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73 Cards in this Set
- Front
- Back
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Where is the first synaptic relay in teh pathway that serves visual perception?
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Occurs in a cell group of the dorsal tahlamus called the lateral geniculate nucleus, or LGN
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What are the highest energy wavelengths?
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Gamma and x-ray
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Which colors are higher in energy, purples or reds?
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Purple
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In a vacuum, a wave of electromagnetic radiation will travel in a straight line and thus can be described as a ____.
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Ray
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What is optics?
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The study of light rays and their interactions
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What is absorption?
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The transfer of light energy to a particle or surface
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What is refraction?
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The bending of light rays taht can occur when they travel from one trasnparent medium to another
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What is the white of the eye called?
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Sclera
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What are inserted into the sclera that move the eyeball in the orbit?
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Extraocular muscles
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What is the membrane called that folds back from the inside of the eyelids that attaches to the sclera?
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The conjunctiva
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What is the optic disk?
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Pale ocular region where retinal vessels originate from; where the ocular nerve fibers exist the retina
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Can the sensation of light occur at the optic disk? Why?
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No, because there are no photoreceptors; nor can it occur where the large blood vessels exist because the vessels cast shadows onto the retina. The brain fill sin our perception of these areas
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What is the macula?
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Place where the vessels originating at the optic disk arc to; also the trajectory of the optic nerve fibers from the nerve fibers en route to the optic disk
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What is the fovea?
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Dark spot about 2 mm in diameter where the retina is thiner
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What is the cornea nourished by?
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The aqueous humor
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What is located behind the iris?
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The lens
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What holds up the lens?
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Ciliar muscles are attached to zonule fibers, which suspend the lens
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Where is the vitreous humor?
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Between the lens and the retina; it serves as pressure to keep the eyeball spherical
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What holds the lens in place?
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The ciliary muslces are attached to zonule fibers, which suspend the lens
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What is the distance called from the refractivge surface to the point where parallel light rays converge?
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Focal edistance
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what does focus distance depend on?
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The curvature of the cornea
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What is the relationship between focus distance and curvature of the cornea?
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The tighter the curve, the shorter the focal distance
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What is diopter and how many diopters does the cornea harness?
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Diopter is the inverse of focal distance; the corena has a refractive power of about 42 diopters.
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What does refractive power depend on?
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The slowing of light at the air-cornea interface; if we replace air with a medium that passes light about the same speed as the eye, refracticve power of the cornea is elminated, which is why things look blurry under water
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What happens to refractive power when one approaches an obejct. How is this change accomplished?
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Light rays originating at a point can lno longer be consideered to be parallel. Rather, these rays diverse ,and greater refractive power is required to bring them into focus on the retina. This is provided by changing the shape of the lens, a process called accommodation.
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What happens during accommodation?
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The ciliary muscles contrac t and swell in sze, amkaing the area inside the muscle smlaler and decreasing the tension in the supersenroy ligaments. This rounding increases the curvature of the lens surface, thereby increasing their refractive power. Relaxation of the ciliary muscles increases the tension in teh suspensory ligaments, and the lens is streteched into a flatter shape, ithereby decreasing the refractive power.
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What is the pupillary light reflex?
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INvolves connections between teh retina and neurons in the brain stem that control the muscles that constrict the pupils. An interesting property of this reflex is that it is consensual; shining light into only one eye causes the constriction of the pupils of both eyes.
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What effect does constricting the pupil have on focus?
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Constricting the pupil increases the depth of focus, just like decreasing the aperture size on a camera lens, such that the image more closely approximates a point.
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Where is the left visual field imaged?
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On the right side of the retina (it is inverted)
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What is visual acuity?
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The ability of the eye to distinguish two nearby points.
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What does it mean to have 20/20 vision?
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You can rexcognize a letter that subtends an angle of 0.083 degrees
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What is the basic arrangement of cells in the pathway for visual processing?
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Photoreceptors to bipolar cells to ganglion cells.
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WHat are horizontal cells?
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Receitve input from teh photoreceptors and project neurites laterally to influence bipolar cells and surrounding photoreceptors
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What are amacrine cells?
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Receive input from bipolar cells and project laterally to influence surrounding ganglion cells, bipolar, and other amacrine cells
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What is the only sources of output from the retina?
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Ganglion cells
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What is one advantage of having an inside-out arrangement for the cells in the eye?
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The pigmented epithelium that lies below the photoreceptors plays a critical role in the maintenance of the photoreceptors and photopigments; absorbs any light that passes entirely through the retina, minimizing the reflection of light within the eye that would blur the image.
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What is the inner plexiform layer?
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Contains the synaptic contacts between bipolar cells, amacrine cells, and ganglion cells.
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What is the outer plexiform layer?
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Where photoreceptors make contact with the bipolar and horizontal cells
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Which contain more photopigment disks, rods or cones?
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Rods ~ rods are 100 times more sensitive to light
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Whyi s the peripheral retain more senstiive to light?
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There are more rods and there are more photoreceptors feeding information to each ganglion cell
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What is the region of hthe retina that is most highly specialized for high-resolution vision?
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The fovea
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Are there mor erods or cones in the fovea?
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All cones, no rods
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What is the membrane potential f the rod outer segment in complete darkness?
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About -30 mV; this depolarization is caused byt eh steady influex of Na+ through special channels in the outer segment membrane
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What is the drak current?
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the movement of positive charge across the membrane in the dark
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What causes the dark current?
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Sodium channels are stimulated to open by cGMP, which is continually produced iin the photoreceptor by the enzyme guanylyl cyclase, keeping the Na+ cahnnels open; (light reduces cGMP, causing the Na+ channels to close and the membrane potential becomes hyperpolarized
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Why does hyperpolarization occur in the membrane potential of retina in the presence of light?
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Absorption of electromagnetic radiation by rhodopsin; rhodopsin consists of a receptor protein called opsin and a prebound agonist called retinal; the absorption of light causes a change in the conformation of retinal so that it activates the opsin (bleaching)
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What is bleaching?
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Change in the conformation of retinal that activates opsin; this changes the wavelength absorbed by the rhodopsin; bleaching stimulates a G-protein called transducin to exchange GDP for GTP and activate the effector enzyme, phosphodiesterase (PDE), which breaks down the cGMP that is normally present in the cytoplasm of the rod; reduction of cGMP causes the Na+ channels to close & membrane to hyperpolarize
G-protein (transducin) --- effector enzyme (PDE) --- breaks down cGMP |
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How does signal amplification relate to our ability to detect a single photon?
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When a single photo is detected, many G-proteins are activated by each photopigment molecule, and each PDE enzyme breaks down more than one cGMP
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What happens in bright light?
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cGMP levels in rods fall to the point where the response to light becomes saturated; additional light causes no more hyperpolarization
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Why does vision during the day depend on cones?
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Rods are saturated; cones take more energy to become bleached (they have different opsins)
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What is the Young-Helmholtz trichromacy theory?
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The brain assigns color based on a comparison of the readout of hte three cone types; when all three types of cones are equally active, we perceive "white."
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Rods are most sensitive to what wavelenght?
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500 nm, perceived as blue-green
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What are the two views regarding the design of the automobile dashboard color?
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1. Light sshould be dime blue-green to take advantage of the spectral sensitivity of the rods
2. The lights should be bright red because this wavelength affects mainly cones, leaving hte rods unsaturated, resulting in better night vision |
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How long does it take to transition from the all-day cone vision to the all-rod nighttime vision?
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About 20-25 minutes.
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How much does sensitivity to light increase during dark adaption?
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A million-fold
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What are two changes that occur during dark adaption?
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1. Dilation of the pupils
2. Regeneration of unbleached rhodopsin and adjustment so that more rods become available to each ganglion cells |
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How long does light-adatation cocur?
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5-10 minutes; counters the changes that occur during dark adaption.
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How does gradual depolarization back to a membrane potential of about -35 mV occur?
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In the dark, Ca2+ has an inhibitory effect on guanylyl cyclase that synthesizes cGMP; when cGMP-gated channels close, the flow of Ca2+ into the photoreceptor is curtialed ~ more cGMP is s yntehsized because the synthetic enzyme is less inhibited, allowing the cGMP-gated channels to open again.
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What is the difference between OFF bipolar cells and ON bipolar cells?
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OFF bipolar cells have glutamate-gated cation channels thatmediate a depolarizing EPSP from teh influex of Na+; ON biopolar cells have G-protein-coupled receptors and resond to glutmaate by hyperpolarizing. Therefore, OFF depolarize when light is off and ON depolarize when light is on
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What neurotransmitter is released by photoreceptors?
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Glutamate
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What does the receptive field of a bipolar cell consist of?
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Receptive field center that provides direct photoreceptor input; receptir field surround that provides input via horizontal cells
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What visual angle is subtended by one millimeter on the retina?
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3.5 degrees
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If a cell is depolarized by its receptive field center changing from light to dark (an OFF receptor), what will happen when the surround becomes dark
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It will hyperpolarize
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How do retinal ganglion cells respond when illumination includes both receptive field center & the receptive field surround?
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Not particularly responsive
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What does OFF/ON indicate about ganglion cells.
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Indicates whether light being turned ON or OFF in the center causes it to depolarize.
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The center-surround organization o the receptive field leads to a neural response that emphasizes _____?
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The contrast at light-dark edges.
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Rank these in order of activation, from least to most, in an OFF-center ganglion cell:
Center and surround both light Center light; surround partially dark Center dark; surround partially light Center and surround both dark |
Center light; surround partially dark
Center and surround both light Center and surround both dark Center dark; surround partially light |
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Are there more P cells or M cells?
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P cells
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What is the difference between M and P-type ganglion cells?
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M cells have larger receptive fields; conduct action potentials more radpily in the optic nerve, and are more sensitive to low-contrast stimuli; M cells respond to stimulation of their receptive field centers with a transient burst of action potentials , while P cells respond with a sustained discharge as long as the stimulus is on
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What type of ganglion cells are sensitive to differences in the wavelength of light?
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Color-opponent cells: Some P cells and nonM-nonP cells
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What are the two types of color-opponency?
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Red vs. Green; blue vs. yellow
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For a cell with a red ON center and a green OFF center, list the order of activity from least to most, regarding the following combinations:
Center and surround have no color Center is red; surround has no color Center and surround are red Center is red; surround is green |
Center and surround have no color
Center is red; surround is green Center and surround are red Center is red; surround has no color |
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What are the three comparisons ganglion cells make to send information to the brain?
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1. Light vs. Dark
2. Red vs. Green 3. Blue vs. Yellow |
