Term
| Explain the basic structure of the retinotopic map on the eye. |
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Definition
- Fovea in back of eye contains greatest density of photoreceptors and is responsible for inversion and left-right switching of actual image projection.
- Vertical and horizontal meridians through fovea create superior and inferior nasal and temporal quadrants (remember, image at superior temporal would actually represent inferior nasal). |
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Term
| What is the information processing path from fovia to cortex? |
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Definition
1) Optic axons exit at disk at back of eye and travel in the optic nerve to the optic chiasm, where they undergo a partial decussation.
2) Fibers from nasal hemiretina cross to opposite side, whereas fibers from temporal hemiretina remain on same side (all structures behind the optic chiasm recieve information ONLY from the CONTRALETAL visual hemifield)
3) This organization is preserved in LGN and visual cortex, but in primary visual cortex (Striate cortex), the hemifield is further mapped onto dorsal and bentral bands of calcarine sulcus.
4) Lingual gyrus of Calcarine sucus gets Superior quadrant and Cuneus gyrus gets Inferior Quadrant.
Ex) Left cuneus gyrus gets information about the inferior right side of the body. |
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Term
What are the clinical manifestations arising from the following lesions?
1) Right optic nerve
2) Optic chiasm
3) Optic tract
4) Fibers exiting LGN (Meyer's Loop)
5) Cuneus or lingual gyrus |
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Definition
1) total blindness of right eye
2) Bi-temporal Heteronymous Hemianopsia (since nasal hemiretinae map to temporal hemiretenae)
3) Contralateral homonymous hemianopsia
4) If in Meyer's Loop of superior visual field fibers, Contralateral homonymous superior quandrantanopsia
5) Inferior and Superior quadrantanopsia, respectively (may have foveal or macular sparing) |
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Term
| Explain the course of the superior and inferior visual field pathways running from the LGN to the visual cortex? |
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Definition
Superior Visual field= Lingual Gyrus
Inferior Visual field= Cuneus Gyrus (located superiorly)
1) Superior visual field information exits the LGN laterally and travels by Geniculocortical fibers in the white matter of the temporal lobe.
- Initially they course anteriorly, in Meyer's loop, and then turn caudally to synapse in the Lingual Gyrus.
2) Inferior visual field fibers exit the LGN vertically and dorsally, and travel back to the posoterior limb of the internal capsule and terminate in the Cuneus gyrus. |
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Term
| The are the major functions of photoreceptors and ganglion cells in the retina? |
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Definition
1) Photoreceptors transduce photic energy into neural signals (rods and cones at back of eye)
2) Ganglion cells convey this information to central visual structures via optic nerve |
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Term
Which artery, when compressed, is responsible for the Papilloedema seen under increased intracranial pressure?
What is the course of this artery? |
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Definition
- Central Retinal Artery, off of the opthalmic artery from the internal Carotid artery (supplies Retina)
- Travels within core of optic nerve, entering eye at optic disk and then fanning out to supply the retinal layers.
- Crossed by central retina vein |
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Term
| What is the difference between Rods and Cones in the retina? |
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Definition
1) Rods (Rhodopsin) are densest in Parafoveal regions
-They operate over a lower range of stimulus intensities and ambient light levels, involving more amplification then cones (good for night or dusk)
**This is why you can't see objects in your fovea at night, and have to shift your eyes to focus image on peripheral retina**
2) Cones (Opsins) are densest in the fovea.
- They are sensitive to a larger range of intensities, but require higher illumination for stimulation and are good for color (Rods are saturated at these illuminations).
- For cones, long-red/yellow, medium-yellow/green, short-blue/green wavelengths |
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Term
| How do Rods transduce light into neural signals? |
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Definition
In para-foveal and peripheral regions of the eye.
SHORT- Rhodopsin breakdown.... Retinal-mediated activation of PDE and cleavage of cGMP..... increased Na permeability and AP generation......signal leaves via ganglion cells.
1) Light absorbed by rhodopsin, causing conformational change in pigment that leads to breakdown to Opsin and Retinal.
2) Retinal activates phosphodiesterase (PDE) that hydrolyzes cGMP in the cell membrane, leading to change in membrane permeability to Na
3) If change in potential is significant enough to generate action potential, NT release occurs and signal is passed to ganglion cells, which transmit to central visual structures (sole output). |
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Term
| What is a receptive field? |
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Definition
Set of receptors within the photoreceptor layer of the retina that lead to a neural response in a particular cell.
Example is a ganglion cell receptive field which can be localized to the retinal surface. |
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Term
| Explain the course of the Retino-Genuculate pathway. |
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Definition
1) Light hits photoreceptors on retina and leaves eye via Ganglion cells, synapsing on the LGN of the thalamus (each LGN gets temporal hemiretina of ipsilateral eye and nasal hemiretina of contralateral eye)
- Even though information in each LGN is from both eyes, it is segregated at the single-cell level with each cell receiving info from one retinal receptive field (Monocular)
2) Axons from LGN sent to layer IV of area 17 (V1 or Striate cortex), re-establishing retinotopic map (Central fovea is at posterior pole of Calcarine cortex and more peripheral regions are mapped anteriorly)
- Cortical magnification of retinotopic map (Foveal info is over-represented) reflects density of ganglion cells across retina (similar to how density of mechanoreceptors is represented in S1)
3) High cortical magnification of foveal, or macular retina helps account for macular sparing in damage to Occipital lobe. |
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Term
| How is Binocular integration achieved in area 17 of the cortex? |
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Definition
Remember, each LGN cell processed information from a single hemifield of a single retina.
1) Afferents from LGN from right and left eyes segregate in layer 4 into Ocular Dominance Columns (1mm wide in humans)
2) Adjacent outputs from ODCs converge on neurons to prove binocular input and these "binocular neurons" are critical for processing binocular depth (Stereopsis) |
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Term
| How do cells in area 17 code for Contour orientation? |
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Definition
Elongated receptive fields increases sensitive to straight bars or edges that are oriented parallel to long axis of RF (Orientation selectivity)
- These cells are organized into orientation column systems that span vertical layers 1-6 of the cortex
-In retina and LGN, cells have circular RFs, but in area 17 they are elongated, reflecting convergence of LGN afferents whose RFs are arranged in a row across the retina. |
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Term
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Definition
| Total blindness in an hemitopic area of V1 affected by a lesion |
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Term
| Where does post-area 17 processing take place? |
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Definition
After going from the LGN to V1, information can pass on to extrastriate processing regions such as
These areas have larger receptive fields and require more complex stimuli (behaviorally relevant stimuli)
1) Anterior occipital cortex (area 18, 19)
2) Posterior temporal cortex
3) Posterior parietal cortex (area 7)
4) Frontal cortex (area 8, frontal eye fields. |
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Term
1) Which area of the brain is responsible for processing color?
2) Which area is processes complex, behaviorally relevant shapes?
3) What about spatial relationships among stimuli? |
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Definition
1) V4, at the occipitotemporal junction (little retinotopic organization because of large receptive fields)
2) Similar organization is seen in Inferior Temporal cortex (IT), where face and hand processing takes place (OBJECT, not location)
3) V5 in the posterior parietal cortex processes trajectory and directionality. |
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Term
A patient is unable to recognize and to name objects or persons presented visually, even though they can describe the objects rudimentary features.
What is going on? |
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Definition
Visual Agnosia from damage to the Posterior and Inferior temporal lobe (IT) near the occipito-temporal junction.
They can still recognize spatial relationships (V5-MT cells)
and perform actions that require hand-eye coordination. |
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Term
| When a patient is asked to draw a picture of a clock, he only draws the right half. What is going on? |
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Definition
Sensory Neglect of the left side, because of a large infarction to the posterior parietal lobe.
Smaller infarcts might lead to patient's misjudging an object's position while reaching for it. |
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Term
| Explain the difference between the so-called "what" pathways and "where pathway" of the visual system? |
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Definition
1) "What Pathway" involves feature extraction and object identifcation (Occipitotemporal stream)
2) "Where Pathway" involves locating an object in space (Occipitoparietal stream). |
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