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69 Cards in this Set
- Front
- Back
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image produced on retina
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inverted and reversed
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fovea
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central 1-2 degrees of visual space
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macula
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oval region that surrounds fovea and has relatively high visual acuity; central 5 degrees of visual space
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optic disc location
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15 degrees medial (nasal) to fovea; creates blind spot 15 degrees lateral and slightly inferior to central fixation point for each eye
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rods
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more numerous than cones 20:1; poor spatial and temporal resolution of visual stimuli; do not detect colors; main fxn in low-level light
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cones
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less numerous; more highly represented in fovea; high spatial and temporal resolution; detect colors
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outmost layer of retina (farthest from lens)
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photoreceptors; other layers in retina not represented in fovea preventing distortion here
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receptive field of a neuron in the visual pathway
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portion of visual field where light causes excitation or inhibition of cell
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what do photorecepors form excitatory or inhibitory synapses onto
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bipolar cells, which synapse onto ganglion cells, which send neurons into optic nerve
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how do photoreceptors and bipolar cells convey information
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passive electrical conduction via 'nontraditional' synapses that release neurotransmitter in a graded fashion that depends on membrane potential
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horizontal and amacrine cells in retina
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lateral inhibitory or excitatory connections with nearby bipolar and ganglion cells
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On-center cells
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excited by light in center of the receptive field and inhibited by light in surrounding area
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Off-center cells
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inhibited by light in center and excited by light in surrounding area
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M cells of retinal ganglion cells
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large receptive fields and respond best to gross stimulus features or movement; large-diameter fibers and porject to magnocellular layers of lateral geniculate nucleus of thalamus
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P cells of retinal ganglion cells
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small receptive areas, more numerous, and are sensitive to fine visual detail and to colors; small-diameter and project to parvocellular layers of lateral geniculate
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right optic tract carrier info from
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right hemiretinas (medial retinal fibers for each eye cross at optic chiasm)
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optic tracts go where
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from optic chiasm, wrap around midbrain laterally and reach the lateral geniculate nucleus (LGN) of the thalamus
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where do neurons project from the LGN
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primary visual cortex
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where else do a minority of optic tract travel
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bypass LGN and enter brachium of superior colliculus; form extrageniculate visual pathways that project to pretectal area and superior colliculus
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pretectal area is important in
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pupillary light reflex and projects to parasympathetic nuclei controlling pupils
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what are superior colliculus and pretectal area important for
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directing visual attention and eye movements toward visual stimuli (retino-tecto-pulvinar-extrastriate cortex pathway of visual attention and orientation)
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6 layers of LGN from ventral to dorsal
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1&2) Magnocellular layers = relay info from M cells of retina 3-6) parvocellular layers = relay info from P cells
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intralaminar neurons are important for
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relaying info about color
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on/off cells
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detect changes and fire transiently to both on and off stimuli
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Meyer's loop
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fibers of the inferior optic radiations arc formard into the temporal lobe; carries info from inferior retina or superior visual field
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what can temporal lobe lesions cause in vision
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contralateral homonymous superior quadrantanopia (pie in the sky) due to Meyer loop disturbance
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where do upper optic radiations pass under
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parietal lobe; lesion can cause contralateral homonymous inferior quadrantanopia (pie on the floor)
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where is the primary visual cortex
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on banks of the calcarine fissure in the occipital lobe; upper portions project to superior bank and inferior optic radiations to lower bank
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upper bank lesions of calcaine fissure cause
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contralateral inferior quadrant defects
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how is the primary visual cortex organized
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retinotopically; fovea near occipital pole and more peripheral regions of ipsilateral retinas and contralateral visual fields more anteriorly along calcarine fissure
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where is most input to primary cisual cortex
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cortical layer 4=relatively thick and subdivided into sublaminae 4A, 4B, 4Calpha, and 4Cbeta
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layer 4B
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contains numerous myelinated axon collaterals resulting in pale-appearing stria of Gennaru
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3 best characterized channels of processing in visual system
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analyzing motion, form, and color
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layer 4Calpha receives info from
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magnocellular layers of LGN conveying info about movement and gross spatial features
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layer 4Cbeta receives info from
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parvocellular layers of LGN carrying spatial info
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cortical layers 2 and 3 (called blobs) receive info from
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parvocellular layers and interlaminar zones with color info
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area 17
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primary visual cortex
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what projects from area 17
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neurons to extrastriate regions of visual association cortex and other regions of parieto-occipital and occipitotemporal cortex
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dorsal pathways of primary/secondary visual cortex project to
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parieto-occipital association cortex; answer 'where?' by analyzing motion and spatial relationships
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ventral pathways of primary/secondary visual cortex project to
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occipitotemporal association cortex; answer 'what?' by analyzing form with specific regions identifying colors, faces, letters, and other visual stimuli
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ocular dominance columns
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contralateral and ipsilateral eye inputs terminate in alternating bands of cortex ~ 1mm wide
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simple cells
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respond to lines or edges that occur at a specific location and with a specific angular orientation within their receptive field
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complex cells
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activity from simple cells summates to generate receptive field properties; respond to lines or edges that occur at any location in their receptive field with a specific angular orientation
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orientation columns
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vertical columns of uniform orientation selectivity
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hypercolumns
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functional units consisting of ocular dominance and orientation columns intersecting
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visual field defects do not typically affect
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visual acuity
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negative phenomenon of visual changes
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scotoma, homonymous visual field defect
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lesions causing blind spot interpreted similar to the physiologic bind spot are generally caused by
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lesion of central visual pathways
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lesions causing black, dark brown, oe purplish scotomas are most often caused by
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retinal lesions
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simple visual phenomena examples and cause
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lights, colors, geometric shapes; caused by disturbances anywhere from eye to primary visual cortex
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important causes of positive phenomena
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light flashes in retinal detachment; rainbow-colored halos around objects in acute glaucoma
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fortification scotoma
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scintillating appearance or consist of jagged alternating light and dark zigzag lines; migraine phenomena
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what should be suspected when pusating colored lights or moving geometric shapes experienced
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occipital seizure
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what do formed visual hallucinations arise from and causes
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inferior temporo-occipital visual association cortex; toxic or metabolic distubances, focal seizuress, complex migraine, neurodegenerative conditions, narcolepsy, midbrain ischemia, psychiatric disorders
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release phenomenon
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deprivation in part or all of visual field caused by either ocular or CNS may occasionally see objects/people/anomals in region of vision loss, especially in early stages of vision deficit
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normal visual field
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~60 degrees nasally and superiorly, and slightly further inferiorly and temporally
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Goldmann perimetry
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small lights of differnet sizes and intensities displayed on a screen in front of patient
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scotomas of retina lesions
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monocular with location, size, and shape depending on location and extent of lesion
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retrochiasmal lesions generally cause
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homonymous
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macular sparing
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occurs in partial lesions of visual pathways due to large representation of fovea; may be due to colateral blood flow in occipital pole by MCA or PCA
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blood supply of retina
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branches of opthalmic artery
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amaurosis fugax
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transient ischemic attack (TIA) of retina; browning out or loss of vision in one eye for ~ 10 minutes
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blood supply of primary visual cortex
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PCA
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what is bilateral altitudinal scotoma strongly suggestive of
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vertebrobasilar insufficiency causing bilateral infarcts or TIAs
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blood supply of inferior occipitotemporal association cortex (what?)
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PCA
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lateral parieto-occipital association cortex (where?)
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MCA-PCA watershed territory
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Optic neuritis
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inflammatory demyelinating disorder of optic nerve; related to MS (50% dvlp); mean onset in 30s
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usual clinical features of optic neuritis
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eye pain, especially with movement and monocular central scotoma, decreased visual acuity, and impaired color vision
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visual evoked potential test
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shown shifting checkerboard pattern which elicits a voltage waveform that can be detected over occipital cortex via electrodes on scalp; normal latency 115 ms
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