
The fovea has a lower ratio of photoreceptors to ganglion cells, making it better at resolving fine detail
The retina is also thinner at the fovea because the nonphotoreceptor cells are laterally displaced. This decreases the scattering of light resulting in greater visual acuity. |
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Term
| What is the result of stimulating a photoreceptor with a flash of light? |
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Definition
| The photoreceptor hyperpolarizes with the degree of hyperpolarization being proportional to the brightness of light. |
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Term
| What is unique about the resting potential of photoreceptors and how is this maintained? |
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Definition
| The resting potential is relatively depolarized. This resting potential is maintained CNG channels that are kept open. The outer segment of the photoreceptor contains many cyclic nucleotide gated ion channels (CNG) and it also synthesizes a high level of cGMP. This means there is a continuous inward current in the dark (dark current). |
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Term
| Do photoreceptors fire an action potential? If not, how do they signal? |
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Definition
| Photoreceptors do not fire an action potential, instead its short axon like processes have presynaptic terminals that contain voltage gated calcium channels that open during depolarization allowing Ca in and exocytosis of glutamate. |
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Term
| Does glutamate release increase or decrease upon light stimulation? |
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Definition
| Decrease in proportion to the degree of hyperpolarization. |
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Term
| Why does light decrease the amount of glutamate released? |
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Definition
| When light strikes a photoreceptor phosphodiesterase is stimulated. PDE degrades cGMP which results in the closing of CNG channels and this causes the cell to hyperpolarize. |
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Term
| Trace the path of phototransduction after light strikes the photoreceptor outer segments. |
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Definition
| Light travels from the photreceptor outer segments where it is transduced into neural activity this signal is sent to the outer nuclear layer, the outer plexiform layer, the inner nuclear layer, the inner plexiform layer and finally to the ganglion cell layer. The axons of the ganglion come together to form the optic nerve. Both optic nerves cross over at the optic chiasm and the continue as the optic tract. |
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Term
| T/F All neurons of the vertical pathway release excitatory glutamate. |
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Definition
| False. All neurons of the vertical pathway release glutamate but it may be excitatory or inhibitory. |
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Term
| When is phototransduction initiated? |
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Definition
| When a photon is absorbed by a photopigment molecule contained in one of the disks in the rod outer segment. |
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Term
| What is the photopigment composed of and what do these subunits do? |
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Definition
1)Retinal-a vitamin A aldehyde-light absorbing chromophore
2) Opsin- tunes retinal's absorbance to a particular range in the light spectrum. |
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Term
| What type of receptor does opsin resemble in structure and function? |
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Definition
| Metabotropic neurotransmitter receptor. |
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Term
| Describe, in detail, phototransduction in rods. |
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Definition
| Light strikes retinal causing it to change confirmation from 11-cis retinal to all-tran-retinal. This causes a confirmational change in opsin which then activate transducin (a g protein). Transducin stimulates PDE which breaks down cGMP causing the CNG channels to close. This prevents Na and Ca (dark current) from flowing into the cell and the cell hyperpolarizes, releasing less NT. |
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Term
| Why is it important that the chain reaction that a light photon starts have so many steps? |
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Definition
| The cascade allows for signal amplification. |
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Term
| How many mV of change can 1 photon of light induce in a photoreceptor? |
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Definition
| 1 photon of light can close 200 CNG channels causing the photoreceptors to hyperpolarize 1 mV. |
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Term
| What is the absorbance of light by retinal called? Why? |
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Definition
| Bleaching, b/c retinal changes color from purple to yellow with its confirmational change. |
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Term
| What happens to retinal once it changes confirmation? |
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Definition
| Once retinal changes from 11-cis to all-tran it dissociates from opsin and diffuses into the cytosol of the outer segment. It is then transporter to the pigmented epithelium where it is converted back to 11-cis-retinal and then transported back to the outer segment to recombine with opsin. |
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Term
| When are photoreceptors most sensitive to light? |
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Definition
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Term
| What regulates light adaptation? |
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Definition
| Calcium concentration--light induced closure of the CNG channels decreases calcium concentration lowering calcium concentration and decreasing light sensitivity. |
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Term
| What is an advantage of light adaptation? |
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Definition
| The decrease in sensitivity of photoreceptors allows them to signal a change in light level over a greater range. |
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Term
| Is retina output relative or absolute for light intensity? |
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Definition
| Relative, due to light adaptation. |
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Term
| What does the pupillary light reflex do? |
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Definition
| Adjusts the amount of light allowed into the eye for stimulation of the retina. |
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Term
| What are the differences between phototransduction in cones and rods? |
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Definition
-Unlike rods, cones have three different types of opsin that allow for spectral sensitivity.
-Cones are less sensitive to light sensitivity than rods. Cones require 100 photons of light while rods only require one. |
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Term
| What are the different cone opsins and what wavelengths are they maximally sensitive to? |
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Definition
Red- long
Green-medium
blue-short |
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Term
| Can the wavelength of light be determined by the response of a single cone? |
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Definition
| No, cone opsins have a great deal of overlap and the only way to determine the wavelength of light is to compare the output of different classes of cones. |
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Term
| Define dyschromatopsia. What causes it? |
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Definition
| Dyschromatopsia is color blindness. It is an inherited sex linked (X) disorder that results from an alteration of one of the genes coding for red or green cone photopigments. |
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Term
| Red or green color deficiencies are often associated with males, while blue color deficiencies affect males and females equally why? |
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Definition
| Red/green is a result of an altered gene on the X chromosome, while blue color deficiencies are a result of the 7th chromosome. |
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Term
| What is the inability to distinguish different colors known as? |
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Definition
Rod monochromacy-lack of cones
Cone monochromacy- only a single cone |
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Term
| Are rods or cones activated under low light conditions? What is this called? |
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Definition
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Term
| Are rods or cones activated at high levels of light? What is this called? |
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Definition
|
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Term
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Definition
| When both rods and cones are activated under intermediate illumination. |
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Term
| Does scotopic vision have high or low acuity? |
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Definition
|
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Term
| Where does the first stage of informational processing of the visual system take place? |
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Definition
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Term
| What does the outer plexiform layer contain? |
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Definition
| Synaptic connections between the photoreceptors, horizontal cells, and bipolar cells. |
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Term
| What are the two classes of bipolar cells? |
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Definition
On-center bipolar cells
Off-center bipolar cells. |
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Term
| What is the effect on on-center bipolar cells when the center of the receptive field is exposed to light. |
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Definition
| The photoreceptors would be hyperpolarized by the light causing them to decrease the amount of glutamate they release. The decrease in glutamate causes the cation channels of on center bipolar cells to open causing depolarization. In summary on center bipolar cells are excited (disinhibited when light is on the center of their receptive field) |
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Term
| Does glutamate release from photoreceptors inhibit or stimulate the opening of cation channels in on-center bipolar cells? |
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Definition
| Glutamate release inhibits the opening of cation channels in on center bipolar cells |
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Term
| What type of glutamate receptors do off center bipolar cells have? |
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Definition
| Off center bipolar cells of ionotropic glutamate receptors. |
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Term
| What is the effect, on off center bipolar cells, of exposing the center of the receptive field to light? |
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Definition
| Light stimulation causes the photoreceptor cell to hyperpolarize and release less glutamate. The decrease in gluatamate causes off-center bipolar cells to close their ionotropic glutamate receptors resulting in hyperpolarization. |
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Term
| Does glutamate release inhibit or allow for the opening of ionotopic glutamate receptors in off center bipolar cells? |
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Definition
| Glutatamate allows ionotropic glutamate receptors to open. |
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Term
| Why are the center and surround regions antagonistic? |
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Definition
| Because they are connected by an inhibitory interneuron, a horizontal cell. |
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Term
| Does glutamate release cause horizontal cells to hyperpolarize or depolarize? Why? |
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Definition
| Glutamate release causes horizontal cells to depolarize, b/c they contain conventional glutamate receptors. |
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Term
| What do horizontal cells release? |
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Definition
| Inhibitory GABA onto laterally adjacent photoreceptors |
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Term
| If a surround photoreceptor is stimulated with light what effect does this have on the center photoreceptor? |
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Definition
| If a surround photoreceptor is stimulated then it will hyperpolarize and release less glutamate. This decrease in glutamate will cause the horizontal cell to hyperpolarize and release less GABA onto the adjacent center photoreceptor, so the center photoreceptor will be less inhibited and release more glutamate. |
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Term
| If light stimulates the surround receptive field what effect does this have on on-center and off center bipolar cells? |
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Definition
| If the surround receptive field is stimulated then the photoreceptor hyperpolarizes and releases less glutamate. The horizontal cell then releases less GABA. This decreases the inhibition on the center photoreceptor causing it to release more glutamate. The increase in glutamate causes the on-center bipolar cell to hyperpolarize. Off center bipolar cells are depolarized by the increase in glutamate. |
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Term
| Off center bipolar cells are ______ when the center of the receptive field is stimulated and are _____ when the surround of the receptive field is stimulated. |
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Definition
| Hyperpolarized; depolarized |
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Term
| What is a difference between bipolar cells and ganglion cells? |
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Definition
| Ganglion cells fire action potentials, bipolar cells have graded potentials. |
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Term
| T/F. Ganglion cells respond best to diffuse illumination? |
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Definition
| False. Ganglion cells respond best to large relative differences in light. |
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Term
| Can absolute intensity of light be used to determine the rate of action potential firing of ganglion cells? |
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Definition
| No, the ratio of the intensities is needed. The same firing rate can be obtained at varying levels of luminance if the brightness of the background is also changed. |
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Term
| T/F. A dim test spot against a slightly dimmer background is equally effective as a bright test spot against a slightly less bright background. |
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Definition
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Term
| What is the M and P-type classification based on? |
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Definition
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Term
| Which is more prevalent, M or P type ganglion cells? |
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Definition
|
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Term
| Which has a larger receptive field size, M or P type ganglion cells? |
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Definition
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Term
| Which ganglion type has a faster conduction velocity M or P type? |
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Definition
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Term
| Which ganglion type is sensitive to wavelength? |
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Definition
| P type and some non-M-non-P type |
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Term
| What is the functional role of M type ganglion cells? P-type? |
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Definition
M-type- movement
P-type precision-detect form and fine detail |
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Term
| What are wavelength sensitive cells called? Why is this? |
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Definition
| Color opponent cells. B/c the center and surround areas of the receptive fields are sensitive to different wavelengths of light |
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Term
| Loss of which cone opsin gene results in an inability to distinguish between between red, yellow, and green colors. |
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Definition
| Alteration of genes coding for red or green cone photopigments results in an inability to distinguish between red, yellow, or green. |
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