vision III and IV - jullet Flashcards
Where do retinal ganglion cell axons project to in the brain? (4)
1) suprachiasmatic nucleus of the hypothalamus, 2) pretectal area, 3) superior colliculus, 4) lateral geniculate (THINK: PLSS)
What are the two types of layers found in the lateral geniculate
Parvocellular and Magnocellular
The lateral geniculates have 6 layers/laminae. How are they structured?
6 layers, 3 from each eye, in an ALTERNATING pattern. The 4 outermost layers are the PARVOcellular layers which receive input from P-type ganglion cells. The 2 inner layers are the MAGNOcellular layers which receive input from M-type ganglion cells
What are P-type ganglion cells?
constitute the outer 4 layers of the lateral geniculate. Have SMALLER receptive fields and is involved in COLOR vision and fine discrimination of shae.
What are M-type ganglion cells
constitute the inner 2 layers of the lateral geniculate. Have LARGER receptive fields and is involved in MOTION detection, DEPTH, and CONTRAST
What is the optic radiation?
Projections of axons from the lateral geniculate to the primary cortex
What are the two tracts of the optic radiation?
1) OVERLAND ROUTE: fibers of superior retinal quadrants (lower half of visual field) terminates in the visual cortex ABOVE the calcarine sulcus. 2) MEYERS LOOP: fibers of inferior retinal quadrants (lower half of visual field) terminates in the visual cortex BELOW the calcarine sulcus
What is the visual field of the OVERLAND ROUTE of the optic radation?
lower half of visual field
What is the visual field of the MEYERS LOOP of the optic radation?
upper half of visual field
Where is the primary visual cortex?
located on the medial surface of the occpital lobe, above and below the calcarine suclus. aka V1 or Area 17
Where does the 1˚ visual cortex project to?
Visual association areas: Depth (V2, V3, V3a), Color (V4), and Motion (MT/V5)
Where is the visual association cortexes located?
surrounds the 1˚ visual cortex - includes the rest of the occpital lobe along with portions of the temporal lobe
What is the laminar organization of the primary visual cortex?
6 layers of cells: 1-6
What two types of cells are present in the 1˚ visual cortex?
1) pyramidal cells (large cells with long spiny dendrites and axons projecting to other regions) 2) non-pyramidal cells (interneurons) that project to other regions of the visual cortex
How does visual information travel through the layers of the 1˚ visual cortex?
Layers: 4 -> 2/3 -> 5 -> 6
What are the input layers of the 1˚ visual cortex?
Layer 4
What are the output layers of the 1˚ visual cortex?
Layers 2/3, 4, 5, 6
A disproportionaltely large volume of the lateral geniculate and 1˚ visual cortex is devoted to information from this region of the retina. Why is that?
foveal region - photoreceptors here are densly packed in this region
How does the lens affect the projection of the image on the retina?
the image on the retina is INVERTED (top to bottom, left to right)
How does eph-ephrin signaling influence axon decussation at the optic chiasm?
ganglion cells in the temporal retina express Ephrin B1 and cells at the optic chaism express Ephrin B2. When Ephrin B1 interact with Ephrin B2, it generates a repulsive signal that prevents the axon from crossing the chiasm
The lateral geniculate receives input from:
both eyes
The lateral geniculate receives information about the visual field from:
contralateral visual field of both eyes
What is monocular blindness?
loss of vision/visual field in one eye
What is anopsia?
large visual field deficits
What is bitemporal hemianopsia?
loss of information from the nasal retina (temporal half of visual fields)
What is homonomous hemianopsia?
loss of vision in one half of the visual field (same half of the visual field in each eye)
Disease in one of the eyeballs (ocular hemorrhage, retinal detachment) or optic nerves (optic nerve tumor) results in this type of blindness:
monocular blindness
Compression of the optic chiasm (pituitary tumor, aneurysm of circle of willis) results in this type of blindness:
bitemporal hemianopsia
Damage of optic tract (post-optic chiasm) can result in this type of blindness
homonomous hemianopsia
What happens if you damage the meyers loop on both sides?
upper half of visual field is gone
What happens if you damage the meyers loop on the R side only?
upper L half quadrant of visual field is gone
What happens if you damage the meyers loop on the L side only?
upper R half quadrant of visual field is gone
What are the 3 categories of cells in the visual cortex?
1) simple cells, 2) complex cells, 3) hypercomplex cells
What are simple cells?
cells with rectangular receptive fields that increase activitiy to a LINE of illumination (may be due to the sum of inputs from several on-center ganglion cells in a row
What are complex cells?
cells with larger receptive fields with a PREFERRED orientation (may be due to combined input from a # of simple cells and are sensitive to the orientation and length of light)
What are hypercomplex cells?
cells that are sensitive to the orientation, length, and motion
What cell types converge as you go higher the visual system?
photoreceptor cells -> ganglion cells -> simple cells -> complex/hypercomplex cells
What are ocular dominance columns?
stripes of neurons in the visual cortex that respond preferentially to input from one eye or the other. Arranged in vertical columns, with cells receiving input from a small piece of the retina in one eye next to cells receiving input from that same piece of retina in the other eye (alternating columns of cells receiving info); spans multiple cortical layers of V1 (1˚ visual cortex).
What are hypercolumns?
pair of L and R ocular dominance columns
What are blobs?
groups of cells that process color information and do NOT have orientation specificity; interspersed among the ocular dominance columns
The cells within an ocular dominance column are arranged vertically according to their orientation selectivity; thus as one goes vertically down the column, how does the orientation change? What if you go across columns?
VERTICALLY: all cells have the same orientation specificity. ACROSS: gradual shift in orientation selectivity of the cells
What molecules generate the attractive and repulsive signals that guide axon projections from the lateral geniculate to the visual cortex?
Eph-Ephrins
What two factors guide patterned development of the ocular dominance columns in the visual cortex? (2)
1) chemical cues (Ephs, Ephrins) guide initial localization 2) final pruning and segregation of inputs via coordinated electrical activity (coincident input)
What is the critical period and how does it affect development of the visual cortex?
CP: period where axons that have grown into the cortex can still undergo plasticity (that is - sort and segregate under the influence of electrical activity).
What happens if there is no active input from one eye? What happens if input is restored during the critical period? What if it is restored after the critical period?
There will be fewer cells that have dual inputs (from both eyes), resulting in a thinner ocular column for that eye. RESTORED DURING: loss of ocular dominance column is REVERSED. RESTORED AFTER: blindness is permanent
What is needed for normal development of the visual cortex?
correlated electrical activity from each eye “coincident” pattern of activity
What is strabismus?
misalignment of the two eyes - usually due to an improper control or weakness/damage to one of the eye muscles, resulting in a lack of coordination between the eyes.
What is esotropia?
convergent strabismus “cross-eyed”
What is exotropia?
divergent strabismus “wall-eyed”
What is amblyopia? What is this a consequence of?
“lazy-eye”, reduced or complete loss of visual input from one of the eyes; caused by prolonged strabismus (misaligned eyes) and/or cataracts (damage to cornea)
What cells are responsible for 2D components of vision? (3) What is responsible for the 3D components of vision?
2D: 1) GANGLION cells: contrast, 2) SIMPLE and COMPLEX cortical cells: lines, boundaries, corners. 3D: visual association areas: (V2-V5)
Given how cells in the eye can detect 2D components of vision, how does the brain abstract information about the 3D component of vision?
the brain divides what it sees into 4 components: motion, depth, color, and shape. It individually analyzes it and compares it to stored memories, and then combines all of these into the field of view that you see and understand. This is done in the visual association areas (V2-V5)
What is V2 and V3 known as? Where can it be found? What is it responsible for?
Area 18. Occiptal lobe. Depth perception
What is V4 known as? Where can it be found? What is it responsible for?
Area 19. Occipital lobe. Color vision
What is V5 known as? Where can it be found? What is it responsible for?
area 19, or MT. Middle temporal area. Motion detection, spatial relationships, depth perception
What is the ventral stream of visual information? What type of information does it convey?
V1 -> V2 -> V4 -> Temporal lobe. Involved in object/form recognition (including color).
What is the dorsal stream of visual information? What type of information does it convey?
V1 -> V2 -> V4 -> MT -> Parietal lobe. Temporal lobe. Involved in motion detection, depth perception.
What is akinetopsia?
motion blindness - patient cannot perceive motion in their visual field but can still see stationary objects without a problem. Due to lesions in MT/V5 region and/or V1
How does lesions to V1 cause akinetopsia?
V1 is where pre-processing of visual information occurs; damage to this area limits motion detection but does not stop it completely
How does lesions to middle temporal cortex cause akinetopsia?
this is where MT/V5 is located, and this region is critical for motion detection and depth perception.
How can the motion of an object be determined? (2)
1) IMAGE movement - change in position of the image on the retina at different times. 2) EYE movement - use of info. about the movement of the eyes and head if we follow the object with our eyes and the position of the image on our retina stays in the same place
What is stereopsis?
ability to perceive depth; done by comparing and reconstructing the image that is projected onto the retina in both eyes
What are some monocular cues that is used in depth perception?
1) previous familarity with size, 2) interposition (overlapping of objects), 3) linear perspectives, 4) shadows and illumination, 5) motion parallax
What areas of the cortex is involved in depth perception?
V2, V3, and V5/MT
Where does color information go in the brain?
Parvocellular Pathway: P ganglion cells receive input from cones»_space; lateral geniculate»_space; “blobs” of the 1˚ visual cortex, layers 3-6»_space; visual association area V4 (area 19) of the occipital lobe
What is color constancy?
feature of the human perception system that ensures that the perceived color of objects remain constant even when the lighting has changed -> helps with object identification
What are the 3 characteristics of color?
1) hue, 2) saturation, 3) brightness
What is hue?
proporton to which 3 cone types (red, green, blue) are stimulated
What is saturation?
amount that all 3 cone systems (red, green, blue) are stimulated to the same degree
What is brightness?
total effect of stimulus on the 3 cone systems (red, green, blue)
What kind of organization do ganglion cells in the RETINA have for color detection? VISUAL CORTEX?
Both have a center-surround organization.
Where would you find single-opponent cells? Double-opponent cells? What type of organization do these cells have?
Both have a center-surround organization. RETINA: single-opponent ganglion cells (ie red+ center, red-surround). VISUAL CORTEX: double-opponent ganglion cells (ie red-/green- center w. red-/green+ surround)
What is the significance of having cells in the retina being single-opponent cells?
they transmit information about color
What is the significance of having cells in the visual cortex being double-opponent cells?
allows for comparison of the contrast of color.
What is the binding problem?
visual features (color/orientation) are coded in independent brain modules but have to be integrated into a single entity (likely via matching the element to a familiar/cued object)
How is visual input integrated step-wise? (3)
features map»_space; master map»_space; image of object
What is a features map?
first map that’s generated after scanning something really quickly and making note of certain characteristics: color, edges, texture, and depth
What is a master map?
map generated from multiple “features map”; contains information in which features have been detected in various locations
How is the image of an object ultimately formed?
via associations made between an object (based on the features map and master map) and prior knowledge to help identify images
What is synesthesia?
involuntary physicla experience of cross-modal linkage (where the stimulation from one sensory modality evokes another different modality) ex: listening to music evokes visions of different colors.
What is a theory that explains for the effects of synesthesia?
there may be EXCESS CONNECTIONS between the different sensory modalities in the brain. Ex: words activate langugage centers of the brain but the vision and color processing ceners as well.
