Flashcards in L12: Central Visual Pathways I Deck (56):
Primary visual cortex has multiple names, such as
The reticulo-geniculo-striate pathway is most important for visual perception. Explain the pathway
retinal ganglion cell, optic nerve, optic chiasm, optic tract, LGN, ultimately to primary visual cortex
Due to the optics of the eye, the visual field mapping onto the retina is inverted and flipped, what does this mean?
-superior nasal field will map onto the inferior temporal retina
-inferior temporal field will map onto the superior nasal retina
The nasal hemiretinal axons that carry info from temporal visual field will cross while the temporal hemiretinal axons do not cross. When exactly does this crossing occur?
The nasal hemiretinal axons cross at the level of the optic chiasm. So visual field representations are crossed at the level of the LGN and cortex
Explain the 6 layers of the LGN
lateral geniculate nucleus is found in the thalamus. It has 6 layers (VENTRAL/Layer 1: 2 magnocellular layers and 4 parvocellular layers: DORSAL/Layer 6). These diff layers are eye-specific/will be innervated by the optic nerve but in this way: Contra, ipsi, ipsi, contra, ipsi, contra)
The output of the LGN then creates the optic radiation that provides input to V1, primarily in which layer of the neocortex?
Layer 4 gives primary visual cortex, V1 its other name as the striate cortex because it forms the macroscopically visible "stria of gennari"
The visual input into V1 is also ordered retinotopically. But far greater cortex is devoted to which parts of the eye?
To central vision -the fovea
The task of vision is divide among several receptor systems. List 3 of them
1) the rods
2) 3 cone classes - L-cones, M-cones, S-cones
3) P, M, K signalling pathways
The magnocellular system is specialized for
high temporal resolution (speed) and higher contrast gain
The parvocellular pathway is specialized for
higher visual acuity and color
The koniocellular pathway is probably also specialized for
Where do you find the koniocellular layers in the LGN?
You find the K layers in btw the 6 layers
Both retinal ganglion cells and LGN cells have what types of receptive fields?
center-surround receptive fields
The input from the two eyes are segregated until V1, forming what types of columns?
ocular dominance (eye preference) columns
Who discovered the ocular dominance columns?
Hubel and Wiesel
V1 output that goes to extrastriate cortex, such as V2 (area 18) comes from which layers of the LGN
layers 2 and 3
Layer 5 of the V1 sends output to
superior colliculus which is a different visual pathway involved in eye movements and head/attentional orientation
Layer 6 of V1 goes
back to the LGN, forming the cortico-thalamic feedback loop
V1 neurons' receptive fields are different from retinal ganglion cells and LGN in that
VI neurons are not center-surround receptive fields; they are orientation-tuned/have oriented receptive fields
What does orientation tuning mean?
orientation tuning refers to the preference of a cell to be stimulated by an edge of light presented at a particular angle
Why does V1 want to create oriented cells?
Orientation tuning is probably the early stage of form vision anaylsis
What are the 2 color opponent signals that are created in the retinal circuitry?
1) Red vs Green
2) Blue vs Yellow
**where yellow is red vs green
Like other sensory neocortex, V1 is columnar. These column of cells will have
similar receptive field properties, similar orientation tuning
The ocular dominance columns of V1 alternate btw left and right eye. The pattern of the OD columns is interwoven with the pattern of the
V1 neurons have simultaneously a receptive field position (retinotopy), ocular dominance and preferred orientation except for
cytochrome oxidase (CO) blobs which are centered on the ocular dominance columns and contain monocular, UNORIENTED COLOR cells.
What makes up the beginnings of the segregation btw the "form" vision an "color" vision pathways?
The segregation btw the color blob cells and the regular, oriented V1 cells found in the interblobs
What is a hypercolumn?
a collection of functional cortical columns that cover the entire range of possible values of one stimulus parameter (e.g. orientation) for a given point in space
Give an ex. of a hypercolumn
a collection of orientation columns that would cover ALL POSSIBLE orientations from horizontal to vertical and back for a given point in visual space.
What is an ocular dominance hypercolumn?
a complete set of cortical columns handling both the right and left eyes.
The orientation column map an the ocular dominance bands run parallel or perpendicular to one another?
a 1mm x 1mm chunk of V1 neurons contains both an ocular dominance hypercolumn and an orientation hypercolumn -this implies that
this 1 mm2 chunk contains all the cortical machinery required to fully analyze one part of a visual scene for all orientation with both eyes
Output of V1 goes to V2 (area 18). Explain the 2 major visual processing streams after V2
1) DORSAL stream goes to PARIETAL cortex & seems most concerned with MOTION PERCEPTION and the LOCALIZATION visual objects (aka "WHERE PATHWAY") -includes visual areas such as MT, MST, Area 7a
2) VENTRAL stream goes to TEMPORAL cortex & seems concerned with object & pattern recognition (aka "WHAT PATHWAY) -includes areas such as V4, PIT, and AIT
areas such as V4, PIT, and AIT are involved in which pathway after V2
they are involved in the what pathway which is the ventral stream that goes to temporal cortex. and is involved in object & pattern recognition
areas such as MT, MST, Area 7a are involved in which pathway after V2
they are involved in the where pathway which is the dorsal stream most concerned with motion perception
T/F lesions in V1 lead to blindness or scotomas
T/F lesions in higher visual areas often manifest as blindness
False -often manifest as specific visual deficits, such as loss of color vision or form vision
In V1, there are CO blobs; are there blobs in V2?
No, there are not blobs. V2 has STRIPES
Explain the 3 types of CO stripes found in V2?
1) thick stripes - contain oriented , disparity cells and are involved in stereo depth vision
2) thin stripes - contain color cells
3) Pale stripes - contain oriented cells, often end-stopped, involved in detection of curvature
V2 is not as retinotopically organized as V1. Compare the receptive fields of V1 and V2
V2 has larger receptive fields
What do the V2 stripes suggest?
There's a separation of the visual processing for form, color and depth.
T/F: When we see an object as being red-colored, it is because our red cones are being stimulated, not our green or blue cones.
False: the 3 different cone types have 3 different spectral sensitivities, but they are very broad and overlapping, not just 3 different wavelengths. SInce our color vision is based on color-opponecy, if we see red-colored object, it is stimulating both the red cones and green cones. So, both red and green cones are stimulated and our visual system must compute the difference (red minus green) to learn that the red cones are stimulated more.
Can we see red and green at the same time? Or blue and yellow at the same time? Why or why not?
We cannnot see green and red or blue and yellow at the same time due to color opponency.
Are the ganglion cells associated with color vision arranged in antagonistic center-surround organization?
Probably, but there's still controversy as to how
What is the evidence that color is organized in an orderly cortical map compared to directionality?
The orderly cortical map is referring to the layout of the CO blobs in V1. CO blobs are patches of cells that lie in the center of OD column bands ("ice cube" model). CO blobs are color selective, monocular and have center-surround receptive fields. No one has demonstrated an orderly map for directionality in V1 in primates.
Do P and M pathways originating from the retina form an orderly cortical map?
No, they do not form an orderly cortical map. They make up all the input that goes to form these cortical maps, but the distribution of M and P are dispersed across all those cells and maps.
Explain where the inputs of P, M and K pathways go into the visual cortex?
K pathway = CO blobs
P pathway = layer 4cb of V1
M pathway = layer 4ca of V1
T/F: Although OD and orientation tuning are both properties found in the neurons of visual cortex, individual V1 neurons only exhibit one or the other property.
T/F: Any single cell resides in both an ocular dominance hypercolumn and an orientation column at the same time
List the 5 general info that's coming from retina to brain.
1 - spatial representation -retinotopy & receptive field
2 - intensity/luminence -rods and cones
3 - differential spectral absorption (s-cones, m-cones, l-cones)
4 - spatial contrast -center surround receptive fields, edge detection
5 - sign of contrast -light on dark or dark on light
retinal axons can go to LGN and then to visual cortex, but some will not synapse at LGN but to other places, such as
1) pretectum = pupillary light reflex
2) superior colliculus = orient movements of head & eyes
3) hypothalamus = regulation of circadian rhythms
Which of the following is the structure where a visuotopically organized, alternating (left/right) pattern of monocular responsiveness is found?
A) Layers 2 and 3 of LGN
B) Layer 4 of LGN
C) Layer 2/3 of visual cortex
D) Layer 4 of visual cortex
E) All layers of visual cortex
D) Layer 4 of visual cortex
Visual area _______ is most specialized for the processing of color information.
Which of the following statements about Primary Visual Cortex is false?
A) Each of the six layers has distinct cell types that send projections both within cortical columns and to structures outside of cortex.
B) Pyramidal cells are the principal projections neurons of visual cortex.
C) Layer 4C is dominated by spiny stellate cells.
D) The upper layers (layers 2 and 3) project to layer 5.
E) The lower layers (layers 5 and 6) project to the brainstem and thalamus.
In relation to other retinal ganglion cells, the P ganglion cells have
finer spatial resolution and better color sensitivity.
Which of the following is not a component of the phototransduction signaling cascade initiated by light falling on a rod?
A) Absorption of a photon of light by 11-cis retinal
B) Activation of protein kinase A
C) A conformational change in the rhodopsin protein
D) Dissociation of the α subunit of transducin from the β/γ complex
E) Increased activity of PDE (phosphodiesterase)